Effect of disturbance, habitat fragmentation and alien invasive plants ...

RAGHUBANSHI & TRIPATHI

Tropical Ecology 50(1): 57-69, 2009 © International Society for Tropical Ecology www.tropecol.com

57

ISSN 0564-3295

Effect of disturbance, habitat fragmentation and alien invasive plants on floral diversity in dry tropical forests of Vindhyan highland: a review A.S. RAGHUBANSHI* & ANSHUMAN TRIPATHI

Department of Botany, Banaras Hindu University, Varanasi 221005, India Abstract: Effect of disturbance, habitat fragmentation and exotic plant species invasion on plant diversity is documented for the dry deciduous forests of India’s Vindhyan highlands. Plant communities were identified with the help of satellite imagery and ground data, and community wise diversity was estimated. Analysis of landscape matrices using satellite images and GIS tools showed deforestation of smaller fragments, and fragmentation and degradation of larger ones. Fragments of smaller size had lesser number of plant communities compared to larger fragments. Generally large size fragments were species rich and more diverse as compared to smaller fragments. Lagestoemia-Acacia-Lannea community was richest for both tree and herb strata, whereas Shorea-Buchanania community was the richest when all vascular flora were considered. The study indicates that species rich communities of the dry tropical forests are not only being reduced in area but they are also becoming species poor and less diverse due to rapid deforestation and forest fragmentation. The communities most vulnerable to alien invasive plants were Acacia-Terminalia, Terminalia-Shorea and Hardwickia-Acacia. Ecological implications of plant invasion are discussed and conservation measures suggested. Resumen: Se documenta el efecto del disturbio, la fragmentación del hábitat y la invasión de especies vegetales exóticas sobre la diversidad vegetal para el bosque seco caducifolio de las tierras altas de Vindhyan, India. Las comunidades vegetales fueron identificadas por medio de imágenes de satélite y datos de campo, y se estimó la diversidad de las comunidades. El análisis de las matrices de paisaje que usaron imágenes satelitales y herramientas de SIG mostraron la deforestación de los fragmentos pequeños, y la fragmentación y degradación de los de mayor tamaño. Los fragmentos pequeños tuvieron un menor número de comunidades vegetales que los fragmentos mayores. En general, los fragmentos grandes fueron ricos en especies y más diversos en comparación con los fragmentos pequeños. La comunidad de Lagestoemia-Acacia-Lannea fue la más rica tanto para árboles como para el estrato herbáceo, mientras que la comunidad de Shorea-Buchanania fue la más rica cuando se consideró a toda la flora vascular. El estudio indica que las comunidades ricas en especies del bosque tropical seco no solamente se están reduciendo en área, sino que además se están haciendo menos ricas en especies y menos diversas debido a la rápida deforestación y fragmentación del bosque. Las comunidades más vulnerables a las plantas invasoras exóticas fueron Acacia-Terminalia, Terminalia-Shorea y Hardwickia-Acacia. Se discuten las implicaciones ecológicas de la invasión de plantas y se sugieren medidas para la conservación. Resumo: O efeito das perturbações, fragmentação do habitat e invasão de plantas exóticas está documentado para a floresta seca decídua das terras altas de Vindhyan na Índia. As *

Corresponding Author; e-mail: [email protected]

58

PLANT DIVERSITY IN VINDHYAN FORESTS

comunidades vegetais foram identificadas com a ajuda de imagens de satélite e dados de campo, e a diversidade alargada foi estimada. As análises das matrizes de paisagem usando imagens de satélite e ferramentas GIS mostrou a desflorestação de pequenos fragmentos, e fragmentação e degradação das maiores. Os fragmentos de menor dimensão apresentavam menor número de comunidades de plantas quando comparadas com os fragmentos maiores. Geralmente os fragmentos de maior dimensão eram rios em espécies e mais diversos em comparação com os menores. A comunidade Lagestoemia-Acacia-Lannea era a mais rica quer no número de árvores e no estrato herbáceo, enquanto que a comunidade Shorea-Buchanania era mais rica quando toda a flora vascular era considerada. O estudo indica que as comunidades ricas em espécies das florestas secas tropicais não só vêm a sua área sendo reduzida mas se vêm tornando mais pobres em espécies e menos diversas devido à rápida desflorestação e à fragmentação da floresta. As comunidades mais vulneráveis à invasão de espécies exóticas invasivas eram a AcaciaTerminalia, Terminalia-Shorea e Hardwickia-Acacia. As implicações ecológicas da invasão de plantas são discutidas e sugeridas medidas de conservação.

