A. africana seed (containing 31% fat) may be a source of seed oil for domestic ...... across a disturbance gradient in Nepalese Sal (Shorea robusta Gaertn. f.) ...
UNIVERSITY OF ABOMEY-CALAVI (UAC) FACULTY OF AGRONOMIC SCIENCES
DEPARTMENT OF NATURAL RESOURCES MANAGEMENT
Effect of Human Disturbance and Climatic Variability on the Population Structure and Habitat Diversity of Afzelia africana Sm., a threatened tree species.
By Sylvanus MENSAH
Thesis presented in the fulfilment of the requirements for the degree of Master of Science in Forest Management
Supervisor: Prof. Romain L. GLELE KAKAÏ
Jury: Chairman Referee 1st examiner 2nd examiner
: Prof. Marcel R. HOUINATO : Prof. Romain L. GLELE KAKAÏ : Prof. Achille E. ASSOGBADJO : Prof. Julien Djego Academic year: 2012-2013
Certification
Certification I attest that the entirety of the present work has been done by Mr. Sylvanus Mensah at the Faculty of Agronomic Sciences of the University of Abomey-Calavi, under my supervision. I attest that it is an original work and that it has not been submitted elsewhere for obtaining any qualification. Signature:
Romain L. GLELE KAKAÏ Associate Professor of
Biometry
and
Forest
Modelling
i
Dedications
Dedications To the almighty for always feeding me on his graces!!! To my beloved late father, only God can repay you for always helping me reach this far!!!
ii
Acknowledgements
Acknowledgements I would like to thank everyone who help me reach this far. Particularly, my thanks go to:
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The International Tropical Timber Organization (ITTO)through a research grant (Ref.
142/12A) provided to Prof. Achille E. ASSOGBADJO -
Prof. Romain L. GLELE KAKAÏ, for his leadership, positive criticism and guidance
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Prof. Achille E. ASSOGBADJO for his advices and contribution
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All the members of departmental staff for providing me with necessary courses and tools
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Dr. Thierry D. HOUEHANOU for his assistance
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Dr. Gerard GOUWAKINOU for his help in climatic data extraction
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PhD students Valère SALAKO, Marcel DONOU and Rodrigue IDOHOU for their
encouragements and contribution -
The local people and foresters who provided help during field research
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My mother Bernadette HOUESSOU, brothers and sisters for their support
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Regine ANATO for her moral support
Finally, I would like to express my sincere gratitude to all postgraduate students in the Department for their encouragements and support.
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Figures and tables
Figures Figure 3.1 : Location of the study area ………………………………………………... Figure 5.1 : Stem diameter structures of A. africana populations according to climatic zone and level of anthropogenic disturbance …………………………….. Figure 5.2 : Projection of plots in the canonical system axis defined by the six most discriminant variables.…………………………………….……………… Figure 5.3 : Non Metric Multidimensional Scaling of plots from Guinean, SudanoGuinean, Sudanian and Sahelo-Sudanian zones……...…………………... Figure 5.4 : Loading of sample units from Guinean, Sudanian, Sudano-Guinean and Sahelo-Sudanian zones in combination with environmental variables...….
Pages 9 16 19 22 23
Tables Pages Table 2.1 : Common names, taxonomic classification and conservation status of A. africana …………………………………………………………………….. Table 2.2 : Different uses of A. africana………………………………………………... Table 3.1 : Characteristics of study area across climatic zones in Benin………………. Table 4.1 : Repartition of sample units according to climatic zones and levels of disturbance………………………………………………………………….. Table 4.2 : Description of dendrometric parameters………………………………......... Table 4.3 : Description of diversity parameters……………………………………........ Table 5.1 : Structural parameters of A. africana populations according to climatic zones and anthropogenic disturbance ……………….................................... Table 5.2 : Canonical loadings of selected variables: correlations between axes and variables ……................................................................................................. Table 5.3 : Significance of correlation coefficients between principal components of structural variables and environmental variables............................................ Table 5.4 : Most ecologically important species in A. africana habitats in the four climatic zones with their IVI………………………………………………..
4 6 8 10 11 14 18 19 21 24
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Abstract
Abstract Data on population structure and habitat species composition of an endangered species has important implications for conservation and restoration. Anthropogenic disturbances and climatic variations were presumed to alter the species population structures. Moreover, the habitat species composition is governed by climate, because functional groups differ in their climatic limits. In this study, we assessed the population structure and the woody flora composition of the habitat of an endangered species, Afzelia africana Sm. across a climatic and human disturbance gradients. Dendrometric variables (diameter at breast height and total height) of A. africana were recorded at national scale in forests reserves of different climatic zones in Bénin. On the first hand, the effects of climatic variations and disturbance were evaluated using structural parameters (regeneration and tree density, mean diameter, basal area and height) and stem diameter structure. To describe the A. africana populations’ structure between climatic zones and disturbance levels, a Canonical Discriminant Analysis after stepwise selection was performed using variables related to the size of trees, their density and their vertical structure. The relationships between the principal components (structural parameters of A. africana stands) and climatic variables were assessed using Pearson correlation. On the second hand, the woody flora composition of Afzelia africana Sm. habitats was assessed along a latitudinal gradient using a Non Metric Multidimensional Scaling on presence-absence data matrix. This helped explore the patterns of woody species composition of natural stands. The observed patterns were correlated with climatic variables (temperature, precipitations etc.) and altitude, through a Canonical Correspondence Analysis. Finally, the Importance Value Index (IVI) was computed for each species recorded within each Bioclimatic zone in order to determine species dominance per zone. Significant differences were found in the structural parameters between the disturbance levels, mostly in the Guinean zone. However, structural parameters differed significantly across the three climatic zones, with the Guinean zone recording the highest values. The effects of disturbance levels on structural parameters depend on the climatic zone, and vice versa. Tree characteristics such as thickness (size) and tallness were found to discriminate the A. africana populations along the disturbance and climatic gradients. In the Guinean zone, the tallest and biggest trees were found at the low disturbance level. However, along the climatic gradient (towards drier regions), trees were shorter and smaller at the low and high disturbance levels. Furthermore, the tallest and biggest trees were found at less drier regions with lower altitudes. The results from multidimensional scaling revealed that plots of Sudanian and SudanoGuinean zones were similar but distinct from those of Guinean and Sahelo-Sudanian zones. This delineation in the habitat diversity of A. africana was further supported by precipitation and temperature regime as revealed by Canonical correspondence analysis. Results also suggest important co-occurring species characteristic for each habitat as inferred from IVI values. The implications for management and conservation of A. africana are discussed. Key words: A. africana, disturbance gradient, climatic gradient, habitat pattern, floristic composition. v
Résumé
Résumé Les données sur la structure des populations et la composition spécifique d’une espèce menacée ont d’importantes implications pour sa conservation et sa restauration. Les pressions anthropiques et les variations climatiques sont supposées modifier la structure des populations des espèces. De plus, la composition spécifique est gouvernée par le climat, parce que les groupes fonctionnels différent dans leur limite climatique. Dans cette étude, nous avions examiné la structure des populations et la composition floristique de l’habitat d’une espèce menacée, Afzelia africana Sm. à travers les gradients climatiques et de pression. Les données dendrométriques (diamètre et hauteur) de A. africana ont été collectées à une échelle régionale dans les réserves forestières des zones climatiques du Benin et du Burkina Faso. Les effets des variations climatiques et de perturbation anthropique ont été évalués en utilisant les paramètres structuraux (densité d’arbre et de régénération, diamètre moyen, surface terrière moyenne et hauteur moyenne) et les structures en diamètre. Pour décrire les structures des populations de A. africana entre zones climatiques et niveaux de pression, une Analyse Canonique Discriminante a été effectuée en utilisant les variables liées à la taille, la densité et la structure verticale des arbres. La relation entre les composantes principales et les variables climatiques ont été examinées à travers le test de corrélation de Pearson. La composition ligneuse floristique des habitats de Afzelia africana Sm. a été explorée le long du gradient latitudinal à travers un Positionnement Multidimensionnel sur les données de présence absence. Ceci a permis d’explorer les patrons de la composition ligneuse spécifique des habitats de l’espèce. Les patrons observés ont été corrélés avec les variables climatiques et l’altitude à travers une analyse canonique de correspondance. Enfin, les indices de valeur d’importance ont été calcules pour chaque espèce dans chaque zone afin de déterminer les espèces importantes des habitats de A. africana. Des différences significatives ont été obtenues entre les niveaux de pressions surtout dans la zone guinéenne. Cependant, les paramètres structuraux diffèrent entre les trois zones climatiques, les valeurs élevées étant obtenues dans la zone guinéenne. Les effets des niveaux de perturbation dépendent de la zone climatique et vice versa. Les caractéristiques telles que la grosseur et la hauteur des arbres discriminent les populations de l’espèce à travers les gradients climatiques et de pression. Dans la zone guinéenne, les arbres les plus hauts et les plus gros sont retrouvés dans les peuplements à faible niveau de pression. Toutefois, vers les régions sèches, les arbres deviennent moins hauts et moins gros dans les deux niveaux de pression. De plus, les arbres les plus gros et les plus hauts sont retrouvés dans les régions moins sèches à faibles altitudes. Les résultats du positionnement multidimensionnel révèlent que les placeaux du Soudanien et du Soudano-Guinéen sont similaires mais distincts du Guinéen et du Sahélo-Soudanien. Cette discrimination dans la diversité de l’habitat de l’espèce est soutenue par les régimes de précipitation et de températures comme le révèle l’Analyse Canonique de Correspondance. Mots clés: A. africana, gradient de pression, gradient climatique, composition floristique vi
Table of contents
