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Home , Disturbance (ecology), Habitat fragmentation, Invasive species

RAGHUBANSHI & TRIPATHI 57

Tropical 50 (1): 57-69, 2009 ISSN 0564-3295 © International Society for Tropical Ecology www.tropecol.com Effect of , fragmentation and alien invasive on floral diversity in dry tropical of Vindhyan highland: a review

A.S. RAGHUBANSHI * & ANSHUMAN TRIPATHI

Department of Botany, Banaras Hindu University, Varanasi 221005, India

Abstract: Effect of disturbance, and exotic 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 wise diversity was estimated. Analysis of landscape matrices using satellite images and GIS tools showed 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 and herb strata, whereas Shorea-Buchanania community was the richest when all vascular 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 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 Acacia- Terminalia, 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, invasion, plant communities.

Introduction substantial portion of these hotspots contain tropical dry forest, which is one of the world’s most Understanding the diversity of in endangered forest types (Janzen 1988a; Lerdau et various forms is a fundamental goal of ecological al. 1991; Mittermeier et al. 1999). Data from most research (Lubchenco et al. 1991). Apart from the threatened dry tropics (Janzen 1988a) are scanty. immense economic, ethical and aesthetical Dry tropical forests once occupied more than half benefits, is essential for the the forest terrestrial tropics (Janzen 1988b; function and stability (Ehrlich & Wilson 1991; Murphy & Lugo 1986). Most of the past studies on Holdgate 1996; Tilman 2000). Biodiversity has habitat fragmentation have focussed their attracted world attention because of the growing attention on animals in temperate forests or in the awareness of its importance on the one hand and rain forests. Little information exists from dry the anticipated massive depletion on the other tropical zone. Few studies have actually measured (Singh 2002). the level of diversity that exists in networks of Globally, concerns are raised over the rapid small forest patches in tropical environments loss of biodiversity in all its forms and at all the (Pither & Kellman 2002; Sagar et al. 2003, 2008). levels. is the chief cause of the Present documents the effects of . can either disappear disturbance, habitat fragmentation and exotic completely or they may become degraded and/or plant species invasion on plant diversity in the dry fragmented, both causing serious impact on deciduous forests of Vindhyan highlands. species as well as ecosystem processes. Regionally, species introductions and altered disturbances Vindhyan highlands rates may favour increased local diversity, but habitat loss or modification, outbreaks of Vindhyan highlands are located in between the introduced or , and management of highly populated Indo Gangetic - plains and the exploitable systems tend to decrease species Narmada valley, and have natural of richness and heterogeneity (Lubchenco et al. tropical dry deciduous forests (Champion & Seth 1991). 1968). Although practically no historical Biodiversity hotspots around the world contain information exists on in the high degree of and are undergoing Vindhyan highlands, it appears that the area was exceptional loss of habitats (Myers et al. 2000). A once very rich in diversity. The potential natural RAGHUBANSHI & TRIPATHI 59 ecosystem of the area is tropical deciduous forest. diversity was estimated. Community classification Earlier ecological work in the Vindhyan highlands was done by cluster analysis procedure as given in shows a profound impact of activity on the PC-ORD software (McCune & Mefford 1999). natural (Singh et al. 1990). There has Hierarchical agglomerative polythetic cluster been rapid industrialization in recent years and analysis was performed with Ward’s divisive activities like quarrying for limestone, method and squared Euclidean distance options to establishment of cement factories, thermal power delineate communities on the basis of basal area stations, widespread mining have resulted in a values of adult for all the sampling units. In rapid build up of human , displacement order to select the appropriate stop level, of original population, deforestation and preliminary cluster was evaluated by Indicator conversion of natural forest ecosystems into Species Analysis procedure in the software as savanna and marginal croplands. Earlier studies proposed by Dufrene & Legendre (1997). indicated that the forested area in the region is Cluster analysis of vegetation data of adult continuously decreasing and the remnant forest tree individuals resulted in 11 plant communities cover exists in the form of non-contiguous patches (Fig. 1). The identified communities were: of varying sizes dominated by single or mixed (1) Tectona-Acacia (2) Lagerstroemia-Terminalia species (Singh et al. 1990). This feature is unique (3) Lagerstroemia-Acacia-Lannea (4) Shorea- and gives a new dimension to the biodiversity Buchanania (5) Tectona-Holarrhena (6) Acacia- pattern. The change detection study (1982-1989) Terminalia (7) Anogeissus-Holarrhena (8) Adina- for a part of Vindhyan hills using satellite images Lannea (9) Terminalia-Shorea (10) Shorea identified that only 31% of the forest area has Community, and (11) Hardwickia-Acacia . Of the 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).

Plant communities in Vindhyan highlands

Classification and mapping of vegetation is needed for 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 Fig. 1. Dendrogram produced from hierarchical cluster imagery and ground data and community wise analysis of vegetation data for classification of forest communities.

60 PLANT DIVERSITY IN VINDHYAN FORESTS identified communities, Lagerstroemia–Acacia– richest when tree and herb strata are considered Lannea community is the most prevalent followed separately, whereas Shorea-Buchanania community by Acacia –Terminalia community (Table 1). was the richest when all the vascular flora were The eleven plant communities recognized in considered together (Table 2). Shorea inhabiting the region reflect variety of patches developed due communities prefer moist north facing slopes that to interaction of edaphic factors, heterogeneity of favour development of rich and diverse vegetation environment and disturbance (Jha & Singh 1990; (Jha 1990). Pausas (1994) reported highest Roy & Singh 1994; Singh et al. 1990). Jha (1990) herbaceous species on shallow north facing slopes classified vegetation of the same area into ten from eastern Pyrenees. Saxena & Singh (1982) community types and advocated soil texture as an opined that intermediate temperature and moist important determinant of the distribution of environmental conditions are favourable for that of communities (Jha & Singh 1990). According to the trees, saplings and herbs. In the Vindhyan them, dry of the Vindhyan area is a highlands the climatic conditions are relatively mosaic of non contagious patches of communities. xeric during the most part of the year and soil The patchy distribution of species emphasizes moisture coupled with aspect plays a critical role environmental determinism, in which forests are in deciding vegetation composition. considered to be mosaic where plant species Opposite to Shorea , Hardwickia and Acacia composition is determined by edaphic and other prefer xeric sites with large canopy openings, thus, environmental characteristics (Gentry 1988; their companion species are also adapted to low Tuomisto et al. 1995; Tuomisto et al. 2003). moisture and high light environments. Intermediate to these extremes is Lagerstroemia– Plant diversity within identified Acacia–Lannea community which occurs on soils communities with moderate moisture regimes created by non- continuous canopy which is not as poor as In order to understand pattern of plant Hardwickia–Acacia community dominated sites. diversity, six communities were selected for Because of intermediate environmental conditions, analysis (Table 2). These communities had a and environmental heterogeneity created by representation in minimum 8 sampling units out canopy gaps, Lagerstroemia–Acacia–Lannea may of total 150 sampling units used for analysis. The be the most diverse community for all trees and for selected community types of this region differed all vascular categories. Environmental with each other in their species composition. heterogeneity has long been recognized as an Lagestoemia -Acacia -Lannea community was the important factor maintaining biological diversity (Huston 1994). The broken canopy creates an internal mosaic involving spatial gradients of Table 1. Plant communities identified in the dry light, soil moisture, soil organic matter, tropical forests of the Vindhyan highlands and % temperature and effective rainfall (Breashears et area occupied by them . al. 1997) which ultimately govern the overall diversity of vascular flora. At the fine scale, Community types Area % Occupied (ha) area comparatively well-developed herbaceous vegetation interferes with the of Tectona-Acacia 892.9 2.8 trees, and has higher species number and diversity Lagerstroemia-Terminalia 2248.7 7.1 per unit area as compared to trees, therefore, it Lagerstroemia-Acacia-Lannea 11150.2 35.3 regulates all vascular richness. Shorea-Buchanania 2385.8 7.6 Whittaker (1972) stated that of one Tectona-Holarrhena 2229.2 7.1 stratum may affect the diversity of another Acacia-Terminalia 5481.3 17.4 stratum. In the present study herbaceous diversity Anogeissus-Holarrhena 901.1 2.9 do not relate well with tree diversity, although Adina-Holarrhena 559.9 1.8 herb layer cover and density were negatively Terminalia-Shorea 3071.1 9.7 influenced by the tree (Table 3). It Shorea robusta 917.1 2.9 seems that greater variety of trees with different Hardwickia-Acacia 1109.7 3.5 canopy architecture may fill canopy gap in a better Vegetated area 30946.8 98.0

RAGHUBANSHI & TRIPATHI 61

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

Communities

Diversity parameters Acacia Buchanania Acacia Acacia Lagerstroemia Lannea Shorea Terminalia Terminalia Shorea Shorea dominated Hardwickia Species Richness (S) 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 Shannon-Wiener’s Index (H') 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

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).

All tree (S) tree All (S) herb All (H') vascular All (H') tree All (H') herb All Adultbasal tree Adultdensitytree density tree All cover basal tree All cover herb All density herb All cover cover All vascular (S) 0.634 0.770 0.888 * 0.800 0.742 0.157 0.313 0.249 0.385 -0.637 -0.339 All tree(S) -0.005 0.807 * 0.876 * 0.368 0.505 0.658 0.765 0.650 -0.944 * -0.902 * All herb(S) 0.482 0.312 0.655 -0.213 -0.137 -0.309 -0.038 -0.045 0.305 All vascular (H') 0.946 * 0.744 0.122 0.281 0.424 0.321 -0.688 -0.549 All tree (H') 0.487 0.201 0.342 0.594 0.383 -0.802 -0.593 All herb (H') -0.091 0.046 -0.086 0.071 -0.196 -0.254 way than a few species with limited canopy shape 1986). Pausas (1994) and Qian et al. (1997) found variations, thereby making ground environment that forest structural variables such as tree basal unfavourable for the effective growth of herb layer. area or tree density are less important in affecting Saxena & Singh (1982) opined that an inverse the diversity of herbs than environmental relation between the two strata would occur only variables. However, negative relationship between after a certain limit in the cover of the tree layer density of trees with herb density and cover has been attained. recorded in the present study was in conformity A comparison of diversity indices calculated for with the findings of Klinka (1996) and Qian et al. different communities suggested that variation in (1997). Considering this relationship and results of the canopy cover and canopy tree species had no present study, it appears that canopy cover in dry significant influence on diversity of herbaceous tropics does influence cover and density but not vegetation (Table 3). This may be due to the open the mathematical diversity indices of herbaceous nature of dry tropical vegetation (Murphy & Lugo vegetation.

62 PLANT DIVERSITY IN VINDHYAN FORESTS

Impact of disturbance on plant trend of α-diversity and its components along the diversity disturbance gradient (Sagar et al. 2003) reflects enhanced utilisation pressure in the form of The plant community organization in cutting and lopping of trees (Bhat et al. 2000) and Vindhyan highlands is also changing in response decreased resource availability with increasing to the increased anthropogenic disturbance due to human disturbance (Brokaw 1985). mining, thermal power generation, cement industry and related build up of human population Deforestation and forest in the area. Disturbance results in extensive fragmentation mortality and failure of of member species in the plant communities. Population Disturbance leads to deforestation, forest analysis indicated that Tectona-Acacia and Adina- fragmentation and degradation and subsequent Lannea communities are the most threatened due exotic species invasion, all of which adversely to disturbance (Tripathi 2003). Fig. 2 shows that affect plant diversity. Yet the causes, rates, the diversity of both overstorey and understorey magnitude, pattern and trends of landscape strata decrease significantly with disturbance changes in the tropics at local and regional levels intensity (Sagar et al . 2003, 2008). The decreasing are not well documented, and lack particularly for dry tropics of India (Ramesh et al. 1997). The alteration of 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 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 Fig. 2. Disturbance reduces diversity of both correspondence was developed with the previously overstorey and understorey plants in dry tropical forests identified plant communities on the basis of of Vindhyan highlands. Sites are: HT-Hathinala, KH- extensive ground survey of the area. The classified Khatabaran, MJ-Majhauli, BK-Bhawani Katariya, KT- data sets were taken and subjected to binary Kota. (Based on Sagar et al . 2003, 2008). RAGHUBANSHI & TRIPATHI 63 conversion (n= 2). Within binary conversion forest ERDAS 8.5. Summary function was executed for was coded as 1 and all non-forest areas as 0. This this file in GIS tools. The output gave how many raster file was converted into format. The communities are there in each fragment and what vector files also give area statistics with the are the areas occupied by each. For the year 1988, polygon ids. Proper symbology was given to Landsat TM (1988) image was used and classified differentiate forest and non-forest. This layer of data was converted in forest and non forest fragments was subjected to conversion from fragments using binary conversion similarly as vector to raster conversion in vector module of 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 Table 5. Increase in edges and edge density in highlands using satellite images and GIS tools fragments of all size classes indicates rapid showed deforestation of smaller fragments, and fragmentation of dry tropical forests in Vindhyan fragmentation and degradation of larger ones (Fig. highlands . 3). In Table 4 changes in number and area of Fragment Edge Edge density fragments belonging to different fragment size size class (km) (metre -1) classes are given for a ten year interval (1988- 1998). Present study found a decrease in number 1988 1998 1988 1998 of smaller (<10 ha) and larger fragments (> 1000 >1000 257.67 397.80 0.01 0.01 ha) and increase in medium sized fragments which 100-1000 26.50 42.04 0.01 0.02 is indicative of deforestation of smaller and 10-100 5.09 6.06 0.02 0.03 fragmentation of larger fragments (Table 4). Most 1-10 1.04 1.06 0.04 0.04 drastic effect was observed in <1 ha size class fragments as their number and area decreased by < 1 0.20 0.24 0.11 0.11 65 and 55%, respectively. Mean fragment area in Total 0.72 1.36 0.01 0.09 this size class decreased by 30%. In the case of 1-

