Diversity of woody vegetation in the buffer zone of silent valley national park, Kerala

Introduction

Silent Valley National Park represents one of the best-preserved Tropical Evergreen forest ecosystems in the Western Ghats and forms an ecologically important part of the Nilgiri Biosphere Reserve. The region is well known for its rich biodiversity, high level of endemism, and significant ecological value (Pascal, 1988;Nair, 1991) [45, 41] . The establishment of buffer zones around protected areas is considered an important conservation approach for reducing habitat degradation, regulating human activities, and maintaining ecological connectivity (Nair, 1991) [41] . The buffer zone surrounding Silent Valley National Park acts as a transitional landscape that safeguards the core rainforest ecosystem from anthropogenic disturbances while supporting long-term ecological stability and sustainable resource management. Woody species diversity is considered a key ecological attribute because it reflects vegetation composition, community structure, regeneration status, and overall ecosystem functioning (Odum, 1971) [42] . Therefore, phytosociological investigations are essential for understanding forest dynamics and for developing effective biodiversity conservation and forest management strategies. Quantitative ecological methods such as quadrat sampling are widely used to analyze vegetation composition, species distribution, and stand structure in forest ecosystems (Misra, 1968) [39] . Diversity indices, including the Shannon-Wiener and Simpson indices, are commonly applied to evaluate species richness, dominance, and community diversity. Earlier studies conducted in the Western Ghats have reported considerable variation in woody species diversity among different forest types. Studies from the Nilgiri Biosphere Reserve and Periyar Tiger Reserve revealed higher species diversity in evergreen and semi-evergreen buffer zone of Silent Valley National Park. A total of 84 quadrats were laid across different forest types within the buffer zone. Among these, 20 quadrats were established in West Coast Tropical Evergreen Forest, 16 quadrats in West Coast Semi-evergreen Forest, 24 quadrats in South Indian Moist Deciduous Forest, 16 quadrats in South Indian Dry Deciduous Forest, and 8 quadrats in Southern Montane Wet Temperate Forest. The quadrats were distributed throughout the study area to assess woody species diversity and vegetation structure across the different forest types (Table 6). Data from each quadrat were collected and recorded individually, and all plant specimens within the sampling plots were documented. Trees and lianas with a girth at breast height (GBH) greater than 10 cm were identified, and girth measurements were taken at 1.37 m above ground level (Parthasarathy and Karthikeyan, 1997) [44] . Species identification was carried out using standard floristic literature and regional taxonomic manuals, including Flora of the Presidency of Madras (Gamble andFischer, 1915-1936) [25] , A Field Key to the Trees and Lianas of the Evergreen Forests of the Western Ghats (Pascal and Ramesh, 1987) [46] , Flora of Silent Valley (Manilal, 1988) [38] , Flora of Palghat District (Vajravelu, 1990) [54] , and Flowering Plants of Kerala (Sasidharan, 2013) [51] . The nomenclature and taxonomic status of the recorded plant species were verified using Plants of the World Online (POWO) and International Plant Names Index (IPNI) to confirm accepted scientific names, author citations, and synonymy (POWO, 2024; IPNI, 2024) [47, 31] . The Red-listed species recorded according to IUCN reports(2024) [35] . Phytosociological analysis of woody species was carried out following standard ecological methods described by Curtis and McIntosh (1950) [15] , Curtis and Cottam (1956) [14] , and Misra (1968) [39] . Quantitative ecological parameters such as density, frequency, abundance, abundance to frequency ratio (A/F), relative density (RD), relative frequency (RF), relative dominance (RDO), Importance Value Index (IVI), and total basal area were calculated to evaluate vegetation structure and species dominance in different forest types of the buffer zone of Silent Valley National Park. Density was calculated as the total number of individuals of a species divided by the total number of quadrats studied. Frequency was determined based on the percentage occurrence of species in the sampling quadrats, while abundance was estimated from the number of individuals present in quadrats where the species occurred. The abundance to frequency ratio (A/F) was used to determine the distribution pattern of species. Basal area was calculated from girth at breast height (GBH) measurements, and total basal area was obtained by summing the basal area of all individuals of a species. The 15 major species based on Density (D) values were selected for regeneration and population structure analysis. Population structure and regeneration status were assessed based on the distribution of individuals among different girth, seedling, and sapling classes to evaluate population stability and regeneration potential of the species. Relative density, relative frequency, and relative dominance were computed to determine the ecological significance of species, and the Importance Value Index (IVI) was derived by summing RD, RF, and RDO values. Family Importance Value Index (FIVI) was calculated as the sum of Relative Density, Relative Diversity, and Relative Dominance of each family (Mori et al., 1983) [40] . The diversity indices, including the Shannon-Wiener index and Simpson index, were calculated using PAST version 5.3 statistical software (Hammer et al., 2001) [29] .