Table of contents Certification ................................................................................................................................. i Dedications ................................................................................................................................. ii Acknowledgements ................................................................................................................... iii Abstract ...................................................................................................................................... v Resume ..................................................................................... Error! Bookmark not defined. 1. INTRODUCTION .................................................................................................................. 2 1.1. Rationale.......................................................................................................................... 2 1.2. Objectives ........................................................................................................................ 4 1. Botanic description, climatic limitations and uses of A. africana .......................................... 5 3. STUDY AREA ....................................................................................................................... 8 3.1. Description of study area ................................................................................................. 8 4. MATERIAL AND METHODS ........................................................................................... 11 4.1. Material ......................................................................................................................... 11 4.2. Sampling design and vegetation and climate data collection ........................................ 12 4.3. Structural characterization of the populations of A. africana along climatic and anthropogenic disturbance gradients .................................................................................... 12 4.4. Discrimination of A. africana populations for structural and distributional variables .. 14 4.5. Climate-related structural variables of A. africana populations ................................... 15 4.6. Assessing the variation in habitat woody species composition along climatic gradient15 4.6.1. Discrimination of habitat of A. africana along climatic gradient .......................... 15 4.6.2. Characterization of species composition of A. africana habitat............................. 16 5. RESULTS............................................................................................................................. 17 5.1. Structural characteristics of A. africana populations along climatic and anthropogenic disturbance gradients ............................................................................................................ 17 5.1.1. Stem diameter structures ........................................................................................ 17 5.1.2. Structural parameters .............................................................................................. 18 5.2. Patterns of structural parameters of A. africana populations across climatic zones ..... 20 5.3. Relationship between structural traits of A. africana populations and climatic parameters ............................................................................................................................ 22 5.4. Discriminated habitats of A. africana along the climatic gradient ................................ 23 5.5. Characterization of A. africana habitats’ woody species composition along latitudinal gradient ................................................................................................................................. 25 6. DISCUSSION AND CONCLUSION .................................................................................. 26 6.1. Anthropogenic disturbance of A. africana natural stands according to climatic zones 26 6.2.Traits variation patterns of A. africana populations across the climatic gradient .......... 27 6.3. Change in woody flora composition of A. africana habitats along the latitudinal gradient ................................................................................................................................. 29 6.4. Conclusions and implications for the conservation of A. africana ............................... 32 References ................................................................................................................................ 34
vii
Chapter 1. Introduction
1. INTRODUCTION 1.1. Rationale Tropical forests are among the most threatened ecosystems in the world, and except for the Congo and Amazon Basins, most remaining forest habitats are now fragmented and turned into human-modified landscapes (Santo-Silva et al. 2013). The growing human population increases the agricultural intensification and the demand for forest products, and as results, African forest degradation and loss of biodiversity occur at a scary rate (Assogbadjo et al. 2009). In the meantime, natural habitats are also negatively impacted by forest degradation (Debinsky and Holt 2000) and therefore, species populations become increasingly vulnerable to extinction. Moreover, the patterns of associated plants are recurrently facing threats from climate change. All these situations raise a critical question regarding the fate of endangered species. Understanding their current status and the factors that affect the preservation of their populations remains a fundamental task of conservation biology (Lindenmayer and Fischer 2006). It is well established that rural and urban people still depend on tropical forest products and particularly on high value plant species (Singh and Singh 1987, Bellefontaine et al. 2000, Glèlè Kakaï et al. 2009, Nacoulma et al. 2011, Shrestha et al. 2013). In Benin, a selective forest harvesting is reported on valuable specieslike Kaya senegalensis (Desr.) A. Juss. (Gaoué and Ticktin 2008), Afzelia africana Sm. (Sinsin et al. 2004) and Pterocarpus erinaceus Poir. (Glèlè Kakaï et al. 2009). Among these species, A. africana is the most threatened species. A. africana is an agroforestry tree species (Assogbadjo et al. 2010) belonging to the Fabaceae-Caesalpinioideae family. The species is widely distributed in several types of natural forests spreading from the northern limit of the species distribution to the Guinean littoral forest (Adomou et al. 2009, Ouédraogo and Thiombiano 2012). It is heavily harvested for timber mainly by indigenous communities and its foliage is an important forage for herders in raising cattle (Onana 1998,Ouédraogo-Koné et al. 2008). Moreover, it seeds are used as thickening agent (African Regional Workshop 1996), and bark, leaves and roots, useful for traditional medicine (Adjanohoun et al. 1989). Because of these uses,A. africana is classified as endangered at country-scale (Adomou et al. 2009).The acknowledged utility of the species in tandem with its insistent harvesting has increased the investigations at local and
2
Chapter 1. Introduction
regional scales (Sinsin et al. 2004, Bonou et al. 2009, Ouédraogo and Thiombiano 2012, Houehanou et al. 2013a). These investigations have elucidated the critical traits of A. africana populations and reported a very weak potential of recruitment. The human disturbances and climate pejoration were documented to have a significant negative impact on A. africana regeneration (Ouédraogo and Thiombiano 2012). In this line, Sinsin et al. (2004) reported increasing anthropogenic disturbances from the Guinean to the Sudanian zones of Benin. Such a situation suggests some combined effects of human disturbances and climate conditions on the species population structure. In the Guinean zone, forests and fallows habitats serve as shelter for the species (Bonou et al. 2009), however, change in its population structure in relation with other climatic zones remain misunderstood. Additionally, many studies on A. africana were limited to a local scale (Bonou et al. 2009, Houehanou et al. 2013a, Chabi et al. 2013) and missed linking the species traits with climatic conditions (Sinsin et al. 2004). Consequently, knowledge on the species traits throughout the three climatic zones of the country is required. It may be worth taking into account these climatic zones, to search out for respective effects of disturbance and climatic gradients as well as their eventual interaction. Furthermore, assessing the traits that discriminate the species populations will provide further details for understanding the impacts of disturbances across climatic gradient and implementing an appropriate management and conservation strategies of the species. In spite of its wide distribution, the species should genetically get used to a local climate. As a result, even slight variations in climate could hamper its capacity to cope with local environmental conditions. The climatic variations may impact the population structure of the species so that it could grow better in some climatic zones than in others ones. Thus, A. africana structural characteristics (tree density, thickness and tallness) are expected to be correlated positively with increasing rainfall. Although Biaou (2009) studied the effect of precipitation (water stress) on recruitment characteristic of some tree species in dry woodlands and savannas of Benin, understanding how climatic drivers could explain A. africana population structure is still lacking. Similarly, A. africana is found with many other species across its habitat (Akoègninou et al. 2006) and such cohabitation is likely governed by climate-driven mechanisms. In addition, vegetation analytic studies that are oriented in a way that makes it possible for decision-makers to take into account an entire habitat and not only the individual species, have the potential of saving species from becoming extinct. We therefore believe that, an 3
Chapter 1. Introduction
analysis of woody floristic composition of A. africana habitat along a latitudinal gradient could reveal the important species assemblages and provide useful information to support conservation strategies. Therefore, our study emphasized a country scale analysis of A. africana population structures and habitat diversity.
1.2. Objectives The global objective of this study is to assess the populations’ structures and habitat species composition of A. africana across a climatic gradient. Specifically the study aimed to: -
Objective 1. Characterize the structure of populations of A. africana along the climatic and disturbance gradients
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Objective 2. Discriminate the populations of A. africana along the climatic and disturbance gradients
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Objective 3. Examine the relationships between the structural and distributional variables of A. africana populations and environmental variables
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Objective 4. Assess the variation in the woody flora composition of A. africana habitat along a latitudinal gradient
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2. BOTANIC DESCRIPTION 2.1. Botanical description, climatic limitations and uses
Chapter 2. Botanic description, climatic limitations and uses of A. africana
1. Botanic description, climatic limitations and uses of A. africana Afzelia africana is a large deciduous tree with a spreading crown, to 30 (-35) m tall in forests and 10-18 m tall in savannah. The average dbh is 1 m. The stem has relatively thick, unequal buttresses with a light concave profile; in general they are 1-1.5 m tall and 1-2 m wide. Twigs are glabrous with lenticels. The bark is a reddish-grey, scaly, about 2 cm thick. It exfoliates in rounded patches, protecting the tree effectively against the frequent bush-fires of the dry season. The rosy slash exudes a dark yellow, highly aromatic resin.