10 ha size class fragments, although mean from surrounding human dominated habitats fragment area is almost same, its number and (Janzen 1986). Fragments can be bombarded with total area fell by half during the 10 years. diaspores of exotic or weedy species that may be Similarly, 10-100 ha size class fragment got incorporated into remaining plant community, reduced in number (37%) and area (29%) due to sometimes eliminating species confined to forest forest conversion. Most remarkable shift was in interiors (Janzen 1986; Tabarelli et al. 1999). the case of 100-1000 ha fragment size category in Rates of mortality and canopy gap formation also which increase in number and area of fragment tend to increase near edges after fragmentation was observed. Although mean area remained (Laurance et al. 1998). same, >1000 ha area size class fragments got Lesser increase in edge density (ratio of reduced by 44% in number as well as size. fragment perimeter to fragment area) for smaller Analysis also indicated that there was general fragment size classes and higher increase for increase in forest edge, and edge density (Table 5). medium and large fragment size classes (Table 5) Smaller size fragments had lesser number of plant indicate that the of small fragment has communities compared to large size fragments. not increased in proportion to the total amount of Generally large size fragments were species rich area removed. It suggests that many smaller and more diverse as compared to smaller fragments have disappeared rather than having fragments. Small forest fragments have a greater been split into smaller habitat fragments. This edge to area ratio and are intrinsically more also explains the lower change in edge density of susceptible to colonization of plants and animals smaller fragments compared to larger ones. Analysis of individual communities show that mean percent area occupied by Shorea dominated, Table 4. Changes in numbers, area and mean Shorea-Buchanania, Terminalia-Shorea, Tectona- area of different fragment size classes during ten years interval . Holarrhena, Adina-Lannea, Tectona-Acacia and Anogeissus-Holarrhena communities increased Fragment Area (ha) positively with log mean area of fragments Number size class Total Mean indicating that these may be most sensitive (ha) communities to deforestation and forest 1988 1998 1988 1998 1988 1998 fragmentation. >1000 9 5 32237 18143 3581.9 3628.6 Our work in the region indicates that species 100-1000 27 30 6162 8186 228.2 272.9 rich communities of the dry tropical forests are not 10-100 270 171 6398 4534 23.7 26.5 only being reduced in area but they are also 1-10 2227 1012 6458 2788 2.9 2.8 becoming species poor and less diverse due to < 1 9399 3298 2149 972 0.2 0.3 rapid deforestation and forest fragmentation. Example of most diverse Lagestoemia -Acacia - Total 11932 4516 53 405 34624 4.5 7.7

RAGHUBANSHI & TRIPATHI 65

Lannea community is given in Fig. 4 which shows isolated forest fragments are likely to have high that this community becomes less diverse as edge to interior ratio (Taberelli et al. 1999). Edge fragment size decreases. Mac-Arthur Wilson model effect may be particularly expressed due to also predicts that small forest remnants will alterations in forest microclimate and greater wind support fewer species. A review of the literature turbulence near edges (Laurance et al. 1998). shows that in nearly all cases tropical rain forest Smaller fragments contain proportionally more fragmentation has led to local loss of species edge habitat and fragments, particularly those (Kellman et al. 1998; Turner 1996). Losses of plant under 10 ha, can effectively consist entirely of edge species have been confirmed in many instances habitat and given time may not be structurally or (Turner 1996; Turner & Corlett 1996 a,b). Isolated floristically distinguishable from forest edge fragments suffer reductions in species richness (Kapos 1989; Viana et al. 1997). Honnay et al. with time after excision from continuous forest, (1999a,b) also supported that in case of smaller and small fragments often have fewer species patches (0.03 to 1.6 ha) it is possible that the recorded for the same effort of observation than entire patch may be subjected to . large fragments or areas of continuous forest Fragment edges may be inhospitable to some and (Turner 1996). possibly a majority of forest species leading to an At cutting rates, smaller, more effective reduction in fragment size, and making fragment shape (via periphery to area relationship) an important determinant of fragment quality. It is because of this that edge density has negative effect on mean community diversity. Leigh et al. (1993) proposed suppressed plant recruitment due to desiccating dry season winds as a hypothesis to account for the rapid loss of tree species on tiny forest fragments in Lake Gatun, Panama. Terborgh et al. (2001) also reported severely depressed recruitment of canopy trees due to forest fragmentation in the Caroni valley of Venezuela. However, the situation may not be as simple because the reduction in tree species at smaller fragments may be a consequence of complex interplay of both increased mortality and a reduced recruitment. Tabarelli et al. (1999) obtained a high density of tree fall gaps and dead trees in smaller fragments in of south-eastern Brazil. In Amazon, where effect of forest fragmentation is much studied, the rates of tree mortality and damage rise sharply in fragmented forests (Laurance et al. 1998, 2001) and higher vulnerability of small and large trees is reported near edges (Laurance et al. 2000). During two years of study, Laurance et al. (2003) reported that mortality rates were over twice as high within 100 metres of forest edges (3.73% yr -1) than farther from edges (1.64% yr -1).