Result and Discussion

Five major forest types were selected for the present study. Each forest type exhibited distinct ecological characteristics, species composition, and vegetation growth patterns. The details of these forest types are described below.

West Coast Tropical Evergreen Forest (1A/C4) A total of 141 woody species were recorded from the study area, comprising 116 trees and 25 lianas. These species belonged to 115 genera distributed among 56 families. The Family Importance Value Index (FIVI) revealed that Phyllanthaceae was the most ecologically dominant family with the highest FIVI value of 19.71, followed by Putranjivaceae (17.46) and Myristicaceae (15.52). In contrast, families such as Olacaceae, Combretaceae, Staphyleaceae, and Erythroxylaceae exhibited the lowest FIVI values (0.31), indicating comparatively lower ecological representation and dominance within the forest. The total stand density was 1035 stems ha⁻¹. Among the woody species, Cullenia exarillata (Figure 1A) exhibited the highest density (61.6 stems ha⁻¹), followed by Garcinia cambogioides var. cambogioides (52.0 stems ha⁻¹), Palaquium ellipticum (48.0 stems ha⁻¹) (Figure 1E), Drypetes oblongifolia (45.6 stems ha⁻¹), and Dimocarpus longan (44.0 stems ha⁻¹)(Figure 1B). Other dominant species included Reinwardtiodendron anamalaiense (41.6 stems ha⁻¹), Drypetes venusta (37.6 stems ha⁻¹), Myristica beddomei (36.0 stems ha⁻¹), Actinodaphne bourdillonii (35.2 stems ha⁻¹), and Mesua ferrea (32.8 stems ha⁻¹). Several species showed very low densities of 0.8 stems ha⁻¹, including Diospyros sylvatica, Holigarna nigra, and Monoon coffeoides indicating poor representation in the forest stand. The abundance-to-frequency (A/F) ratio revealed that the majority of species exhibited a contiguous (clumped) distribution pattern in the study area. Out of the 140 recorded species, 129 species (92.14%) showed contiguous distribution with A/F values greater than 0.05, while 9 species (6.43%) exhibited random distribution with A/F values ranging between 0.025 and 0.05. Only 2 species (1.43%) showed a regular distribution pattern with A/F values less than 0.025, indicating a highly aggregated distribution of species within the forest stand. In this forest type, 2 species were categorized as Data Deficient (DD), 3 species as Endangered (EN), 7 species as Near Threatened (NT), and 10 species as Vulnerable (VU) according to the International Union for Conservation of Nature (IUCN) Red List criteria (Table 7).