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Chapter 2. Botanic description, climatic limitations and uses of A. africana
Photo 2.1. Tree (up), trunk (left), branch (right) and flowers (down) of A. africana tree. The leaves bright green, paripinnate, to 30 cm long each with 7-17 pairs of elliptic or ovate glabrous leaflets. Petioles are long from 0.4 to 1 cm. The flowers hold variable colors from white to yellowish and 1.5 cm of length, with one single red striped petal, set in terminal panicles to 20 cm long. Fruit is a flat pod 12-17 x 5-8 x 3.5 cm hard, slightly rounded, dark brown to black, glabrous with a distinct beak at one end. Each pod contains several black seeds. Seeds are poisonous, about 2-3 cm long, with sweet bright orange edible aril in onethird of its length from the base. 6
Chapter 2. Botanic description, climatic limitations and uses of A. africana Flowering occurs at the end of the dry season (April-May in Guinea) and fruiting takes place from December to February. Wind and animals disperse the seeds. A. africana could be found at 200-1200m altitude range under 20 up to 35°C temperature. Annual rainfall could fluctuate between 1000 and 1800 mm. The species prefers deep sandy soil in well-watered sites but tolerates seasonally hydromorphic and lateritic soils. The uses of A. africana are summarized in table 2.1. Currently the species is mainly used for its good timber for materials making and house building and its leaves for cattle feeding. Table 2.1. Different uses of A. africana Uses Food Fodder
Timber Lipids
Medicine
Other products Erosion control Shade or shelter Soil improver Other services
Description The flour from seeds is used as a substitute for wheat flour in biscuits and doughnuts Leaves, fruits and seeds are browsed by wildlife, particularly before the regrowth of grass in the early rainy season. Wild animals browse the arils, and antelopes eat the young shoots. Flying foxes and bats eat the flowers. - Used in carpentry, canoe and house building, furniture making, flooring, and heavy construction. - Also used in woodcarvings, mortars, poles, pilings and other traditional uses. A. africana seed (containing 31% fat) may be a source of seed oil for domestic and industrial use - Infusion of bark is used against paralysis - Decoction of bark is used against constipation. - The maceration is a remedy for leprosy. - Crushed bark, mixed with honey, is used in veterinary medicine. - Ash of the bark, prepared with Shea butter as soap, is used against lumbago. - In decoction or prepared with food, it is a treatment for back-ache. - Roots are pulverised with millet-beer in Côte d'Ivoire and used to treat hernias - Roots in a decoction with pimento is a remedy against gonorrhoea and stomach-ache. - Leaf decoction mixed with Syzygium guineensis leaves and Xylopia fruit forms a drink to treat oedema. The ash of the fruits is rich in potassium salts and is mixed with millet for veterinary purposes. Pods are rich in ashes used for making soap. The seeds are used as a soup thickener As a leguminous tree A. africana is used for soil conservation and improvement. In Upper Guinea the tree serves as a site for hunters and provides shelter The tree associated with ectomycorrhizal fungi, which improve the soil. The nitrogen-rich leaves are used to enrich the soil especially when mulching and littered In many villages it is a sacred tree and is often situated more or less in the middle of the sacred village forest.
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3. STUDY AREA 3.Description of the study area
Chapter 3. Study area
3. STUDY AREA 3.1. Description of study area This study is carried out in the Lama Forest Reserve (LFR, 16.250 ha) located in the Guinean zone, the Wari-Maro Forest Reserve (WMFR, 120.686 ha) and the Bélléfoungou Forest Reserve (BFR, 709 ha) located in the Sudano-Guinean zone, the Pendjari Biosphere Reserve (PBR, 466.640 ha) situated in the Sudanian zone (Figure 1)and the Sudanian phytogeographical areas of eastern Burkina Faso (Sahelo-Sudanian zone). The LFR spreads from 6°55’-7°00’ N to 2°04’-2°12’ E and is characterized by a Guinean climate with a bimodal rainfall regime. The two rainy seasons occur from April to July and from September to November. The driest and coldest periods occur from February to March and August. The LFR is dominated by dense forests and fallows (Bonou et al. 2009). The fallows are subject to human disturbances through agricultural settlings of dwellers around the reserve. Many edible and medicinal plants that these people depend on, come from the fallows. The consequences are that different portions of fallows are characterized by an open vegetation predominated by Chromolaena odorata (L.) King & H.E. Robins., and some tree species such as A. africana, Diospyros mespiliformis Hochst. ex A. DC. And Dialium guineense Wild. (Bonou et al. 2009). However, the dense forest (in the core of the reserve) received much attention in terms of protection from the National Timber Office (Office National du Bois). The dense forest within the LFR is among the most important remaining natural forests of the Benin Guinean zone and is considered as at a low level of disturbance, by opposition to the fallows, seen as at high level of disturbance. The BFR (9°46’40’’- 9°49’00’’N to 1°42’00’’- 1°45’00’’E) is under a Sudano-Guinean climate with two contrasting (dry and rainy) seasons of 6 months each. Woodlands are the main vegetation type of this zone. Although local communities were not allowed to enter into BFR, protection fell and the forest suffered uncontrolled human activities, especially lands ploughing, pastoral installations and grazing, branches and trees logging. These activities have all taken their toll on the important and valuable species of the area. Thus, this forest is seriously perturbed (Houeto et al. 2012). The WMFR (8°80’-9°10’N to 1°55’-2°25’E) is also characterized by a Sudano-Guinean climate with two contrasting seasons of 6 months each. The WMFR is also perturbed, but is considered as at low level of disturbance mainly due to the presence of foresters. Traditional and cultural management practices are permitted around the forest and local people are also
8
Chapter 3. Study area allowed to collect only forest products such as firewood and wild foods products. However, trees logging and pasture are prohibited. The PBR (10°30’-11°30’ N and 0°50’-2°00’E) is characterized by a Sudanian climate, with two contrasting seasons of 6 months each. The rainy season lasts from April or May to October and the dry season covers the period from November to March or April. The reserve is managed for in-situ biological conservation goals but its surrounding zones are often subjected to human disturbances (tree logging, agricultural activities and pasture). The main vegetation types are savannas. The surrounding areas were considered as at a high level of disturbances oppositely to the reserve that is protected The Sudanian phytogeographical areas of eastern Burkina Faso (Sahelo-Sudanian zone) are characterized by a Sahelian climate with an uni-modal rainfall regime. The dominant vegetation are trees and shrub savannah. Across the study area, the Sudanian and Sudano-Guinean zones are characterized by open vegetation (woodlands and savannas) that offers considerable herbaceous cover, which is extremely vital for the herds during the active period of vegetation. In these areas, transhumance is highly developed and both rural and urban people exploit the important species as sources of raw materials, medicine and foods. The Sudanian and Sudano-Guinean zones are also known for their marked drought period lasting 6 months. These areas are drier than the Guinean region that displays a bimodal rainfall regime. Climatic parameters of each zone are summarized in table 3.1. Table 3.1. Characteristics of study area.
Characteristics
LFR
WMFR
PBR
Location
6°55’-7°00’ N 2°04’-2°12’ E 1900 Guinean
8°80’-9°10’N 1°55’-2°25’E 120686 SudanoGuinean Woodland
10°30’-11°30’ N 0°50’-2°00’E 266040 Sudanian
Area (ha) Climatic zone Dominant vegetation Rainfall regime Rainfall range Temperature range
Dense forest and fallows Bimodal 1000-1400 25-29
Unimodal 1100-1300 25-29
Savannah and woodland Unimodal 800-1100 24-31
Sudanian phytogeographical areas 12°35’-11°14’ N 0°10 - 2°30 E 4669400 Sahelo-Sudanian Tree and shrub- savannah Unimodal 600-900 25-35
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Chapter 3. Study area
Sudanian zone
Sudano-Guinean zone
Guinean zone
Figure 3.1. Location of the study area Source: Mensah, 2013.
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4. METHODS 4.1. Material 4.2. Sampling design and vegetation and climate data collection 4.3. Structural characterization of the populations of A. africana along climatic and anthropogenic pressure gradients 4.4. Discrimination of populations of A. africana for structural and distributional variables 4.5. Linking structural and distributional variables of A. africana populations with environmental variables 4.6. Assessing change in habitat species composition along climatic gradient
Chapter4. Material and methods
4. MATERIAL AND METHODS 4.1. Material The material of this study is constituated of the african mahogany A. africana populations. A. africana is a tree species belonging to Fabaceae-cesalpinioideae family. The genus Afzelia encompasses eleven (11) taxa of which seven (7) occur in the tropical Africa and the remaining are growing in the East of Southern Asian. A. africana is vulnerable in the world according to IUCN criteria, and endangered in Benin. Table 4.1 shows species common names, taxonomic classification and conservation status as well. As species of the semideciduous forest and Sudano-Guinean savannas up to the southern border of the Sahel, A. africana is encountered in many countries of East and western Africa such as Benin, Burkina Faso, Cameroon, Central African Republic, Chad, Congo, Côte d'Ivoire, Ghana, Guinea, Guinea-Bissau, Mali, Niger, Nigeria, Senegal, Sierra Leone, Sudan, Togo and Uganda. In Benin, the species is found in the three climatic zones (Guinean, Sudano-Guinean and Sudanian zones), precisely in southern Benin (Lama Forest Reserve, Pobè), in the centre (Regions of Monts Kouffé, Wari Maro, Djougou) and in North of the country (Niger basin). Table 4.1. Common names, taxonomic classification and conservation status of A. africana Languages English French Fon Holli Nago
Names African mahogany, Afzelia Lingué, Doussié, Afzélia kpakpagidé Kpakpa akpakpa Taxa Kingdom Plantae Sub-kingdom Tracheobionta Class Magnoliopsida Sub-Class Rosidae Order Fabales Classification Family Fabaceaea Subfamily Caesalpinioideae Genus Afzelia Species A. africana Afzelia africana Smith ex Pers. Binomial name Vulnerable (World; IUCN) Conservation status Endangered (Benin)
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Chapter4. Material and methods
4.2. Sampling design and vegetation and climate data collection A total of 220 plots of various sizes (table 4.2) were established in the forest reserves by means of stratified random sampling design. The reason for the differences in plot sizes is attributed to the vegetation of Sudanian zone, which is composed of savannas, while woodland and dense forest characterized the vegetation of Sudano-Guinean and Guinean zones, respectively. Within each plot, presence or absence of A. africana was noted. Species names as well as diameter at breast height (dbh) and total height were recorded from all tree species having dbh ≥10 cm. The number of recruits of the species (dbh < 10 cm) were also recorded in a 400 m2 quadrat set up at the corner of each 1 ha plot. In the Sudanian zone, the quadrats considered have 10 m side and were also set up at the corner of each main plot. Quadrats in the Sudanian zone have different size, because of the basic plot size that was considered in this zone. Table 4.2. Repartition of sample units according to climatic zones and levels of disturbance Guinean zone
Sudano-Guinean zone Sudanian zone
Total
48 (100 x 100 m2)
35 (100 x100 m2)
35 (30 x 30 m2)
118
High disturbance 52 (100 x 100 m2)
15 (100 x100 m2)
35 (30 x 30 m2)
102
50
70
220
Low disturbance
Total
100
Climate was expected affect the ecology of plant species due to their long time action (Thuiller et al. 2004, Algar et al. 2009, Record et al. 2013, Braunisch et al. 2013). Climatic variables were extracted from WorldClim data base using DIVA-GIS 7.5 (Hijmans et al. 2005). Variables included; annual mean temperature, mean temperature of wettest quarter, mean temperature of the driest quarter, mean temperature of warmest quarter, mean temperature of coldest quarter, total annual precipitation, precipitation of wettest quarter, precipitation of driest quarter, precipitation of warmest quarter, precipitation of coldest quarter and altitude. Quarter refers to four months.
