Introduction

Tropical forests are the wealthiest biological communities on earth, and these forests have been recognized to harbour a significant proportion of global biodiversity (Naidu and Kumar 2016). Tropical forests occupy 7% of the earth’s land surface but constitute more than half of the world’s species. However, these forests are currently disappearing at an alarming rate of 0.8 and 2% per year (Naidu et al. 2018). Biodiversity maintenance is important to attain forest sustainability, and diversity of a community can be assessed using several nonparametric measures such as diversity indices and these measures have gradually gained credibility (Yam and Tripathi 2016). The study of forest community structure is essential to manage forest resources on a sustainable basis, which includes critical features such as structural type, size, shape, and both vertical and horizontal spatial distribution (Napit et al. 2015). The structure and function of the forest ecosystem are determined by the plant component more than any other living component of the system. The protected area network was initiated to extend legal protection to the forest in undisturbed conditions and facilitate human forest interaction under specific rules and regulations. Under the Wildlife (Protection) Act 1972, State governments are empowered to declare any areas as a Sanctuary to protect, develop wildlife therein or its environment (Soni and Modak 2014). But recent global studies have demonstrated that biodiversity and ecosystem services continue to be lost, despite constant growth in the number of protected areas (Negi et al. 2018). So, the diversity and community characteristics of the tree species in protected areas also should be monitored to know about the ecological health of that particular forest, and also to manage that forest better (Naidu et al. 2018). Tripura is a tiny (10,492 square km) hilly state in the North Eastern Region of India. The state is bordered by Bangladesh on the west, south, and some part of the north, by Assam on the extreme north and Mizoram on the east (Das and Datta 2018). Tripura is a landlocked Indian state located in the 9B-North-East Hills biogeographic zone (Champion and Seth 1968). It falls in the Indo-Burma hotspot region of the world (Debbarma et al. 2016). Tropic of Cancer crosses Tripura through South Tripura District, and hence we can find tropical forest in this state with considerable tree diversity (Lodh and Agarwal 2016). Tripura has four wildlife sanctuaries, namely Gomati, Trishna, Sepahijala and Rowa; among them, Rowa is the smallest. Diversity and community characteristics of tree species and their regeneration status of Rowa Wildlife Sanctuary are not adequately known till now. So, an attempt has been made with objectives of assessing the status of tree diversity and community characteristics, and also to get an idea about the regeneration status of those available tree species, to facilitate better management of forest in this Wildlife Sanctuary.

Materials and methods

Study Site Rowa WLS (area: 0.85 km2; latitude: N 24°17′, longitude: E 92°10′) is located in the Northern District of Tripura and is a part of the Western fringe of the Indo-Burma biodiversity hotspot (Fig. 1). The sanctuary is characterized by undulating land with narrow and wide ditches between rising hillocks (maximum height: 77 m above mean sea level), the presence of several perennial water bodies, and the occurrence of sparse clusters of human habitation on the surrounding areas dominated by indigenous natives dependent on natural resources. The approximate canopy heights of wooded regions range from 5 to 20 m, and leaf litter of the forest floor ranges from 3 to 5 cm in thickness. Forest of this area is classified as ‘Cachar tropical evergreen forest (I/I/IB/C3)’ (Bhomik and Chakraborty 2011). A botanical garden on a small patch of the sanctuary has been constructed for the conservation and exhibition of floral diversity for educational purposes and tourism.

Fig. 1
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Location Map of Rowa Wildlife Sanctuary

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Climate The province has a tropical climate, with four main seasons: winter (late November to February), summer (March to May), monsoon (June to September) and autumn (October to mid-November). The average annual temperature varies from a minimum of 10 °C in winter to a maximum of 35 °C in summer. During the monsoon season, the South-Western monsoon brings the majority of the precipitation (65%) to a province that receives 2000–2500 mm average annual rainfall and has an average humidity of 50–74% during the summer and above 85% in the monsoon season.

Sampling design and data collection

The present study was carried out from April 2019 to October 2019. A simple random sampling quadrate method was used to determine the regeneration status and community structure of the tree species of Rowa Wildlife Sanctuary. A total of 50 sample plots of 20 m × 20 m in size were taken from the whole area of RWS. The number of trees (> 30 cm gbh—girth at breast height) for each species in each quadrat and their girth was recorded for all the quadrates. The characteristics and quantity of all seedlings (˂ 40 cm height), saplings, and coppices (˃ 40 cm height and ˂30 cm gbh) of both indigenous and exotic species were recorded (Bharathi and Prasad 2015). Saplings and seedlings were analyzed by 5 m × 5 m and 1 m × 1 m sized quadrats, respectively, which were randomly laid out within each 20 m × 20 m sized quadrat on the survey site, to study the status of the regeneration. For analysis of saplings and seedlings, 50 and 100 numbers of quadrats were laid out, respectively. Circumference at breast height (1.37 m) was taken for the determination of tree basal area and calculated as πr2 (where r is the radius). Basal area (m2/ha) was used to determine the relative dominance of a tree species.

Data analysis

For many years, species richness, various diversity indices, species density, stem density, species importance value index are used to assess population dynamics and their diversity (Gimaret et al. 1998). The field data were compiled and analyzed for family relative density (FD%), family relative diversity (Fr%), family importance value (FIV), density, relative density (RD%), frequency, relative frequency (RF%), abundance, relative abundance (RA%), and importance value index (IVI). The Eqs. (1–10, Table 1) used for calculating phytosociological characteristics are listed in Table1. However, the Eqs. (11–17, