Vegetation Structure and Prioritizing Plants for Eco-Restoration of Degraded Wildlife Corridor in Dry Tropical Forest of South India

2.1. Study site

The study has been carried out at Edeyarahalli-Doddasampige wildlife corridor (ED corridor), which is one of the degraded but ecologically important functional corridors between Biligiri Rangaswamy Temple Tiger Reserve (BRT) and Malai Mahadeswara Hills Wildlife Sanctuary (MM Hills) (Figure 1). The dimension of the ED corridor is 0.5 km in length and 2 km in width and the geographical coordinates are 11°55′15″ to 11°56′15″N and 77°15′20″ to 77°15′45″E. The corridor landscape is largely in the dry deciduous and scrub forest type. It harbors rich floral and faunal diversity, mainly IUCN red listed mammal species such as Asian elephant (Elephas maximus), Bengal tiger (Panthera tigris), Indian leopard (Panthera pardus) and Indian wild dog (Cuon alpinus). In addition to this, the corridor landscape is inhabited by Soligas, an indigenous tribal community and a few other non-tribal communities.

The corridor landscape is severely degraded due to unplanned land-use practices, past forest management activities- logging and shifting cultivation -and the problem of invasive/exotic plants species [24, 25]. Apart from that, the villagers use this corridor regularly for livestock grazing and fuel wood collection [6]. In addition to this, the state highway (SH-17A) is passing through this wildlife corridor and an average of one vehicle per minute was recorded on this road [26]. This could be an additional threat to the movement of wildlife in this corridor. Irrespective of various threats, ED corridor provides space and passage for more than 15 mammal species (large, medium and small) to move from Western Ghats to forested landscapes of Eastern Ghats [27]. Adjacent to this corridor, in 2007 approx. 25.5 acres of private land was purchased from local farmers to widen the corridor by WTI (Wildlife Trust of India) and its international partner organization International Fund for Animal Welfare (IFAW), with financial support from US Fish and Wildlife Services (USFWS). The land was then handed over to the Karnataka State Forest Department to augment the corridor. This was a pioneering move in corridor conservation in India [6].

2.2. Vegetation enumeration

Transect method was used to enumerate vegetation in the corridor landscape. There were 64 belt transects of 0.1 ha (10 × 100 m), 128 plots of 10 m² and 512 plots of 1 m² were established to enumerate trees, shrubs and herbaceous plants respectively in the study area (Figure 2). Each sampling transect was marked with red ribbons, and the GPS coordinates were recorded at the center of each transect for future study purpose. The sampling was carried out in the month of October, which is the peak wet season in the study area. This is because during the wet season the chances of finding herbaceous species as well as seedlings of woody species in the study area are higher.

3.1. Plant community structure

Species richness and Shannon’s diversity H′ is relatively higher in tree class compared to shrub and herbaceous class. The evenness J is more or less similar between shrub and herbaceous class but relatively higher than tree class (Table 1). The corridor landscape had 92 tree species (belonging to 39 families), 75 shrub species (belonging to 41 families) and 185 species (belonging to 65 families). About 73.9% stems belong to different shrub species and 26.1% are saplings of woody species. In terms of total herbaceous stems enumerated in the study area, around 77.8% are herbaceous plants and 22.2% are woody seedlings.

3.2. Species importance value or IVI

The study site was evaluated for importance value index of each species. For tree species, the top ten most common species found in the sampled area were Anogeissus latifolia, Chloroxylon swietenia, Erythroxylon monogynum, Dalbergia lanceolaria, Strychnos potatorum, Naringi crenulata, Acacia chundra, Diospyros montana, Canthium travencoricum and Ixora arborea (Table 3). Among 92 species, these 10 species contribute 52% of the total IVI (Appendix A).

For non-tree forms such as shrubs/saplings, the top ten and most common species found in the corridor landscape were Lantana camara, Pterolobium hexapetalum, Dodonaea viscosa, Randia dumetorum, Chloroxylon swietenia, Erythroxylon monogynum, Zizyphus oenoplia, Fluggea leucopyrus, Eupatorium odoratum, Dolichandrone falcata and Pavetta indica (Table 4). Among 75 species, these 10 species contribute 70% of the total IVI, of which Lantana camara alone contributes 32% (Appendix B).

For the seedlings/herbaceous plant group, the top ten most important species found in the corridor landscape were Leucas martinicensis, Oxalis corniculata, Eupatorium odoratum, Lantana camara, Evolvulus alsinoides, Atylosia lineata, Randia dumetorum, Justicia simplex, Crotalaria calycina and Ziziphus oenoplia (Table 4). Among 185 species, these 10 species contribute 38% of the total IVI (Appendix C).

The problematic invasive weeds of the landscape, such as Lantana camara and Eupatoruim odoratum are contributing significantly toward total IVI in both shrubs and herbs categories. Lantana camara contributes 32.30% and 5.47% for total IVI of shrubs and herbs respectively, whereas Eupatoruim odoratum contributes 2.82% and 5.89% for total IVI of shrubs and herbs respectively. This indicates the extent of invasion of weeds in the landscape.

3.3. Relationship between vegetation diversity and habitat characteristics

The data was analyzed for relationships between one of the community variables such as vegetation diversity - of trees, shrubs and herbs - (as a response variable) with three habitat covariates such as fuelwood collection, livestock grazing intensity and invasive species – Lantana camara density (as predictor variables). The (four) models were developed to test the relationship between Diversity (H′) of- (i) trees vs. fuelwood collection, (ii) shrubs vs. Lantana camara density, (iii) herbs vs. Lantana camara density and (iv) herbs vs. grazing intensity of livestock.