The analysis revealed active natural regeneration within the forest, as most species were predominantly represented by seedlings, saplings, and lower girth classes. Among the upper canopy species, Knema attenuata (97.66%), Palaquium ellipticum (95.94%) (Figure 1E), Canarium strictum (95.64%), and Dimocarpus longan (95.32%)(Figure 1B) exhibited high seedling representation, indicating successful regeneration status. In the lower canopy, Gomphandra tetrandra recorded the highest seedling percentage (98.48%), followed by Garcinia cambogioides var. cambogioides (83.85%). Species such as Agrostistachys borneensis and Paracroton pendulus subsp. zeylanicus were mainly concentrated in lower girth classes, reflecting a developing population structure with comparatively fewer mature individuals. The dominance of species within the stand was reflected by their total basal area contribution. Among the recorded species, Cullenia exarillata (Figure 1A) exhibited the highest basal area (101,025 cm² ha⁻¹), contributing approximately 19.37% of the total stand basal area. This was followed by Bischofia javanica (13.79%), Palaquium ellipticum (8.65%) (Figure 1E), Lagerstroemia microcarpa (8.28%), and Homalium ceylanicum (6.84%). Collectively, these five species accounted for about 56.93% of the total stand basal area, indicating their strong ecological dominance in the evergreen forest stand. In contrast, the remaining species together contributed 43.07% of the total basal area, reflecting a comparatively lower individual contribution but substantial cumulative representation of species diversity within the community. The ecological dominance of species within the forest was assessed using the Importance Value Index (IVI). Among the recorded species, Cullenia exarillata (Figure 1A) exhibited the highest IVI value (27.63), followed by Palaquium ellipticum (15.37), Dimocarpus longan (11.63) (Figure 1B), Drypetes venusta (10.40), and Mesua ferrea (8.54). Other ecologically important species included Drypetes oblongifolia (7.73), Canarium strictum (7.31), Myristica beddomei (6.72), Antidesma montanum var. montanum (6.49), and Reinwardtiodendron anamalaiense (6.37). The higher IVI values of these species indicate their greater ecological significance and dominance within the forest stand. In contrast, several species exhibited very low IVI values (<0.50), reflecting their poor representation and lower ecological influence within the community structure. Overall, the IVI distribution suggests that a few dominant species contribute substantially to stand structure, while a large number of species occur with relatively low ecological importance, highlighting the diverse nature of the tropical evergreen forest community. 1D) exhibited the highest density (52 stems ha⁻¹), followed by Xanthophyllum flavescens (50 stems ha⁻¹), Antidesma montanum var. montanum (30 stems ha⁻¹), Dimocarpus longan (Figure 1B) and Syzygium laetum (25 stems ha⁻¹ each). Moderate density values were recorded for Drypetes oblongifolia and Atalantia monophylla (21 stems ha⁻¹ each), Drypetes venusta (20 stems ha⁻¹), Mesua ferrea (18 stems ha⁻¹), Kunstleria keralensis and Paramignya monophylla (17 stems ha⁻¹ each). Several species, such as Aglaia lawii, Albizia chinensis, Artocarpus lacucha, Cassia fistula, and Hydnocarpus alpinus, showed very low density values with only 1 stem ha⁻¹, indicating poor representation within the forest stand. Analysis of the abundance to frequency (A/F) ratio showed that the majority of species exhibited contiguous (clumped) distribution patterns in this forest. Out of the total recorded species, 77 species (95.06%) showed contiguous distribution with A/F values greater than 0.05, while 4 species (4.94%) exhibited random distribution with A/F values between 0.025 and 0.05. No species showed regular distribution (<0.025). The predominance of contiguous distribution indicates heterogeneous habitat conditions and patchy regeneration patterns within the forest ecosystem. This forest type contained 1 Data Deficient (DD) species, 2 Endangered (EN) species, 4 Near Threatened (NT) species, and 6 Vulnerable (VU) species, indicating the conservation significance of the forest and the presence of several threatened woody taxa (Table 7). The regeneration and population structure analysis indicated active natural regeneration, as most species were dominated by seedlings, saplings, and lower girth classes. Among the upper canopy species, Mesua ferrea (98.58%), Dimocarpus longan (97.00%) (Figure 1B), and Drypetes oblongifolia (96.75%) showed high seedling representation, while Myristica malabarica (91.44%) and Drypetes venusta (77.64%) were mainly represented by saplings. In the lower canopy, Xanthophyllum flavescens recorded the highest seedling percentage (87.89%), followed