Key words: Diversity, deforestation, forest fragmentation, Lantana invasion, plant communities.

Introduction Understanding the diversity of nature in various forms is a fundamental goal of ecological research (Lubchenco et al. 1991). Apart from the immense economic, ethical and aesthetical benefits, biodiversity is essential for the ecosystem function and stability (Ehrlich & Wilson 1991; Holdgate 1996; Tilman 2000). Biodiversity has attracted world attention because of the growing awareness of its importance on the one hand and the anticipated massive depletion on the other (Singh 2002). Globally, concerns are raised over the rapid loss of biodiversity in all its forms and at all the levels. Habitat destruction is the chief cause of the biodiversity loss. Habitats can either disappear completely or they may become degraded and/or fragmented, both causing serious impact on species as well as ecosystem processes. Regionally, species introductions and altered disturbances rates may favour increased local diversity, but habitat loss or modification, outbreaks of introduced or native species, and management of exploitable systems tend to decrease species richness and heterogeneity (Lubchenco et al. 1991). Biodiversity hotspots around the world contain high degree of endemism and are undergoing exceptional loss of habitats (Myers et al. 2000). A

substantial portion of these hotspots contain tropical dry forest, which is one of the world’s most endangered forest types (Janzen 1988a; Lerdau et al. 1991; Mittermeier et al. 1999). Data from most threatened dry tropics (Janzen 1988a) are scanty. Dry tropical forests once occupied more than half the forest terrestrial tropics (Janzen 1988b; Murphy & Lugo 1986). Most of the past studies on habitat fragmentation have focussed their attention on animals in temperate forests or in the rain forests. Little information exists from dry tropical zone. Few studies have actually measured the level of diversity that exists in networks of small forest patches in tropical environments (Pither & Kellman 2002; Sagar et al. 2003, 2008). Present paper documents the effects of disturbance, habitat fragmentation and exotic plant species invasion on plant diversity in the dry deciduous forests of Vindhyan highlands.

Vindhyan highlands Vindhyan highlands are located in between the highly populated Indo Gangetic - plains and the Narmada valley, and have natural vegetation of tropical dry deciduous forests (Champion & Seth 1968). Although practically no historical information exists on species diversity in the Vindhyan highlands, it appears that the area was once very rich in diversity. The potential natural

RAGHUBANSHI & TRIPATHI

ecosystem of the area is tropical deciduous forest. Earlier ecological work in the Vindhyan highlands shows a profound impact of human activity on the natural ecosystems (Singh et al. 1990). There has been rapid industrialization in recent years and activities like quarrying for limestone, establishment of cement factories, thermal power stations, widespread mining have resulted in a rapid build up of human population, displacement of original population, deforestation and conversion of natural forest ecosystems into savanna and marginal croplands. Earlier studies indicated that the forested area in the region is continuously decreasing and the remnant forest cover exists in the form of non-contiguous patches of varying sizes dominated by single or mixed species (Singh et al. 1990). This feature is unique and gives a new dimension to the biodiversity pattern. The change detection study (1982-1989) for a part of Vindhyan hills using satellite images identified that only 31% of the forest area has remained unchanged since 1982. About 40% of the total forest area was converted from mixed forest with crown cover > 50% to mixed forest with crown cover 30 - < 50%. The good to poor forest conversion occurred at a rate of 6.6% of the forested area each year and savannization in the forested area took place at a rate of 3.3% per year (Raghubanshi et al.1991).