4.3. Structural characterization of the populations of A. africana along climatic and anthropogenic disturbance gradients The three climatic zones were compared according to the structural parameters of A. africana populations along a disturbance gradient. Mean and coefficients of variation of tree-
12
Chapter4. Material and methods density (N, trees ha-1), mean diameter (Dm, cm), basal area (G, m2 ha-1) and Lorey’s mean height (HL, m) were calculated (See table 4.3 for parameter formulas). Table 4.3. Description of dendrometric parameters Dendrometric parameters
Description
Tree-density (N, trees.ha-1)
Average number of trees per hectare
Mean diameter (Dm, cm)
Diameter of the tree with the mean basal area in the stand
Dm
Sum of the cross-sectional area at 1.3 m above the ground level of trees
G
Basal area (G, m2.ha-1)
Formulas n N S n
1 n
d i 1
0.0001 4s
n
Lorey’s mean height (HL, m) (Philips et al. 2002)
Average height of all trees found in the plot, weighted by their basal area
Density of regeneration (Nr, plants.ha-1)
Mean density of species regeneration within stand
Limitation of regeneration (Lr, %) (Muller-Landau et al. 2002)
Proportion of regeneration subplots where regeneration did not occur; high values indicate very low presence of species.
HL Nr
i 1 n
1 np
Lr
2 i
n
d i 1
gihi
i 1
gi
np
y i 1
pi
r n
Differences of mean values were tested between levels of disturbance along climatic gradient through 2-way Analysis of Variance (ANOVA) applied to log-transformed data (log(x+1)). Since interaction between the two factors was significant, the slice option of the SAS software was used to compare effect of disturbance levels within each climatic zone and vice-versa. Density and limitation of regeneration (Lr, %; Muller-Landau et al. 2002) were also quantified. Limitation of regeneration is the proportion of quadrats where recruits did not occur; high values indicate very low presence of individuals. Stem diameter structures were established for each disturbance level within each climatic zone and adjusted to the 2parameters Weibull distribution (Bailey and Dell 1973). Its probability density function for a random variable x (diameter) is (Jonshon and Kotz 1970):
bx f(x) aa
b 1
x b exp a
(1)
where, a the scale parameter, b the shape parameter, and exp [ ] is the exponential function. The Weibull parameters were estimated with R.2.15.3 softwareand diameter distributions with the Weibull fitting were implemented using Minitab 14.However, due to some pitfalls 13
2 i
Chapter4. Material and methods inherent from the flexibility of the Weibull distribution (Feely et al. 2007), the skewness coefficient was additionnaly computed. The skewness is a simple measure of the asymmetry of distribution and defined as (Bendel et al.1989):
δ
n i
x m
3
i
n 1n 2s3
(2)
where n is the number of stems and xi, m and s are the log(dbh) of culm i, the mean of xi, and the standard deviation of xi, respectively. A negative value of δ indicates skewness to the left and distribution with few small stems and many large stems whereas positive value of δ indicates skewness to the right and distribution with relatively few large stems and many smaller sized stems.
4.4. Discrimination of A. africana populationsfor structural and distributional variables Tree density (N), mean diameter (Dm), basal area (G), contribution to stand basal area (Gc) and Lorey’s mean height (HL) were used as structural variables. A second group of variables related to the proportion of density and basal area was additionnaly set (Reque and Bravo 2008): 10-30 cm dbh, 30-60 cm dbh and ≥ 60 cm dbh. These dbh classes constitute respectively fine, medium and thick trees. Similarly, three height classes namely < 10 m (stratum 1), 10-15 m (stratum 2) and ≥ 15 m (stratum 3) were considered. Thereafter, we computed the relative density Ni (%) and relative basal area Gi (%) as density and basal area of A. africana trees belonging to a given diameter class i reported to the overall density and overall basal area of species within a given plot; the relative density Nj (%) as density of A. africana trees belonging to a given height class j reported to the overall density of species within a given plot. A total of 14 quantitative structural variables were computed per sample unit of A. africana in each climatic zone associated with each disturbance level. Because of the large number of initial candidate variables, we selected the most dscriminant through a stepwise discriminant analysis. Canonical Discriminant Analysis (CDA) was thereafter applied for a better presentation and easier interpretation of the data. This multivariate method is useful to identify the variable that describe better the variation in species populations structures, but also allows to gather the plots into subgroups on the basis of the differentiating variables. The multivariate analyses were implemented usingthe R2.15.3 package (R Development Core Team, http://www.Rproject.org). The stepwise discriminant 14
Chapter4. Material and methods analysis was performed using greedy.wilks function under klaR package. This procedure provides directly the model formula with only the relevant variables. The canonical discriminant analysis and cluster analysis were run with candisc function under candisc and heplot packages.
4.5. Climate-related structural variables of A. africana populations Principal Component Analysis was performed on structural data, and the first 4 principal components were selected according to the latent root criterion (Hair et al. 2009). These components were correlated with altitude and bioclimatic variables gathered from 2.5 x 2.5’ Wordclim data set using DIVA-GIS 7.5 (Hijmans et al. 2005). The statistical analyses were
performed
using
the
R2.15.3
package
(R
Development
Core
Team,
http://www.Rproject.org).
4.6. Assessing the variation in habitat woody species composition along climatic gradient 4.6.1. Discrimination of habitat of A. africana along climatic gradient The woody flora diversity was described by calculating the species richness, the Shannon diversity index and the Pielou evenness in the vegetation stands (table 4.4). Richness (S, number of tree species found in the whole stand) is a primary measure of diversity and the most recommended one for multi-scale comparison (Kluth and Bruelheide 2004). Shannon index of diversity (H) is related to species richness but also influenced by the distribution of species abundance (Magurran 1988). The pattern of habitats was investigated by performing a Non Metric Multidimensional Scaling (NMDS) on presence-absence data matrix. The NMDS was run using the “metaMDS” function from the vegan package. The function automatically applies a square root transformation and calculated the Bray-Curtis distances for the data matrix. This function has the benefit of providing solutions with smallest stresses using several random starts, thing that may be tedious when using other function like isoMDS (Oksanen 2013). The plots were mapped onto two axes to enable each one to be assigned to a cluster. The nearest plots of the map were considered as the most similar in species composition. The NMDS makes it possible to detect an explanatory gradient of the variation of woody flora composition. Afterwards, a Canonical Correspondence Analysis (CCA) was performed on the presenceabsence data in combination with climatic variables to explain the pattern of habitats. These
15
Chapter4. Material and methods multivariate analyses were carried out using the R 2.15.3 statistical software package (R Development Core Team, http://www.Rproject.org). Table 4.4. Description of diversity parameters Parameters Shannon diversity index (H, bits)
Pielou evenness (Eq)
Importance Value Index (IVI, %)
Description Based on the proportional abundance of species; ni= individual number of species i; n = total number of trees inventoried in the plot. Measure of the diversity degree; Hmax= maximum value of the Shannon’s index of the stand, S= number of tree-species recorded in the considered plot Measure of the importance level of a plant species; ni, fi and ci are respectively the density, frequency and basal area of species i.
Formulas s
H i 1
Eq
ni n Log 2 i n n
H Hmax
IVI
ni
; Hmax Log 2 S
s
fi
s
ci s
n f c i 1
i
i 1
i
i 1
i
4.6.2. Characterization of species composition of A. africana habitat To characterize the habitat species composition, we assessed the relative availability of woody species in each habitat, by computing the Importance Value Index (IVI) (Curtis and Macintosh 1951). IVI was computed for each species as sum of its relative frequency, density and dominance (basal area) in each climatic zone. Species with IVI > 10% were retained to be ecologically important in each climatic zone (Reitsma 1988).
16
5. RESULTS 5.1. Structural characteristics of A. africana populations along climatic gradient and anthropogenic pressure 5.2. Discrimination of populations of A. africana for structural and distributional variables 5.3. Links between structural and distributional traits of A. africana populations and environmental parameters 5.4. Discrimination and characterization of A. africana habitats along the climatic gradient
Chapter5. Results
5. RESULTS 5.1. Structural characteristics of A. africana populations along climatic and anthropogenic disturbance gradients 5.1.1. Stem diameter structures Except for the high disturbed stand of the Guinean zone, a bell-shaped distribution of size class diameter was observed in all climatic zones (Figure 1). In the Guinean zone, low disturbed stand showed a slight left asymmetric stem diameter structure (δ ˂ 0) with a high value of the shape parameter (5.76). Both values revealed a distribution with relatively few young individuals compared to adult ones. The 55-75 cm dbh class was the most represented, with more than 30 individuals. In high disturbed stand, the skewness (0.39) and the shape parameter of Weibull (1.49) revealed a right symmetric distribution characterized by several gaps and relatively many young individuals. Both low and high disturbed stands of the Sudano-Guinean zone exhibited similar trends, with stem distribution characterized by many young individuals. The 20-35 cm dbh class was the most represented with 20 to 32 individuals at the low disturbance level and less than 10 individuals at the high disturbance level. These diameter classes revealed population structures with gaps after 80 cm diameter. Positive skewness and 1-3.6 values of the shape parameter of the Weibull distribution characterized the observed patterns of diameter structure at low and high disturbance levels of the Sudanian zone. Stands at the two levels of disturbance showed distribution with relatively many young individuals. The most represented dbh class was 25-55 cm for the low disturbed stand and 15-35 cm at the high disturbance level. Moreover, 70-95cm dbh was still observed in the low disturbed stand whereas the highest values of diameters still fluctuated around 70 cm in the high disturbed stand.
17
Chapter5. Results 40
Weibull a : 32.77cm b : 5.76 δ : - 0.33
30
Frequency
Frequency
30
20
20
0 0
10
20
30
40
50
60
70
80
90
0
10
20
30
40
Diameter (cm)
60
70
80
Weibull a : 53.10cm b : 1.49 δ : +0.39
10
0 20
30
40
50
60
70
80
10
20
30
40
50
90 100 110 120 130 140
Weibull a : 38.61cm b : 2.40 δ : +0.87
30
20
70
80
90 100 110 120 130 140
Weibull a : 32.27cm b : 2.56 δ : +1.30
30
20
10
0
0
0
10
20
30
40
50
Diameter (cm)
Guinean zone
60
Diameter (cm)
10
10
0 40
Frequency
20
0
0
90 100 110 120 130 140
40
Frequency
Frequency
50
Diameter (cm)
40
30
20
10
0
100 110 120 130 140
Weibull a : 43.92cm b : 2.05 δ : +0.59
30
10
10
B
40
Weibull a : 23.71cm b : 1.91 δ : +2.25 Frequency
40
A
60
70
80
90 100 110 120 130 140
Diameter (cm)
Sudano-Guinean zone
0
10
20
30
40
50
60
70
80
90 100 110 120 130 140
Diameter (cm)
Sudanian zone
Figure 5.1. Stem diameter structures of A. africana populations according to climatic zone and level of anthropogenic disturbance. A: low disturbed stands; B: high disturbed stands. 5.1.2. Structural parameters The mean values of the structural parameters of A. africana in each climatic zone and according to disturbance level are mentioned in the table 5.1. Significant interactions were noted between climatic zones and anthropogenic disturbance from results of the ANOVA. Indeed for most of the considered structural parameters, the effects of the disturbance levels depend on the climatic zones and vice-versa. Tree density A. africana tree density did not differ between the levels of disturbance within each climatic zone. However, the effect of climatic zone on tree density was significant at some level of disturbance. The interaction between climatic zones and disturbance levels showed an influence of climatic zone when increasing in the disturbance gradient. Indeed, at the low disturbance level, tree density was the same for the three climatic zones (P 0.05). However, when increasing in the disturbance gradient (i.e. at the high disturbance level), tree density became highest in the Sudanian zone (17.14 stems ha-1; 133.71%) and lower in the Guinean (1.46 stems ha-1; 126.83%) and Sudano-Guinean zones (1.17 stems ha-1; 133.35%). Basal area Basal area was sensitive to the level of disturbance at some locations (Guinean zone), while the disturbance effect was absent at others ones (Sudano-guinean and Sudanian zones), showing a direction in the climate-related effect of disturbance on basal area. Indeed, along 18
Chapter5. Results the climatic gradient, the basal area decreased significantly from the low disturbance level (7.98 m2 ha-1) to the high disturbance level (2.76 m2 ha-1) in the Guinean zone, but did not differ in the Sudano-guinean and Sudanian zones. There was also significant difference among climatic zones, especially at low disturbance level, showing that the climatic zone effects vary with the disturbance level. At the low level, basal area significantly decreased from the Guinean zone (7.98 m2 ha-1) to Sudanian and Sudano-guinean zones (2.19 m2 ha-1 and 1.95 m2 ha-1 respectively). When increasing in the disturbance gradient, i.e. in the high disturbed stands, basal area did not vary among climatic zones (P = 0.081). Mean diameter Mean diameter differed significantly between the disturbance levels at some locations (Guinean and Sudano-Guinean zones) while the effect was absent at another one (Sudanian zone). This climate-related effect of disturbances showed, in the Guinean zone, a significant decrease of mean diameter from the low disturbance level (70.88 cm) to the high disturbance level (52.90 cm), contrary to the Sudano-guinean zone where the higher value (65.18 cm) was found at the high disturbance level. However in the Sudanian zone, the mean diameter did not change according to the levels of disturbance. For the two levels of disturbance, the mean diameter differed significantly among climatic zones, with the highest value found at the Guinean zone (70.88 cm) and the lowest one at the Sudanian zone (18.56 cm and 20.58 cm). Mean height Mean height differed significantly between the levels of disturbance within climatic zones (Guinean and Sudanian) and among climatic zones for each level of disturbance. The interaction effect between climatic and disturbance gradients was not perceptible. The mean height decreased significantly from the Guinean zone (17.70 m and 13.46 m) to the Sudanian one (11.25 m and 8.87 m), whatever the disturbance levels. Within each climatic zone, the mean height increased significantly from the high disturbance level to the low one. Density of regeneration There was a climate-related effect of disturbance on the density of regeneration. Indeed along the climatic gradient, the density of regeneration decreased significantly in the Guinean zone, from the low disturbance level (23.46 plants ha-1) to the high disturbance level (9.54 19
Chapter5. Results plants ha-1), but did not differ in the Sudanian zone. Differences were also significant among climatic zones, for each level of disturbance. The highest values of the density of regeneration were found in the Guinean zone while the lowest ones were recorded in the Sudanian zone. Limitation of regeneration In both Sudanian and Sudano-Guinean zones, stands exhibited higher limitation of regeneration (94.28-97.14) than in the Guinean zone (31.25-53.84), confirming the lower density of regeneration (0.63 to 1.27 stems ha-1) observed in Sudano-Guinean and Sudanian zones. The effects of anthropogenic disturbance were more perceptible in the Guinean zone, the highest values of structural parameters being recorded at low disturbance level. Table 5.1. Structural parameters of A. africana populations according to climatic zones and anthropogenic disturbance: mean (m), coefficient of variation (cv) and P-values of ANOVA.
Parameters Density, N (stems.ha-1)
Mean Diameter, Dm (cm)
Basal area, G (m².ha-1) Mean height, HL (m)
Density of regenerations (plants.ha-1) Limitation of regenerations (%)
Disturbance Low High P-value Low High P-value Low High P-value Low High P-value Low High P-value Low High P-value
Guinean m cv (%) 3.48 84.47 1.46 126.83 0.111 70.88 17.69 52.90 53.75 0.000 7.98 77.05 2.76 129.9 0.000 17.70 16.77 13.46 26.09 0.000 21.46 113.33 11.54 190.03 0.015 31.25 53.84 -
Sudano-guinean m cv (%) 4.87 162.88 1.17 133.35 0.074 23.77 89.57 65.18 16.99 0.000 1.95 181.83 0.89 137.77 0.189 11.09 20.25 0.71 591.61 97.14
Sudanian m cv (%) 13.97 122.54 17.14 133.71 0.907 20.58 82.21 18.56 93.35 0.625 2.19 134.5 1.28 147.11 0.250 11.25 23.42 8.87 26.53 0.010 0.63 412.13 1.27 463.56 0. 910 94.28 94.28 -
P-value 0.260 0.000 0.000 0.000 0.000 0.081 0.000 0.000 0.000 0.001
P-values are computed from log-transformed data (y=log(x+1)) for the comparison of the 3 climatic zones (last column) and levels of disturbance (lines) according to structural parameters.
5.2. Patterns of structural parameters of A. africana populations across climatic zones Six variables were selected from stepwise discriminant analysis: tree density (Nt), basal area (G), contribution to stand basal area (CG), thick wood (dbh ≥ 60 cm) contribution to basal area (Gi3), stratum 1 (H˂10m) and stratum 3 (H ≥ 15m) relative proportions. They significantly add a predictive power to the discriminant function (P ˂ 0.05).The results of the canonical discriminant analysis on these selected variables indicated two significant axes, the
20
Chapter5. Results first axis explaining 71.7 % of the total variance and the second one accounting for 19 %. Both axes explained well the differences between the climatic zones associated with the disturbance levels. The coefficients of correlation were significant (P < 0.05). On the first canonical axis, tree-density and relative proportion of stratum 1 were opposed to basal area, thick stem contribution to basal area and relative proportion of stratum 3. The second canonical axis took into account the basal area and the contribution to stand basal area (Figure 5.2). Moreover, the first canonical axis discriminated the low disturbed stand of the Guinean zone from stands of the Sudano-Guinean and Sudanian zones. At the low disturbance level of the Guinean zone, A. africana populations had greater size (G, Gi3) and height (NH3) while higher tree density (Nt) and lower mean height (HL ˂ 10 m) characterized the populations in the Sudanian zone. Moreover, highly disturbed stand of the Guinean zone showed similar traits (for the low basal area and the low contribution to the stand basal area) with the SudanoGuinean stands (Figure 5.2). Table 5.2. Canonical loadings of selected variables: correlations between axes and variables
6
Structural and distributional variables Can1 (71.7%) Can2 (19%) Tree density 0.719 0.454 Basal area -0.529 0.510 Contribution to stand basal area -0.049 0.852 Thick wood (DBH ≥ 60 cm) contribution to basal area -0.733 0.417 Proportion of stratum 1 (H ˂ 10 m) 0.584 0.251 Proportion of stratum 3 (H ≥ 15 m) -0.810 0.359
4
Gc
2
NH3 Gi3
SHp SLp
+
+
NH1
Nt
++
GLp
0
Can2 (19%)
G
GHp
+ +
-2
SGLp SGHp
-10
-5
0
5
10
Can1 (71.7%)
Figure 5.2. Projection of plots in the canonical system axis defined by the six most discriminant variables. Nt: tree-density, G: Basal area, Gc: Contribution to stand basal area, NH1: proportion of 21
Chapter5. Results stratum 1 (trees with H ˂ 10 m), NH3: proportion of stratum 3 (trees with H ≥ 15 m) and Gi3: thick wood (tree with dbh ≥ 60 cm) contribution to basal area; Lp: low disturbance, Hp: high disturbance, G: Guinean zone, SG: Sudano-Guinean zone and S: Sudanian zone.