by Syzygium laetum (79.23%) and Garcinia gummi-gutta (63.44%). Species such as Celtis timorensis, Heynea trijuga, and Prunus ceylanica were mainly confined to intermediate girth classes, indicating fewer regenerating individuals. The dominance of species was reflected by their basal area contribution. Syzygium gardneri showed the highest basal area contribution (8.56%), followed by Lagerstroemia microcarpa (7.65%), Antidesma montanum var. montanum (7.62%), Mesua ferrea (5.18%), and Myristica beddomei (5.07%) (Figure 1D). Moderate basal area contribution was recorded for Drypetes oblongifolia (4.08%), Machilus macranthus (4.23%), Heynea trijuga (4.03%), and Dimocarpus longan (3.08%)(Figure 1B). In contrast, several species exhibited very low basal area contribution, indicating poor representation within the stand structure. The Importance Value Index (IVI) analysis revealed that Antidesma montanum var. montanum exhibited the highest IVI value (16.13), followed by Myristica beddomei (15.88)(figure 1D), Xanthophyllum flavescens (12.03), Syzygium gardneri (11.96), Dimocarpus longan (10.88) (Figure 1B), and Drypetes oblongifolia (10.04). Moderate IVI values were recorded for Mesua ferrea (9.86), Lagerstroemia microcarpa (9.77), and Syzygium laetum (9.56), indicating their ecological importance within the forest community. In contrast, several species such as Aglaia lawii (0.57), Paracroton pendulus subsp. zeylanicus (0.57), and Tetrastigma leucostaphylum (0.57) showed very low IVI values, reflecting poor ecological representation in the forest stand (Table 2). The Shannon-Wiener diversity index (2.496) and Simpson diversity index (0.895) indicated high woody species diversity and ecological stability within the semi-evergreen forest. Analysis of the abundance to frequency (A/F) ratio revealed that the majority of species exhibited contiguous (clumped) distribution patterns. Out of the total recorded species, 64 species (94.12%) showed contiguous distribution with A/F values greater than 0.05, while 4 species (5.88%) exhibited random distribution with A/F values between 0.025 and 0.05. No species showed regular distribution (<0.025). According to the IUCN Red List categories, this forest type contained 3 Data Deficient (DD) species, 4 Near Threatened (NT) species, and 1 Vulnerable (VU) species, indicating the presence of several conservation-priority woody taxa within the forest ecosystem (Table 7). The regeneration and population structure analysis indicated active regeneration in most species. Among the upper canopy species, Schleichera oleosa (93.95%), Drypetes venusta (93.70%), Albizia odoratissima (90.82%), and Mallotus philippensis (85.36%) showed high seedling representation, while Grewia tiliifolia (95.42%) and Litsea ghatica (80.33%) were mainly represented by saplings. Species such as Terminalia paniculata, Dalbergia lanceolaria, and Dimocarpus longan (Figure 1B) were mainly confined to intermediate girth classes. In the lower canopy, Holarrhena pubescens (94.55%) showed the highest sapling representation, whereas Chionanthus ramiflorus (88.32%) and Tetrapilus dioicus (73.24%) exhibited high seedling percentages, indicating continuous recruitment and stable regeneration dynamics. The Xylia xylocarpa showed the highest basal area contribution (19.36%), followed by Terminalia paniculata (12.83%), Terminalia bellirica (10.43%), Dalbergia lanceolaria (8.16%), and Grewia tiliifolia (7.66%). Moderate basal area contribution was recorded for Mallotus philippensis (3.26%), Pterocarpus marsupium (3.20%), and Schleichera oleosa (3.41%). Together, the major dominant species contributed a substantial proportion of the total stand basal area, indicating their ecological importance within the forest structure. In contrast, several species exhibited very low basal area contribution, reflecting poor representation in the forest stand. Among the recorded species, Xylia xylocarpa exhibited the highest IVI value (41.20), indicating its strong dominance within the forest stand. This was followed by Terminalia paniculata (22.52), Terminalia bellirica (20.09), Grewia tiliifolia (17.97), Dalbergia lanceolaria (16.11), and Wrightia tinctoria (15.03). Moderate IVI values were recorded for Mallotus philippensis (10.07), Terminalia anogeissiana (5.82), Holarrhena pubescens (5.91), and Albizia odoratissima (5.67). In contrast, several species such as Pterygota alata (0.55), Ziziphus oenopolia (0.55), and Connarus wightii (0.56) showed very low IVI values, reflecting comparatively poor ecological representation and lower dominance within the forest (Table 3). The Shannon-Wiener diversity index (2.253) and Simpson diversity index (0.877) indicated moderate to high woody species diversity and ecological stability within the South Indian Moist Deciduous Forest.