59

diversity was estimated. Community classification was done by cluster analysis procedure as given in PC-ORD software (McCune & Mefford 1999). Hierarchical agglomerative polythetic cluster analysis was performed with Ward’s divisive method and squared Euclidean distance options to delineate communities on the basis of basal area values of adult trees for all the sampling units. In order to select the appropriate stop level, preliminary cluster was evaluated by Indicator Species Analysis procedure in the software as proposed by Dufrene & Legendre (1997). Cluster analysis of vegetation data of adult tree individuals resulted in 11 plant communities (Fig. 1). The identified communities were: (1) Tectona-Acacia (2) Lagerstroemia-Terminalia (3) Lagerstroemia-Acacia-Lannea (4) ShoreaBuchanania (5) Tectona-Holarrhena (6) AcaciaTerminalia (7) Anogeissus-Holarrhena (8) AdinaLannea (9) Terminalia-Shorea (10) Shorea Community, and (11) Hardwickia-Acacia. Of the

Plant communities in Vindhyan highlands Classification and mapping of vegetation is needed for resource management, environmental assessment, monitoring and planning. In an ecological study, it is of paramount importance to know the plant communities of an area, how the different communities are related to one another in the whole range of vegetation and express their environment and how do they organize themselves to function as living systems (Whittaker 1978). For Vindhyan highlands, need of strict conservation measures are highlighted by Sagar et al. (2003) to save remnant non–contagious patches of parent forests experiencing strong biotic and anthropogenic stress. Before fulfilling this need, baseline information is needed on the vegetation of the area. For this, plant communities in Vindhyan forests were identified with the help of satellite imagery and ground data and community wise

Fig. 1. Dendrogram produced from hierarchical cluster analysis of vegetation data for classification of forest communities.

60

PLANT DIVERSITY IN VINDHYAN FORESTS

identified communities, Lagerstroemia–Acacia– Lannea community is the most prevalent followed by Acacia –Terminalia community (Table 1). The eleven plant communities recognized in the region reflect variety of patches developed due to interaction of edaphic factors, heterogeneity of environment and disturbance (Jha & Singh 1990; Roy & Singh 1994; Singh et al. 1990). Jha (1990) classified vegetation of the same area into ten community types and advocated soil texture as an important determinant of the distribution of communities (Jha & Singh 1990). According to them, dry tropical forest of the Vindhyan area is a mosaic of non contagious patches of communities. The patchy distribution of species emphasizes environmental determinism, in which forests are considered to be mosaic where plant species composition is determined by edaphic and other environmental characteristics (Gentry 1988; Tuomisto et al. 1995; Tuomisto et al. 2003).

Plant diversity within identified communities In order to understand pattern of plant diversity, six communities were selected for analysis (Table 2). These communities had a representation in minimum 8 sampling units out of total 150 sampling units used for analysis. The selected community types of this region differed with each other in their species composition. Lagestoemia-Acacia-Lannea community was the

Table 1. Plant communities identified in the dry tropical forests of the Vindhyan highlands and % area occupied by them. Community types Tectona-Acacia Lagerstroemia-Terminalia Lagerstroemia-Acacia-Lannea Shorea-Buchanania Tectona-Holarrhena Acacia-Terminalia Anogeissus-Holarrhena Adina-Holarrhena Terminalia-Shorea Shorea robusta Hardwickia-Acacia Vegetated area

Area (ha) 892.9 2248.7 11150.2 2385.8 2229.2 5481.3 901.1 559.9 3071.1 917.1 1109.7 30946.8

% Occupied area 2.8 7.1 35.3 7.6 7.1 17.4 2.9 1.8 9.7 2.9 3.5 98.0

richest when tree and herb strata are considered separately, whereas Shorea-Buchanania community was the richest when all the vascular flora were considered together (Table 2). Shorea inhabiting communities prefer moist north facing slopes that favour development of rich and diverse vegetation (Jha 1990). Pausas (1994) reported highest herbaceous species on shallow north facing slopes from eastern Pyrenees. Saxena & Singh (1982) opined that intermediate temperature and moist environmental conditions are favourable for that of the trees, saplings and herbs. In the Vindhyan highlands the climatic conditions are relatively xeric during the most part of the year and soil moisture coupled with aspect plays a critical role in deciding vegetation composition. Opposite to Shorea, Hardwickia and Acacia prefer xeric sites with large canopy openings, thus, their companion species are also adapted to low moisture and high light environments. Intermediate to these extremes is Lagerstroemia– Acacia–Lannea community which occurs on soils with moderate moisture regimes created by noncontinuous canopy which is not as poor as Hardwickia–Acacia community dominated sites. Because of intermediate environmental conditions, and environmental heterogeneity created by canopy gaps, Lagerstroemia–Acacia–Lannea may be the most diverse community for all trees and for all vascular categories. Environmental heterogeneity has long been recognized as an important factor maintaining biological diversity (Huston 1994). The broken canopy creates an internal mosaic involving spatial gradients of light, soil moisture, soil organic matter, temperature and effective rainfall (Breashears et al. 1997) which ultimately govern the overall diversity of vascular flora. At the fine scale, comparatively well-developed herbaceous vegetation interferes with the regeneration of trees, and has higher species number and diversity per unit area as compared to trees, therefore, it regulates all vascular richness. Whittaker (1972) stated that dominance of one stratum may affect the diversity of another stratum. In the present study herbaceous diversity do not relate well with tree diversity, although herb layer cover and density were negatively influenced by the tree species richness (Table 3). It seems that greater variety of trees with different canopy architecture may fill canopy gap in a better

