Plant Ecology (2005) 181:69–84 DOI 10.1007/s11258-005-3497-8
Springer 2005
Predicting species response to disturbance from size class distributions of adults and saplings in a Jamaican tropical dry forest K.P. McLaren1,2, M.A. McDonald1,*, J.B. Hall1 and J.R. Healey1 1
School of Agricultural and Forest Sciences, University of Wales, Bangor, Gwynedd, LL57 2UW, UK; Present address: The Department of Life Sciences, University of the West Indies, Mona Campus, Mona, Kingston 7, Jamaica; *Author for correspondence (e-mail:
[email protected]) 2
Received 19 May 2004; accepted in revised form 8 March 2005
Key words: Functional groups, Regeneration, Resilience, Seedlings, Size class distribution
Abstract The diversity of tropical dry forests is poorly described and their regeneration ecology not well understood, however they are under severe threat of conversion and degradation. The Hellshire Hills constitute a dry limestone forest reserve on the south coast of Jamaica that is of high conservation value. In order to describe the structure and composition of this forest and assess the extent to which the population structures of its tree species do characterize their regeneration ecologies, pre-disturbance structure, floristics and seedling populations were compared with post-disturbance species responses in twelve 15 m · 15 m permanent sample plots which were laid out in a blocked design in April 1998, giving a total sample area of 0.27 ha. These plots were subjected to disturbance in April 1999 (cutting) with each of four blocks being assigned with two randomly allocated treatment plots (partially and clear cut) and one control plot (uncut). A total of 1278 trees (‡2 cm DBH) and 7863 seedlings and saplings (0–2 mm and 2–20 mm root collar diameter (RCD) respectively), comprising 60 and 52 species, respectively, were sampled in the plots prior to disturbance. The species-area curve for trees reached a maximum at 0.20 ha, and abundance was widely distributed amongst the species (26 had importance values greater than 1%); four species were notably codominant (with importance values between 7 and 8%). The forest stand structure had a reverse J-shaped curve for tree and for seedling/sapling size-class distributions, which indicated that the forest as a whole was probably regenerating adequately. From an analysis based on adult and sapling size-class distributions (SCDs), 21 species with 15 or more individuals were classified into 3 groups. Many of the species (15 of the 21), had flat adult SCDs that deviate from the whole-community reverse J-shaped SCD. However, sapling SCDs for 6 of the 15 species were strongly positive indicating the potential for their populations to be sustained by recruitment from the saplings present. No general association was found between these SCD species groupings and the actual ability of the species to recover from disturbance. Analysis of post-disturbance response revealed that for only 9 of the 21 species did adult SCDs provide adequate prediction, but for an additional 6 of the species information on sapling SCDs improved the accuracy of prediction if the ‘release’ of saplings or smaller individuals predominated recovery. However in this forest, recovery following disturbance which left stem and roots in place is predominantly by coppice regrowth, and there were no significant correlations found between adult SCDs and the species’ ability to coppice.
70 Introduction Tropical and subtropical dry forest accounts for a larger area than do moist and wet forests (42% of total global forest cover (Murphy and Lugo (1986a)) but have received much less attention from ecologists. Their location and accessibility makes them more susceptible to human disturbance and as a result they have been exposed to severe, large-scale changes through the cutting of valuable trees, creation of pastures, accidental or intentional fires (Gerhardt and Hytteborn 1992) and harvesting of fuel wood (Murphy and Lugo 1986a). They continue to support a larger human population than do the humid forest areas. Despite their overexploitation there have been relatively few studies of tropical dry forests, especially their recovery following man-made disturbance. This is in spite of the fact that less than 2% of tropical dry forests in Central America have remained intact and less than 0.1% have protected area status (Janzen 1998a, b). Most studies on tropical dry forest have generated information on forest structure and floristics (e.g. Kapos 1986; Murphy and Lugo 1986a, b; Lott et al. 1987; Swaine et al. 1990; Ross et al. 1992; Sabogal 1992; Smith and Vankat 1992; Swaine 1992; Sussman and Rakotozafy 1994; Gentry 1995; Gonzalez and Zak 1996) or documented a change in floristics and structure across a gradient, usually of rainfall or substrate (Holdridge et al. 1971; Borhidi 1987; Kelly et al. 1988) due to the relative ease with which the data can be collected and in some cases the availability of data in forestry records (Obiri et al. 2002). There are, however, fewer studies examining dynamics following disturbance (Murphy et al. 1983; Miller and Kauffman 1998; Miller 1999; Gould et al. 2002; Kennard 2002; Kennard et al. 2002; McLaren and McDonald 2003a, b) although the number has risen steadily over the last few years. If conservation objectives are to be realized more studies of tropical dry forest dynamics are needed. This is a priority aspect for current research to better understand the capacity of forests to recover from disturbance. Nevertheless, forest dynamics are most often inferred from a single survey and the analysis of static forest inventory data by constructing species’ population size-class distributions (SCDs), and grouping species with similarities in distribution on the assumption that they share a similar
regeneration ecology (and so comprise a ‘functional group’) has often been recommended (e.g. Campbell 1989). A major weakness of previous approaches has been the lack of data on smaller trees, saplings and seedlings; often gross assumptions had to be made that population structure across these sizes reflected that of the larger trees. Despite this Lykke (1998) has demonstrated in savannas and dry tropical forest systems that SCDs give good indications of the impact of disturbance and of successional trends. Conversely, in moist forest, Condit et al. (1998) found that sizeclass distribution (SCD) did not correlate with the growth rate of larger individuals or with survival. They concluded that static information on species’ SCDs was not a good predictor of their future population trends, while demographic information was. This study examines how well species’ SCD correlates with, or predicts, their response to disturbance in a dry tropical forest. SCDs were obtained from an inventory of seedlings, saplings and adults before experimental gaps were created. A feature of this study is that a full inventory of saplings and seedlings (and their sizes) was carried out. It is hypothesised that the static information of the slopes of plotted species’ adult and sapling SCDs integrate assessment of whole-population structure sufficiently well to indicate their dynamics and predict their response to disturbance.
Methods Study area This study was conducted in the Hellshire Hills (1754¢ N, 7758¢ W) forest reserve in Jamaica which is an extensive area of tropical dry forest over limestone. The central and most of the western (seaward) sections of the reserve are covered with little-disturbed, primary tropical forest, currently subject to little human-caused disturbance, but the unregulated actions of charcoal burners over the last 10–20 years on more accessible fronts have resulted in a significant reduction of the forest area. Dry forests on limestone, similar to those in Jamaica occur in the Caribbean in Cuba, Puerto Rico, Bahamas, Dominica, St. Lucia and St. Kitts and Nevis and extensively in Central America (Kapos 1986) but the Hellshire Hills may
71 contain one of the last substantial remaining areas of primary, undisturbed dry forest in the Caribbean (Vogel et al. 1995). As such (and being the last remaining habitat for the endangered Jamaican iguana, Cyclura collei) the Hellshire Hills have recently been designated as a protected-area. However little is presently known about the forest, and if the forest is to be effectively managed information on its structure, composition and the regeneration ecology of its species is urgently needed. The Hellshire Hills are a gently rolling range (mean elevation of 140 m) of dry forest over hard white limestone from the mid-Eocene to lower Miocene. They are located on the south coast of Jamaica, just to the west of the capital Kingston, forming a peninsula of about 10 km in the North–South and 15 km in the East–West direction covering an area of 114 km2 (Vogel et al. 1995). Weathering of the limestone terrain has produced a coarse reddish soil of the ‘terra rossa’ type which is usually found in small volumes in crevices and pockets and in larger volumes in patches down-slope (Adams and du Quesnay 1970). Most of the ground surface is bare rock, generally covered with a layer of partially decomposing organic matter. The vegetation was described by Adams and du Quesnay (1970) and corresponds to the ‘Dry Evergreen Formations’ on limestone of Beard (1944, 1955). The long-term (20 year) average annual rainfall at Great Salt Pond, an area within the Hellshire Hills, is 780 mm (1961–1981). Actual rainfall measured within the study site was 917 mm in 1999 and 650 mm in 2000. The main rainy season peaks in September (which has an average monthly rainfall of 180 mm). This is usually followed by a dry period of 4–5 months with 2 mm RCD but less than 1.3 m in height (‘saplings’)). The saplings were further subdivided into two size classes: 2–4 mm RCD (which were estimated to be 1–6 years old, depending on species); and 4–20 mm RCD (which were estimated to be generally >6 years old). The SCDs of trees and saplings were analysed using a method proposed by Condit et al. (1998), Lykke (1998) and Obiri et al. (2002). For tree SCDs 16 DBH size classes were used between 2 and 32 cm with each size class being 2 cm wide, and for sapling SCDs 21 RCD size classes were used between 2 and 44 mm with each size class being 2 mm wide (the 0–2 mm class was omitted because the density of seedlings changed so rapidly
72 over time that it was not considered to be meaningful). For each of 21 species represented by 15 or more tree individuals a least-squares linear regression was calculated with the size-class midpoint as the independent variable and the average number of individuals in that class (Ni) as the dependent variable. The size-class midpoints were not transformed, however in order to derive straight-line plots of the SCD the average number of individuals (Ni) in each size class was transformed by ln (Ni + 1) because some classes had zero individuals (Obiri et al. 2002). The slopes of these regressions are referred to as SCD slopes and were used as an indicator of population structure (Lykke 1998; Obiri et al. 2002). The adult SCD slope values, along with a scatter plot of all 21 species’ adult and sapling slope values, were used to separate the species into groups indicative of their regeneration potential. To examine the impact on species classification of applying different minimum size thresholds to the ‘‘sapling’’ data we also tested the slope of sapling size class distributions on populations of >4 mm RCD (i.e. excluding any individuals that might be 4 mm RCD had no impact on the classification of the species; their position in the rank order of regression slopes corresponded closely to that for the >2 mm RCD data set. In April 1999, two treatments were applied to one plot within each block, with one uncut plot acting as a control. The trees were cut in a manner similar to the practice of the charcoal burners (either selective tree removal or complete clearance depending on the implements available to the charcoal burner): 1. Clear cut, where all individuals ‡2 cm diameter were removed. 2. Partial cut, where 50% of stems ‡5 cm in the plot on a size class basis were removed. 3. Uncut. Trees were cut at a standard height of 0.5 m. The trees that were removed in the partially cut plots were selected randomly irrespective of species and if they were represented by more than two individuals within all plots. The plot size was intended to be of similar size to the patches cleared in the forest by charcoal burners. Clearance is generally conducted with machetes and axes
(although some clearance by chain-saw occurs) and consequently cleared areas are relatively small in size. Large or hard stems (or species that do not make good charcoal) may be left in place resulting in partially cleared patches.
Results Floristic composition and dominance In the 0.27 ha sampled, a total of 1278 trees (4733 trees ha 1) (‡2 cm DBH) of 60 species (including one unidentified species) belonging to 27 families, comprising a total basal area of 7.7 m2 (29 m2 ha 1), were recorded. A species-area curve using all 12 plots reached an asymptote at ca. 0.20 ha, showing that a sufficient area had been sampled to include most species in the community. Of the 60 species, 8 (comprising 13.3% of the individual trees sampled) were endemic (Figure 1), and 1 species, Reynosia guama Urb. (Rhamnaceae), was recorded for the first time in Jamaica. The shape of the species rank/abundance curve was shallow, indicating a high degree of evenness amongst the species (Figure 1). In terms of IVI four species were dominant, each with values between 14 and 16 (or 7–8% of the total IVI) (Figure 1). Whilst the top nine species accounted for half (52.2%) of the total IVI, 26 species had values of at least 1% of the total (Figure 1). In terms of frequency eight species occurred in all 12 plots while nine species occurred in only one plot.