5.3. Relationship between structural traits of A. africana populations and climatic parameters The first four principal components, extracted from the PCA, expressed 87.8 % of the total variance of the considered structural data (Table 5.3). The first component is significantly correlated with mean diameter, basal area, mean height of A. africana trees. It separated populations with fine stem (10-30 cm dbh) and height stratum 1 (Height < 10 m) from populations with thick stem (≥ 60 cm dbh) and height stratum 3 (Height ≥ 15 m). The second component is constituted of medium stem individual proportion and medium stem individual contribution to basal area. The third axis takes into account the tree-density, the basal area and the contribution to stand basal area. Finally, the fourth factor showed a positive correlation with height stratum 2. Most of the structural traits of A. africana appeared to be correlated with the environmental factors, even if the correlation coefficients were relatively low (0.20 to 0.67; Table 5.3). The species tree size was higher in colder and wetter lands (lower temperatures and higher precipitations) with lower altitude. These areas were also favorable for taller trees of A. africana (height 15 m). The second component (medium stem proportion and medium stem contribution to basal area) was not correlated with any climatic parameter. Tree-density (from PC3) showed very low and positive correlation with mean temperature and negative correlations with rainfall, indicating that A. africana tends to be at lower density in more humid stand than in drier stand (low rainfall and high temperature). The last component, showing highly positive correlation with stratum 2 (10-15 m height class) proportion, was correlated with all environmental parameters. This result also revealed that rainfall is negatively linked with proportion of stratum 2 contrary to temperature.
22
Chapter5. Results
Table 5.3. Significance of correlation coefficients between principal components of structural variables and environmental variables PC1 PC2 Structural and distributional variables Tree-density Mean diameter + Basal area + Lorey height + Contribution to stand basal area Proportion of fine wood (10-30 cm dbh) Proportion of medium wood (30-60 cm dbh) Proportion of thick wood (≥ 60 cm dbh) + Fine wood contribution to basal area Medium wood contribution to basal area Thick wood contribution to basal area + Proportion of height stratum 1 (Height < 10 m) Proportion of height stratum 2 (10-15 m Height) Proportion of height stratum 3 (Height ≥ 15 m) + Environmental variables Altitude Annual Mean Temperature Temperature Annual Range Maximum Temperature of Warmest Month Minimal Temperature of Coldest Month Mean Temperature of Warmest Quarter Mean Temperature of Coldest Quarter Annual rainfall Precipitation of Warmest Quarter Precipitation of Driest Quarter Precipitation of Wettest Quarter Precipitation of Coldest Quarter
- 0.62*** + 0.07 - 0.63*** - 0.53*** + 0.66*** - 0.29** + 0.20* + 0.21* + 0.67*** + 0.64*** - 0.62*** - 0.65***
PC3
PC4
+ + + + +
+ + 0.07 + 0.10 + 0.11 + 0.13 - 0.08 + 0.15 + 0.09 - 0.15 - 0.02 - 0.09 + 0.12 + 0.09
- 0.01 + 0.26* + 0.13 + 0.20 - 0.05 + 0.26** + 0.23* - 0.26** - 0.09 - 0.13 + 0.12 + 0.10
+ 0.24* + 0.21* + 0.34*** + 0.37*** - 0.28** + 0.34** + 0.14 - 0.32** - 0.29** - 0.34*** + 0.33*** + 0.32**
*: Prob. ˂ 0.05; *: Prob. ˂ 0.01; ***: Prob. ˂ 0.001; + positive significant correlation; -: negative significant correlation.
5.4. Discriminated habitats of A. africana along the climatic gradient According to the NMDS results, plots from the same climatic zone were clustered (Figure 5.3). A low stress value was obtained (0.103), revealing an excellent representation into reduced dimensions. Three clusters were discriminated: the first one was made up of only samples from Guinean zone, the second dominated by plots of Sudanian and Sudano-Guinean zones and the third cluster constituted by plots from Sahelo-Sudanian zone. These three groups were designated to constitute the three habitats patterns of A. africana along the latitudinal gradient.
23
Chapter5. Results
Figure 5.3. Non Metric Multidimensional Scaling of plots from Guinean, Sudano-Guinean, Sudanian and Sahelo-Sudanian zones. The results of CCA indicated that the first two axes accounted for 57 % (39 % for the first axis and 19 % for the second one) of the total variance. Most of the climatic variables showed high correlations (0.62 to 0.98) with the two axes. The projection of these climatic variables onto the two CCA axes (Figure 5.4) showed gradients of altitude and precipitation on axis 1 while the axis 2 revealed temperature and precipitation gradients. The habitat of A. africana in the Guinean zone is discriminated by precipitation of warmest quarter (PWmQ), total annual precipitation (AnP) and precipitation and temperature of driest quarter (PDrQ, MnTDrQ). The discrimination of A. africana habitat in the Sahelo-Sudanian zone is supported by influence of mean temperatures of the coldest and warmest quarters (MnTWmQ, MnTCoQ) and annual mean temperature (AnMT). The habitat in Sudanian and SudanoGuinean zone is discriminated by total annual precipitation (AnP) and precipitation of coldest quarter (PCoQ).
24
Chapter5. Results
Sahelo-Sudanian Guinean
Sudanian and Sudano -Guinean
Figure 5.4. Loading of sample units from Guinean, Sudanian, Sudano-Guinean and SaheloSudanian zones in combination with environmental variables. Alt= Altitude, AnMT= Annual Mean Temperature, MnTWmQ= Mean Temperature of Warmest Quarter, MnTCoQ= Mean Temperature of Coldest Quarter, MnTWtQ= Mean Temperature of Wettest Quarter, MnTDrQ= Mean Temperature of Driest Quarter, AnP= Annual Precipitation, PWmQ= Precipitation of Warmest Quarter, PWtQ= Precipitation of Wettest Quarter, PDrQ= Precipitation of Driest Quarter, PCoQ= Precipitation of Coldest Quarter.