RAGHUBANSHI & TRIPATHI

61

Table 2. Plant diversity in different stratum of selected communities (per 800 m2 sampling area) in dry tropical forests of Vindhyan highlands.

Lagerstroemia Acacia Lannea

Shorea Buchanania

Acacia Terminalia

Terminalia Shorea

Shorea dominated

Hardwickia Acacia

Communities

All vascular species

75

79

61

65

70

63

All trees

15

17

8

18

18

9

All herbs

60

62

53

47

52

54

All vascular species

1.551

1.531

1.313

1.455

1.446

1.29

All trees

1.099

1.026

0.669

1.008

0.973

0.77

All herbs

1.401

1.433

1.353

1.299

1.315

1.206

Diversity parameters

Species Richness (S)

Shannon-Wiener’s Index (H')

All tree basal cover

All herb cover

All herb density -0.902*

-0.137

-0.309

-0.038

-0.045

0.305

0.122

0.281

0.424

0.321

-0.688

-0.549

0.201

0.342

0.594

0.383

-0.802

-0.593

-0.091

0.046

-0.086

0.071

-0.196

-0.254

Adult tree density

0.650

Adult tree basal cover

0.765

All herb (H')

All tree (H')

-0.339

-0.944*

All tree (H')

All vascular (H')

-0.637

All vascular (H')

All herb(S)

0.385

All herb (S)

All tree(S)

0.249

All tree (S) All vascular (S)

All tree density

Table 3. Relationships between diversity indices (Species Richness, S; Shannon-Wiener Index, H') and phytosociological parameters of various categories. Values are Pearson correlation coefficients. (*=correlation is significant at 0.05 level).

0.634

0.770

0.888*

0.800

0.742

0.157

0.313

-0.005

0.807*

0.876*

0.368

0.505

0.658

0.482

0.312

0.655

-0.213

0.946*

0.744 0.487

All herb (H')

way than a few species with limited canopy shape variations, thereby making ground environment unfavourable for the effective growth of herb layer. Saxena & Singh (1982) opined that an inverse relation between the two strata would occur only after a certain limit in the cover of the tree layer has been attained. A comparison of diversity indices calculated for different communities suggested that variation in the canopy cover and canopy tree species had no significant influence on diversity of herbaceous vegetation (Table 3). This may be due to the open nature of dry tropical vegetation (Murphy & Lugo

1986). Pausas (1994) and Qian et al. (1997) found that forest structural variables such as tree basal area or tree density are less important in affecting the diversity of herbs than environmental variables. However, negative relationship between density of trees with herb density and cover recorded in the present study was in conformity with the findings of Klinka (1996) and Qian et al. (1997). Considering this relationship and results of present study, it appears that canopy cover in dry tropics does influence cover and density but not the mathematical diversity indices of herbaceous vegetation.

62

PLANT DIVERSITY IN VINDHYAN FORESTS

Impact of disturbance on plant diversity The plant community organization in Vindhyan highlands is also changing in response to the increased anthropogenic disturbance due to mining, thermal power generation, cement industry and related build up of human population in the area. Disturbance results in extensive mortality and failure of recruitment of member species in the plant communities. Population analysis indicated that Tectona-Acacia and AdinaLannea communities are the most threatened due to disturbance (Tripathi 2003). Fig. 2 shows that the diversity of both overstorey and understorey strata decrease significantly with disturbance intensity (Sagar et al. 2003, 2008). The decreasing

Fig. 2. Disturbance reduces diversity of both overstorey and understorey plants in dry tropical forests of Vindhyan highlands. Sites are: HT-Hathinala, KHKhatabaran, MJ-Majhauli, BK-Bhawani Katariya, KTKota. (Based on Sagar et al. 2003, 2008).

trend of α-diversity and its components along the disturbance gradient (Sagar et al. 2003) reflects enhanced utilisation pressure in the form of cutting and lopping of trees (Bhat et al. 2000) and decreased resource availability with increasing human disturbance (Brokaw 1985).