Southern Dry Mixed Deciduous Forest (5A/C3)

A total of 52 woody species, comprising 48 trees and 4 lianas, belonging to 45 genera and 21 families, were recorded. The Family Importance Value Index (FIVI) revealed that Fabaceae was the most ecologically dominant family with the highest FIVI value (105.84), followed by Combretaceae (60.17) and Apocynaceae (35.12). Moderate FIVI values were observed for Malvaceae (16.72), Sapindaceae (15.12), Phyllanthaceae (11.41), Meliaceae (11.40), and Rubiaceae (11.37), indicating their substantial ecological representation within the forest community. In contrast, families such as Salicaceae (0.80), Ulmaceae (0.78), and Connaraceae (0.77) exhibited very low FIVI values, reflecting comparatively lower ecological dominance and poor representation in the forest stand. The total stand density was 488 stems ha⁻¹. Species density values ranged from 1 to 101 stems ha⁻¹. Among the recorded species, Xylia xylocarpa exhibited the highest density (101 stems ha⁻¹), followed by Wrightia tinctoria (52 stems ha⁻¹), Terminalia paniculata (45 stems ha⁻¹), Terminalia anogeissiana (44 stems ha⁻¹), and Dalbergia lanceolaria (35 stems ha⁻¹), indicating their ecological dominance within the forest stand. In contrast, several species such as Combretum latifolium, Vepris bilocularis, Antidesma acidum, Connarus wightii, Holoptelea integrifolia and Psydrax dicoccos showed very low density values (1 stem ha⁻¹ each), reflecting poor representation and lower ecological dominance in the forest. Analysis of the abundance to frequency (A/F) ratio showed that most species exhibited contiguous (clumped) distribution patterns. Species such as Ochlandra scriptoria (0.64), Sterculia guttata (0.48) (Figure 1F), Dalbergia horrida (0.48), Atalantia racemosa (0.32), Falconeria insignis (0.32), and Flueggea virosa (0.32) showed high A/F values, indicating strongly contiguous distribution. Several species including Albizia odoratissima (0.06), Bauhinia racemosa (0.06), Grewia tilifolia (0.058), Terminalia anogeissiana (0.058), Terminalia paniculata (0.060), and Wrightia tinctoria (0.058) also exhibited clumped distribution patterns. Random distribution was observed in a few species such as Dalbergia lanceolaria (0.039), Dalbergia sissoides (0.042), Bombax ceiba (0.040), Pterocarpus marsupium (0.040), Sapindus trifoliatus (0.040), and Phyllanthus emblica (0.027). No species showed regular distribution (<0.025). According to IUCN status, 3 Data Deficient (DD) species, 1 Endangered (EN) species, 4 Near Threatened (NT) species, and 1 Vulnerable (VU) species recorded in this forest (Table 7). The regeneration and population structure analysis indicated active regeneration in several dominant species. Albizia odoratissima (85.04%), Terminalia anogeissiana (77.36%), and Wrightia tinctoria (72.87%) showed high seedling representation, while Holarrhena pubescens recorded both high seedlings (51.76%) and saplings (46.01%), indicating successful regeneration. Species such as Xylia xylocarpa (95.22%) and Dalbergia lanceolaria (79.87%) were mainly represented by saplings, reflecting developing population structures. Several species including Bombax ceiba, Terminalia bellirica, and Toona ciliata were concentrated in middle girth classes, indicating comparatively fewer regenerating individuals. In the lower canopy, Vitex altissima was entirely confined to the 10-30 cm girth class (100%), representing an early population stage. Among the recorded species, Xylia xylocarpa contributed the highest share of the total basal area (14.77%), followed by Terminalia paniculata (14.28%), Dalbergia lanceolaria (14.20%), Terminalia bellirica (5.25%), and Wrightia tinctoria (5.23%). Together, these dominant species accounted for a major proportion of the total stand basal area, indicating their strong ecological dominance within the forest community. In contrast, species such as Combretum latifolium (0.004%), Antidesma acidum (0.004%), Connarus wightii (0.004%), and Vepris bilocularis (0.007%) contributed very low basal area percentages, reflecting poor representation in the stand structure. The ecological dominance of woody species was assessed using the Importance Value Index (IVI). Xylia xylocarpa recorded the highest IVI value (43.94), followed by Terminalia paniculata (29.72), Dalbergia lanceolaria (28.15), Wrightia tinctoria (22.66), and Terminalia anogeissiana (18.18), indicating their strong ecological dominance and greater influence on the forest structure. Other species with comparatively high IVI values included Holarrhena pubescens (12.46), Terminalia bellirica (11.50), Dalbergia sissoides (11.00), and Toona ciliata (10.07). In contrast, species such as Combretum latifolium (0.77), Antidesma acidum (0.77), Connarus wightii (0.77), and Vepris bilocularis (0.78) showed very low IVI values, reflecting poor ecological representation within the forest community (Table 4). The Shannon-Wiener diversity index (2.318) and Simpson diversity index (0.888) indicated moderately high woody species diversity and ecological stability within the Tropical Dry Deciduous Forest. The phytosociological analysis of the five major forest types in the buffer zone of Silent Valley National Park showed considerable variation in species diversity and stand structure. The West Coast Tropical Evergreen Forest (1A/C4) recorded the highest species richness (142 taxa), stand density (1035.0 stems ha⁻¹), basal area (584175.6), and Shannon diversity index (2.767), indicating high ecological stability and structural complexity. The Southern Montane Wet Temperate Forest(11A/C1) also exhibited high stand density (1032.0 stems ha⁻¹) and diversity values, reflecting its unique montane vegetation characteristics. In contrast, Moist Deciduous (3B/C2) and Dry Deciduous Forests (5A/C3) showed comparatively lower diversity and stand density, indicating lower floristic heterogeneity and dominance of fewer species. Overall, evergreen and shola forests exhibited greater ecological stability and woody species diversity than the deciduous forest types (Table 6).

Acknowledgement

AcknowledgementsThe authors sincerely thank the Director, Group Coordinator Research (GCR), Institute of Forest Genetics and Tree Breeding (IFGTB), Coimbatore, and the Head of the Forest Ecology and Climate Change Division (FECC), for their support and guidance throughout the study. The authors also acknowledge the Librarian of IFGTB for providing access to International Journal of Biology Sciences https://www.biologyjournal.net relevant literature and reference materials. Gratitude is extended to the Kerala Forest Department, forest officials, and watchers of Silent Valley National Park for funding the project, permission to work in the forest and all the help and support during the field survey and data collection.

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