5.5. Characterization of A. africana habitats’ woody species composition along latitudinal gradient The most dominant species in A. africana habitats based on their IVI, are presented in table 5.4. A. africana was present in the four climatic zones, with the highest IVI being recorded in the Sudanian zone. Apart from A. africana, only one species, Anogeissus leiocarpa, was common in the four climatic zones. Also, apart from the Guinean zone, four important species (A. africana, Pterocarpus erinaceus, Burkea africana and A. leiocarpa) were simultaneously found in the Sudano-Guinean, the Sudanian and the Sahelo-Sudanian zones. Moreover, the Sudano-Guinean and Sudanian zones shared many species. The most characteristic species in the Sudanian zone were A. africana, P. erinaceus, Vitellaria paradoxa, Lannea acida, Crossopteryx febrifuga, Daniellia oliveri and B. africana whereas the most important ones in the Sudano-Guinean zone were A. africana, P. erinaceus, V. paradoxa, L. acida, B. africana, Isoberlinia doka, Isoberlinia tomentosa, Monotes kerstingii, 25
Chapter5. Results and A. leiocarpa. All these species were associated with A. africana in both Sudanian and Sudano-Guinean zones. In addition to P. erinaceus, B. africana and A. leiocarpa that were common, other characteristic species associated with A. africana in the Sahelo-Sudanian zone were Bombax costatum, Sterculia setigera and Boswelia dalzielii. In the Guinean zone, the important species were Dialium guineense, Diospyros mespiliformis, Drypetes floribunda, Mimusops andongensis, Ceiba pentandra and Celtis brownii, and seemed to be restricted to this habitat. Table 5.4. Most ecologically important species in A. africana habitats in the four climatic zones with their IVI; only species with IVI greater than 10 % are shown. Guinean Sudano-Guinean Sudanian Afzelia africanaSm. Pterocarpus erinaceus Poir. Burkea africana Hook. Anogeissus leiocarpaGuill. & Perr. Vitellaria paradoxa C.F.Gaertn. Lannea acida A.Rich. Isoberlinia doka Craib & Stapf. Isoberlinia tomentosa (Harms) Craib & Stapf. Monotes kerstingiiGilg. Crossopteryx febrifuga Benth. Danielia oliveri (Rolfe) Hutch. & Dalziel Bombax costatum Pellegr. & Vuillet. Sterculia setigera Del. Boswellia dalzielii Hutch. Dialium guineense Wild. Diospyros mespiliformis Hochst. Drypetes floribunda Hutch. Mimusops andongensis Hiern. Ceiba pentandra(L.) Gaertn. Celtis brownii Rendle
16.7 6.3 68.9 49.2 32.7 19.1 18.5 17.4
16.4 14.2 13.5 19.5 27.7 15.4 27.2 22.3 21.2 5.7 7.4 3.0 -
65.2 52.6 14.1 1.0 43.7 18.8 17.0 15.3 -
SaheloSudanian 40.2 50.1 34.7 58.7 37.6 33.3 45.2 -
26
6. DISCUSSION AND CONCLUSION 6.1. Anthropogenic disturbance of A. africana natural stands according to climatic zones 6.2.Traits variation patterns of A. africana populations across the climatic gradient 6.3. Changes in woody flora composition of A. africana habitats along latitudinal gradient 6.4. Implications for the conservation of A. africana
Chapter 6. Discussion and conclusion
6. DISCUSSION AND CONCLUSION Biodiversity is an important source of people’s well-being and full attention is required as far as endangered tree-species are concerned, for protecting them from broad scale extinction. In the present study, we investigated the current structure of A. africana’s natural stands across the climatic and human disturbances gradients. Indeed, understanding the template generated by both human disturbance and climatic variability on A. africana population structure should be essential to draw up new directives for better conservation at country scale
6.1. Anthropogenic disturbance of A. africana natural stands according to climatic zones In general, the human disturbances often lead to altered environmental conditions, which influence the process that can affect species diversity and structure in a forest community (Sagar et al. 2003). Dendrometric parameters like tree-density, stem diameter, basal area, mean height and regeneration were used in this study to examine the effect of human disturbance on A. africana populations structure across climatic gradient. The results showed that the effect of disturbance levels on A. africana populations depends on the climatic zone. Indeed, levels of disturbance were reported as having much influence on plant species population structure (Sakpota et al. 2010). Populations’ structures of A. africana were negatively affected at high level of disturbance, mostly in the Guinean zone, that has the highest density of inhabitants in Bénin (INSAE 2003, 2008). In the other climatic zones, dendrometric parameters’ values were the same along the disturbance gradients, except for the tree density in the Sudanian zone and the mean height in the Sudano-Guinean zone. These results may first mean a lower human density. The same values obtained for the dendrometric parameters, at high and low disturbance levels, revealed the importance of the protection in these zones. The weakness of the forests’ protection is manifested by unlawful exploitations of valuable species mainly for timber production, pasture, firewood, medicine, fodder and crafting in the Sudanian regions (Houehanou et al. 2011). The density of regeneration was not affected by the levels of disturbance, except for the Guinean climatic zone, where the higher values of both regeneration and adult tree density were observed at the low disturbance level. This might be likely because the regeneration density must be closely related to adult tree density. In the Sudano-Guinean zone, low values of regeneration density recorded are explained by the absence of strengthened protection measures. This leads to intensive pastures and herbivore disturbances that could greatly and
26
Chapter 6. Discussion and conclusion negatively impact the process of seed production and regeneration (Hall and Bawa 1993, Ouédraogo-Koné et al. 2008). In the Sudanian zone, low values of regeneration density were also recorded. A similar situation was observed in Sudanian zones of Burkina Faso (Ouédraogo and Thiombiano 2012). The authors reported very rare saplings of A. africana with recruitment and growth difficulties in natural stands. The low regeneration density observed in the Sudanian zone (even in the protected area) suggests that, protection would not be always a sufficient action to conserve some threatened tree species; other factors like climate pejoration and occurrence of potential bush fires may also compromise the viability of the threatened tree species (Biaou 2009, Nacoulma et al. 2011, Ouédraogo and Thiombiano 2012, Houehanou et al. 2013b). Effects of disturbances were also examined through the establishment of stands diameter structures. These structures showed a bell-shaped distribution for all disturbance levels, with small number of individuals of 10-20 cm dbh. Such observations are supported by the regeneration patterns reported in this study. Except for the low disturbed stand in the Guinean zone, all the graphs showed unstable population structures characterized by absence of some diameter classes. That is explained by the constant illegal logging of individual with diameter above 50 cm, as already reported by Sinsin et al. (2004).
6.2.Traits variation patterns of A. africana populations across the climatic gradient The most discriminant variables across the climatic zones were tree-density, basal area, contribution to stand basal area, thick stem (dbh ≥ 60 cm) contribution to basal area, and height strata 1 (Height< 10 m ) and 3 (Height ≥ 15m) relative proportions. These variables have been reported as the main drivers of stand structures (Gonzales 2008). The canonical loadings showed clear distinction between climatic zones with smaller individuals of A. africana in the Sudanian and Sudano-Guinean zones and taller and bigger individuals in the Guinean zone. Significant differences were found in basal area, mean height and density of regeneration between the three climatic zones. The biggest and tallest individuals of A. africana were encountered in the Guinean zone, as previously reported (Sinsin et al. 2004). Moreover, the highest density of regeneration was observed in this climatic zone. Thus, the species is supposed to be more adapted to the climatic conditions of the Guinean zone (Houehanou et al. 2013b) even though it has been reported as being mostly valued in the Sudanian regions (Adomou et al. 2009, Nacoulma et al. 2011). In fact, Sudanian and Sudano-Guinean zones are 27
Chapter 6. Discussion and conclusion still the potential areas for the seasonal movements of herds. These areas are also known for the noteworthy human activities such as pruning for cattle feeding and harvesting for medicines purposes (Sinsin et al. 2004, Houehanou et al. 2011). The relationships between the principal components (structural traits of A. africana stands) and climatic variables revealed significant correlations, suggesting that the variation in species’ traits is supported by the variation in climatic factors. Similar observations were done with other tree species such as Vitellaria paradoxa C.F. Gaertn., in the Sudano-Guinean and Sudanian zones of Benin (Glèlè Kakaï et al. 2011). Likewise, Ouédraogo et al. (2013) reported significant variations in morphological traits and population structure of the widespread species, Anogeissus leiocarpa (DC.) Guill. &Perr., along a climatic gradient in Burkina Faso. Our findings are consistent with the fact that climate has potential impact on ecosystem and species. It was showed that A. africana individuals tend to be taller and bigger in colder and wetter areas with lower altitude, consistent with other studies on shea butter tree (Glèlè Kakaï et al. 2011) and plant strategy schemes (Westoby 1998). At community and species levels, organisms in more humid sites were found to have greater performances than species occurring at more xeric sites (Fonseca et al. 2000, Thuiller et al. 2004). For example, when water is limiting, species may develop water-use efficiency that may allow them to compete and withstand during harsh droughts. Unfortunately, during such periods, the drought may compromise the species potential of foliation resulting in wilting (Engelbrecht and Kursar 2003). The drought may also cause decline in species traits (Kleidon and Mooney 2000) and induce a decreased growth of the tree species (Condit et al. 1995). At six months age, A. africana seedlings developed an expanded root system with long branching roots (Ouédraogo and Thiombiano 2012). However, such a strategy was not effective for the seedlings’ adaptation to drought. In tropical forests, the intensity of seasonality can lead adult tree species to develop several more characteristics (deep development of root systems, root morphological plasticity, leaves dropping, better stomatal control, etc) that allow them to compete and persist (Borchert 1998, Eamus et al. 2001, Ostonen et al. 2011). These ecophysiological abilities developed by tree species to meet the environmental stress and thus pass through the unfavorable conditions might well explain the negative correlation of increasing tree-density with decreasing precipitation, and the positive correlation with increasing temperature. The fact that tree density is negatively correlated with increasing temperature and decreasing precipitation is also supported by the finding that tree density is higher at the drier sites (table 4; figure 2). The highest tree density observed in the drier regions has been reported by Biaou (2009) in the Sudanian and Sudano-guinean zones of 28
Chapter 6. Discussion and conclusion Benin, but contrary to our results, the author did not find any significant correlation between water stress and tree density. The analysis of the relationship between structural traits and climatic factors has showed some low correlation values. This was the case of the relationship between the climatic variables and the second principal component, which is related to the individual proportion and the contribution to basal area of the medium stem. The medium stem reflects the 30-60 cm diameter class, which was found along the whole climatic gradient, making it less related to a given level on this gradient. The low correlation values obtained could also be the result of strong effects of other factors. Indeed, in this study, climatic variables were used to only examine their relationships with A. africana populations’ structures. However, climate only cannot readily predict the detailed patterns in population structures, since the growth of an organism may be strongly influenced by factors such as resources, disturbance and their competitive interactions (Huston 1979, Hobbie et al. 1993). Some environmental variables such as topography (slope and relief), soil properties (soil types, litter cover, moisture), bush fire and herbivory can also act as other potential factors that have been showed to interact and reduce the woody cover (Frost et al. 1986, Bond et al. 2005, Sankaran et al. 2005). Therefore, soil properties might well interact with climatic factors to better explain species population structures in the present study. This might be reinforced by the fact that soil conditions in Benin differed significantly across the three climatic zones (Adomou 2005).