Even though no statistically significant linear dependence of the mean of y on x was detected (the p-values are >0.05 for all relationships except for Shannon’s diversity vs. Lantana camara density in shrubs) the slope (regression coefficients) shows a negative trend (Figure 3). The negative (marked in minus symbol) slope coefficient value for (i) trees vs. fuelwood collection is −0.007, (ii) shrubs vs. Lantana camara density is −0.006, (iii) herbs vs. Lantana camara density is −0.001 and (iv) herbs vs. grazing intensity of livestock −0.005. This indicates that fuelwood collection, cattle grazing and the density of invasive species like Lantana camara affects the species diversity (H′) of life forms (trees, shrubs and herbaceous species) in the corridor landscape.

4.1. Prioritized plant species for eco-restoration: a socio-ecological approach

Globally, conceptual models for restoration of biodiversity have highlighted the importance of regional plant source pool and framework species in restoration [54, 55, 56]. Regional plant species are more important for eco-restoration, because the type of forest occurring naturally in a place is the result of climatic, edaphic, topographic, and biotic factors of the locality [22, 23].

Out of 92 tree species, 10 species contribute 52% of the total IVI of the corridor landscape. Among the 10 species Anogeissus latifolia, Canthium travancoricum, Erythroxylon monogynum and Ixora arborea are the top five species which have been exploited for fuelwood. People prefer these trees as firewood due to their calorific value, ease of carrying as headload, and frequency of availability. Though species such as Cassia spectabilis and Eucalyptus sp. could form good fuelwood and timber trees respectively they are not collected by people as they are planted by the Forest Department. Some of the other tree species with high IVI in this landscape are not preferred either as fuelwood species or as domestic timber requirements due to multiple reasons. For instance, Chloroxylon swietenia, Acacia chundra, and Strychnos potatorum are tree species with thick/rough bark and are uncomfortable to carry as headload. Similarly Diospyros montana is not harvested for fuelwood because of the belief that doing so could splinter the family by inciting fights between family members. Similarly, people believe that Terminalia bellerica is one of the sacred trees in the landscape and belongs to the god Shani Devaru, (a local deity regarded as an incarnation of Shiva). Hence, we have shortlisted Anogeissus latifolia as a dominant and firewood tree species, and Terminalia crenulata, Dalbergia lanceolaria and Albizia odoratissima as timber tree species for vegetation enrichment. Since Phyllanthus indofischeri and Terminalia bellerica are major NTFP species that serve as a source of livelihood for local people [41] and also form part of the dietary requirement for ungulates during the lean season, people generally do not cut these trees for fuelwood. So, we have shortlisted these two species also for vegetation enrichment. Since honey is a major NTFP in this landscape, people suggested the planting of one nectar yielding tree species for honey bees in the landscape such as Pterocarpus marsupium. In addition to these, Acacia chundra, Hardwickia binata and Bambusa arundinacea were identified and shortlisted as important plant sources of elephant’s food in the landscape [33, 34, 35].

Ten suitable native plant species were identified for vegetation enrichment based on their Important Value Index, ecological importance and recommendation by the community. Our research prioritized similar plant species for restoration such as Anogeissus latifolia (dominant tree and source of firewood), Terminalia crenulata, Dalbergia lanceolaria and Albizia odoratissima (timber trees), Phyllanthus indofischeri and Terminalia bellirica (NTFP trees), Pterocarpus marsupium (nectar source for honey bees), Acacia chundra, Hardwickia binata and Bambusa arundinacea (elephant food plants).

4.2. Species selected for clonal propagation

The plant species such as Bambusa aurindinacea, Tectona grandis, Gmelina arborea and Dalbergia sissoo in the corridor landscape may have the capability to propagate through clonal methods. Clonally propagated species (CPS) have the capacity to tolerate adverse conditions and give significantly better growth rates, and better disease resistance with most desirable timber traits [57]. In addition to this, clonal propagation trait not only could persist and maintain species richness but also retain genetic diversity of the species in the forests even after experiencing disturbance in the form of forest fire, grazing, and harvesting pressure from fuelwood collection [58, 59]. Since clonal propagation of dry tropical forest trees influence the tree species composition and demography, we suggested planting CPS, including bamboo along the forest boundary and teak in the farmland of the study area.

4.3. Nursing plants

Most of the forest landscapes in BRT have been subjected to different kinds of forest management practices such as shifting cultivation, logging, monoculture plantation, etc., both by the indigenous community and the State Forest Department in the past. This makes it more complex when it comes to understanding the structure, composition and successional status of native species [24, 25]. However, in eco-restoration, in order to improve the performance of target species, the “nursing” procedure seems to be promising, and shows enhanced plant survival and growth [18]. Therefore, in the same landscape, two native species, Pterolobium hexapetalum and Dodonaea viscosa were identified. These could play the role of nursing plants as they cover the native shrub and sapling communities extensively in more open forested areas. Being a prickly straggler, Pterolobium hexapetalum is not grazed by cattle and other ungulates. Likewise, Dodonaea viscosa, a bushy plant, is a pioneer species that is not eaten by cattle or other ungulates. Based on our field observations, we believe that these two native plants P. hexapetalum and D. viscosa could play the role of nursing by protecting seedlings from grazing and browsing, and influence the regeneration of tree seedlings and saplings.