Exotic plant invasions Fig. 4. Fragment size in dry tropical forest positively influences % area occupied by most widespread Ever since the studies reported by Elton (1958) Lagerstroemia-Acacia-Lannea community (a) and its tree to those reported by Burke & Grime (1996) species richness (b) .

66 PLANT DIVERSITY IN VINDHYAN FORESTS researchers support that plant communities are generally more susceptible to invasion when they are subjected to some form of disturbance. Seasonal deciduous forests in the tropics are thought to be more susceptible to invasion and domination by weedy species (Janzen 1988 a,b). Invasion by non-native species is also a problem faced by most of the communities of the region. In the Vindhyan highlands, communities most vulnerable to exotic plant invasions were Acacia- Terminalia, Terminalia-Shorea and Hardwickia- Acacia . Canopy openings, resulting from local disturbance, create patches of greater light availability. These canopy openings can act as windows for (Johnstone 1986), particularly Lantana in dry tropical forests, Fig. 5. Relationship between % Lantana cover and because it provides pertinent light regime and Shannon-Weiner diversity in Vindhyan dry tropical forest suitable moisture level that is lacking in the open (Sharma 2007). as well as closed patches of the forests. Disturbance patch invasion model (Gentle & similar trend was followed for herb species Duggin 1997) states that the removal of diversity and richness (Sharma and Raghubanshi, competitive and disruption of inter unpublished). Although tree species diversity was specific competitive interactions create patches of not significantly affected but the tree species increased resources. The presence of disturbance richness showed a significant decrease with in the form of canopy openings increases resource increasing Lantana cover. The results of the availability and also modifies the microclimate. present study are in conformity with the Many of the exotic benefit from edge experimental model of most of the studies environments and exert substantial pressures, (Stohlgren 2002), that with increasing invasive including , on the range of native cover the diversity decreases. Study of tropical species (Fischer et al. 2006; Ries et al. 2004). The semi-evergreen forest in Bangladesh also revealed success of invader depends on the disturbance that invasion of Lantana in forest gaps after (Rejmanek 1989) and the community involved human perturbations cause an abrupt decline in (Forcella & Harvey 1983; Hobbs 1989; Orians species richness (Islam 2001). 1986). Subsequent to plant species invasion marked Conclusions changes in community composition, diversity and functioning occur (D’Antonio & Vitousek 1992). In The overall analysis indicates that species rich Vindhyan dry deciduous forests we observed that communities of the dry tropical forests are not only sites with high Lantana cover contain little or no being reduced in area but they are also becoming understorey vegetation whereas sites with low species poor and less diverse due to rapid Lantana cover had some understorey species below deforestation and forest fragmentation. The their canopies. Dense cover created by horizontal community organization is also changing in stratification of Lantana reduces the intensity and response to increased anthropogenic disturbance. duration of light, preventing the establishment of Lantana invasion is changing the forest structure, tree species seedlings (Sharma & Raghubanshi leading to species diversity loss and creation of a 2006), resulting in marked changes in structural homogeneous, mono-specific Lantana invaded and floristic composition. Therefore, diversity and understorey in the forest. Serious measures are species richness showed a decreasing trend with needed to preserve and conserve whatever intact increasing Lantana cover (Fig. 5). All species forest is left in the Vindhyan highlands so that diversity and all species richness significantly this alarming trend of disappearance of species decreased with increasing Lantana cover and a rich communities is halted. RAGHUBANSHI & TRIPATHI 67

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