Deforestation and forest fragmentation Disturbance leads to deforestation, forest fragmentation and degradation and subsequent exotic species invasion, all of which adversely affect plant diversity. Yet the causes, rates, magnitude, pattern and trends of landscape changes in the tropics at local and regional levels are not well documented, and lack particularly for dry tropics of India (Ramesh et al. 1997). The alteration of land use pattern has resulted in fragmentation of habitats, ecosystems and landscapes in most parts of the world. Habitat fragmentation is a leading cause of biodiversity loss (Singh 2002). The study of the temporal changes of spatial patterns in landscapes is important to understand the underlying factors and the functional effects (Mendoza & Etter 2002). Vindhyan is regarded as one of the sensitive areas under high risk of deforestation and is adjacent to the hot spots of the deforestation (Roy et al. 2002). Therefore, understanding deforestation and forest fragmentation in this region is important. With the collaboration of Forestry and Ecology division of National Remote Sensing Agency (NRSA) the vegetation of Vindhyan highlands was mapped in 2001 and defined the community types (mapping units) employed in the study. These vegetation maps for Vindhyans serve as a primary data source in this study. In this analysis, multiseasonal data (IRS, LISS 111 December 1998, April 1999 and March 2000) were acquired to represent more recent landscape composition. These data were layer stacked. Differential GPS was used in field for mapping communities on ground. This data was used to give training sets while classifying the vegetation. The entire area was delineated into eleven communities by using supervised most likelihood classification (MLC). A correspondence was developed with the previously identified plant communities on the basis of extensive ground survey of the area. The classified data sets were taken and subjected to binary

RAGHUBANSHI & TRIPATHI

conversion (n= 2). Within binary conversion forest was coded as 1 and all non-forest areas as 0. This raster file was converted into vector format. The vector files also give area statistics with the polygon ids. Proper symbology was given to differentiate forest and non-forest. This layer of fragments was subjected to conversion from vector to raster conversion in vector module of

63

ERDAS 8.5. Summary function was executed for this file in GIS tools. The output gave how many communities are there in each fragment and what are the areas occupied by each. For the year 1988, Landsat TM (1988) image was used and classified data was converted in forest and non forest fragments using binary conversion similarly as done for 1998.

Fig. 3. Map showing a part of Vindhyan forest indicating extent of deforestation and fragmentation between 1988 (top) and 1998 (bottom).

64

PLANT DIVERSITY IN VINDHYAN FORESTS

Analysis of landscape matrices in Vindhyan highlands using satellite images and GIS tools showed deforestation of smaller fragments, and fragmentation and degradation of larger ones (Fig. 3). In Table 4 changes in number and area of fragments belonging to different fragment size classes are given for a ten year interval (19881998). Present study found a decrease in number of smaller ( 1000 ha) and increase in medium sized fragments which is indicative of deforestation of smaller and fragmentation of larger fragments (Table 4). Most drastic effect was observed in 1000 ha area size class fragments got reduced by 44% in number as well as size. Analysis also indicated that there was general increase in forest edge, and edge density (Table 5). Smaller size fragments had lesser number of plant communities compared to large size fragments. Generally large size fragments were species rich and more diverse as compared to smaller fragments. Small forest fragments have a greater edge to area ratio and are intrinsically more susceptible to colonization of plants and animals Table 4. Changes in numbers, area and mean area of different fragment size classes during ten years interval. Fragment size class (ha) >1000 100-1000

Area (ha)

Number

Total

1988 1998 1988 9

Mean

1998

1988

1998

5 32237 18143 3581.9 3628.6

27

30

6162

8186

228.2

272.9

270

171

6398

4534

23.7

26.5

1-10

2227 1012

6458

2788

2.9

2.8

1000 100-1000

Edge (km)

Edge density (metre-1)

1988

1998

257.67

397.80

1988 0.01

1998 0.01

26.50

42.04

0.01

0.02

10-100

5.09

6.06

0.02

0.03

1-10

1.04

1.06

0.04

0.04