Forest structure and sapling and seedling populations The stand tree and sapling SCD shows a classic positive, reverse-J shape (Figure 2a, b). In the 0.27 ha sampled, a total of 2234 individual saplings (RCD 2–20 mm) comprising 53 species (excluding the unidentified species) were recorded; this density is equivalent to (8274 individuals ha 1 or 0.8 individuals m 2). In addition 5629 individual seedlings (RCD 0–2 mm) comprising 38 species (excluding the unidentified species) were recorded. Only 72 of the sapling and seedling individuals (0.9% of the total) could not be identified. Three species were
73
Figure 1. Abundance of the 59 species identified in 0.27 ha of dry limestone forest sampled in Jamaica: relative basal area, relative density and Importance Value Index (IVI) for trees ‡2 cm DBH. Endemics are denoted by *.
recorded in the sapling and seedling survey but were not present as adults (Guanicum officinale L., Randia aculeata L., and one unidentified species) and of the 60 species recorded as trees, nine were absent from the sapling and seedling survey. Saplings and seedlings of tree species were more or less the only component of the ground layer of the undisturbed forest (i.e. there were no herbs, grasses, and very few woody shrubs that exceeded 2 cm DBH (only two species Phyllanthus angustifolius (Sw.) Sw. and Adelia ricinella L.). As the sapling and seedling populations accounted for a very high proportion
of the ground layer vegetation, competition with non-tree species presumably plays a very minor role in tree regeneration in this forest (in contrast to savannah and some moist forests). The dominant species in the sapling and seedling flora was Diospyros tetrasperma Sw. with a relative density of 25% (Figure 4). A further four species (Metopium brownii (Jacq.) Urb., Krugiodenron ferreum (Vahl) Urb., Thrinax parviflora (Sw.) and Comocladia velutina Britton) had relative sapling and seedling densities between 13 and 9% (more than twice that of the next species). All of these species were notable for having
74 sapling and seedling relative densities more than twice that of their tree relative densities.
Species population structures Group 1 comprises six species with adult tree SCD slope values ranging from 0.1 to 0.08 (Table 1). They show a clearly positive shape (a reverse-J shaped curve indicating a steep decline in numbers with increasing size (Figure 4) and their sapling size-class distributions also had steep slopes (especially so for Diospyros tetrasperma and, to a lesser extent, Krugiodendron ferreum; Figure 3).
All species in this group had a high density of seedlings (0–2 mm RCD) except Drypetes lateriflora (Sw.) Krug and Urb. (Table 1). Group 2 consists of 14 species all of which have a relatively flat tree SCD with slopes ranging from 0.063 to 0.029 (Table 1). The species within this group have a wide range of sapling SCDs (Figure 3): six species with a more negative slope value (ranging from 0.05 to 0.03, indicating a high rate of sapling recruitment), five species with a less negative slope value (ranging from 0.01 to 0.026, indicating that there had been a lower recruitment rate) and three species with virtually no sapling recruitment (slope < 0.01). Only the six
Figure 2. Size-class distributions of all tree species in 0.27 ha of dry limestone forest sampled in Jamaica: (a) diameter at breast height of all individual trees ‡2 cm DBH; (b) root collar diameter of all seedlings and saplings of 1.3 m tall) and height of the leading coppice shoot per stump, and the average percentage of the individual’s pre-cutting stem diameter recovered by coppice growth (Table 1). Of the species that failed to coppice well, including Bursera lunanii, Capparis cynophallophora, Clusia flava and Linociera sp., all four had below average height and diameter of their coppice shoots (except Bursera lunanii for diameter) and percentage diameter recovered (Clusia flava being a hemi-epiphyte did not coppice at all) (Table 1), Also three of these four species had below average numbers of coppice shoots per stem and percentage of cut stems with shoots (which was