6.3. Change in woody flora composition of A. africana habitats along the latitudinal gradient Conservation biologists have often been concerned with species diversity dynamics. In this study, a variation in species richness of A. africana habitat was observed regarding the bioclimatic zones, but this should be related to the vegetation type where A. africana occurs. In other words, this variation in species richness of A. africana habitat, is not indicative of the trend in plant diversity of each bioclimatic zone. The observed trend where diversity parameters were high in A. africana natural stands of the Sahelo-Sudanian, Sudanian and Sudano-Guinean zones and relatively low in the Guinean one has also been reported by previous studies (Ouédraogo 2006, Bonou et al. 2009, Houéto et al. 2012, Houehanou et al. 2013b). This may be explained by the fact that the environments in Sudanian and Sahelian areas are more heterogeneous, and may contain more niches that can support more species. From past studies, (Borchert 1998, Eamus et al. 2001, Thuiller et al. 2004), cooccurrence or assemblage life is the outcome of functional strategies (e.g. competiveness 29
Chapter 6. Discussion and conclusion reduction, depth of soil exploited) developed as responses to natural selection and climatic specificity. Indeed, climate creates environmental conditions that can enable life community for some species, depending on their needs. Because different species may require different physiological needs, species community with neighboring tolerance is possible. Our results revealed a discrimination of habitat woody species diversity through the distribution range of the same species along a latitudinal gradient. Indeed, the Sahelo-Sudanian, the Sudanian and the Sudano-Guinean zones are known for their marked long dry seasons and irregular precipitations. These last areas are drier than the Guinean zone that displays a bimodal rainfall regime with high and regular precipitations. The habitats of semi-arid zones may enable some functional strategies (Thuiller et al. 2004), so that from Sudano-Guinean to Sudanian zone, many common species occur freely. This may explain why plots from the Sudanian and the Sudano-Guinean zones were grouped together into the same cluster (Figure 1), indicating a habitat differentiation with the other zones. The high discrimination between the Guinean and the Sahelo-Sudanian zones reflects the limits of the distribution range of A. africana. Indeed, there is presumably a high number of uncommon species in each zone. The South-Sahel (Sahelo-Sudanian zone) is the northernmost limit of the species distribution in the semi-arid savannas (Terrible 1984). The species’ natural populations in this zone are in jeopardy due to the combined effects of climate pejoration and anthropogenic disturbance (Ouédraogo and Thiombiano 2012). On the contrary, the species seems to have the most favorable growing conditions in the Guinean zone characterized by relatively low human disturbance (Bonou et al. 2009). A similar pattern was observed for the widespread species, Anogeissus leiocarpa, across its distribution range in semi-arid areas (Ouédraogo et al. 2005, Ouédraogo et al. 2013). The results of CCA supported previously observed patterns and confirmed the influential role of climate in governing species assemblages. These findings are in line with the ones of Pyke et al. (2001) in a Neotropical lowland forest of Panama canal, where both precipitation and geology have been shown as useful in predicting species-level floristic variations at broader scales. Rainfall is often recognized as good predictor of variation in plant species diversity (Gentry 1988). Moreover, that precipitation and temperature of the warmest quarter were discriminative of habitat of A. africana, appears congruent with the findings of Gwitira et al. (2013) which reported that, precipitation and temperature of the warmest period are important to understand the effect of climate change on plant species diversity in Southern African savannah.
30
Chapter 6. Discussion and conclusion In finding an influence of temperature and precipitation on plant diversity and distribution, we have confirmed the truism that plants respond to warming and precipitation, and changes in climate may directly affect plant species vital rates (Holbrook et al. 1995, Adler et al. 2012) and may drive occurrence pattern of species. Climate has always been a reasonable predictor of the distribution of individual species, because each species has its own realized niche which respects climatic limits. Moreover, variation in climate may directly alter the abiotic environment (e.g. soil moisture, nutrient cycling, or resources availability) and thus may influence the local patterns of functional groups and habitats (Hobbie et al. 1993). But in reality, the change in habitat diversity cannot be explained only by considering climatic factors. The variation in habitat species composition may be adequately evidenced, when considering additional factors such as resources availability, disturbance regime and soil properties. For example, our study revealed, inter alia, that the Sahelo-Sudanian zone sheltered many species, and temperatures were found to discriminate this habitat. Indeed, in dry climate, increased temperatures have the potential to decrease soil moisture, but drought stress may be counterbalanced by increased water use efficiency (Hobbie et al. 1993), thus promoting occurrence of many species. Soil properties and disturbance regime could potentially interact with climate to influence the variability in habitat composition (Huston 1979). The characteristic species were the ones having an IVI greater than 10. According to Reitsma (1988), species with IVI > 10 were often considered as ecologically important in its habitat. The importance values index of species in each climatic zone revealed that the Sudanian and Sudano-Guinean zones shared many important species that are different from the ones observed in the Guinean zone. Likewise four different important species were also common to the Sudanian, Sudano-Guinean and Sahelo-Sudanian zones. These observations may seem self-evident. Indeed, the Guinean zone shared only two important species (A. africana and A. leiocarpa) with the remaining zones (Sudanian, Sudano-Guinean and SaheloSudanian). The most important species of these habitats are not likely to maintain a wide distribution range as for A. africana itself. This is probably because the abilities to reach a community differ greatly among individual species (Pimm 1993).
Moreover, important
species of Guinean habitats are restricted to their geographical distribution because of climate-related limits to their regeneration that does not enable them to colonize the whole distribution range of A. africana (Ouédraogo and Thiombiano 2012). Also in the habitat of Guinean zone, the density of vegetation cover could induce high disturbance of competition that may not allow non-competitive species (Baumberger et al. 2012). 31
Chapter 6. Discussion and conclusion Knowledge on important species co-occurring in a habitat is essential for two main reasons. First, it provides evidence in validating ecological assumptions of biotic interactions such as mutualism, neutralism, facilitation etc. But also the possibility that, these characteristic species likely hinder the immigration of invasive species, thus maintaining habitat integrity. This can aid in conservation processes that seek to promote conservation of the habitat for native species by restricting the chance of invasion. Conservation biologists will also be concerned about important species because conservation actions would be of advantage for these associated tree species. The habitat integrity could likely contribute to mitigate undesirable effects of changing climate conditions. This is all the more true to conserve widespread species like A. africana in the face of eminent threat from harsh environmental conditions and human disturbance (Ouédraogo and Thiombiano 2012).
6.4. Conclusions and implications for the conservation of A. africana Several insights have been provided from the results of this study: first, the effects of disturbance levels depend on climatic zones and were more perceptible in Guinean zone. Second, Sudanian and Sudano-Guinean zones recorded the lowest values of structural parameters. Third, thickness and tallness discriminate the species population across the climatic zones. Fourth, the tallest and biggest trees were found at less drier regions with lower altitudes (Guinean zone). Fifth, there is a clear separation of samples of Sudanian and Sudano-Guinean zones from those of Guinean and Sahelo-Sudanian zones. Sixth, this distinction in the habitat diversity of A. africana is supported by precipitation and temperature regime. Seventh, some co-occurring species are characteristic for each habitat. The management and preservation of populations of A. africana under differential ecological conditions require not only understanding how climatic and human-induced factors affect these populations’ structures, but also how is the pattern of habitat species composition. Protection actions must be enforced in the Sudanian and Sudano-Guinean zones to prevent herders and farmers’ penetration into forests’ reserve. Harvesting species must be fully prohibited. Moreover, low density of regeneration in the three climatic zones could be overwhelmed through protection of seeds and seedling from predators. First, seeds, seedlings and saplings could be protected through protecting forests from incursion of herders and farmers. Second, seedlings and saplings could be tagged and protected from herbivores using metallic barriers. Third, since seeds and seedlings near parent trees are known to be more vulnerable (Janzen 1970, Connell 1971), seeds could be collected under seed bearer and dispersed away from adult trees. This may help remove seeds from the trajectory of natural 32
Chapter 6. Discussion and conclusion enemies (predators) and may contribute to assist the natural regeneration. This might also help reduce conspecific competitions. Moreover, for seed collection and artificial seed dispersal, seed bearers should be selected per climatic zone, since the provenance of seeds may have an effect on the seedlings traits (Weber et al. 2008). Indeed, recent ex-situ experimental study on A. africana germination ability and seedling growth has showed variation in seeds according to the climatic zones (Padonou et al. 2013). As a strategy for species conservation, seed collection under excellent seed bearers will help establishing seed orchard for further plants production and uses in forest enrichment. Pure or mixed stands must be installed and managed following appropriate spacing and silviculture rules. More specifically, adequate monitoring program through ectomycorrhizal fungi inoculation, regular watering and pesticide treatments may be helpful to increase A. africana seedling growth (Villenave and Duponnois 2002, Artursson et al. 2006) and to assist its natural regeneration. The present study illustrated also that A. africana co-occurs with some important species in three different habitats across the four bioclimatic zones. The variation in habitat composition should be also taken into account when promoting the species conservation. Indeed, distinction in the habitat may suggest different considerations while proposing guidelines for management and conservation of A. africana within its habitats. Results suggest that, the Sudanian and Sudano-Guinean zones could use similar management schemes for the species habitat while the Guinean and the Sahelo-Sudanian zones should be treated as distinct habitats. In the Sudanian and Sudano-Guinean zones, enrichment planting program could be embarked upon for A. africana with species like P. erinaceus, V. paradoxa, L. acida, C. febrifuga, D. oliveri, B. africana and A. leiocarpa. Species such as Pterocarpuserinaceus, B. africana and A. leiocarpa should be considered in the Sahelo-Sudanian zone, but with other important species such as B. dalzielii, B. costatum, S. setigera, Pericopsis laxiflora, Combretum molle, Pavetta crassipes, Stereospermun kunthianum and Ziziphus abyssinica. However, in Guinean zone, species such as D. guineense, D. mespiliformis, D. floribunda, M. andongensis, C. pentandra and C. brownii should be considered together with A. africana. Moreover, actions should be taken to protect these important species for maintaining the interspecific relationships and habitat integrity. Most of these actions should be combined with local communities’ awareness for seedlings protection in reserve forests and in surrounding areas. Similarly, conservation education through sensitization may help populations to understand the critical state of the species in forest stands. This should be
33
Chapter 6. Discussion and conclusion achieved in a participative way through interactive discussion in groups and support of technical
manuals.
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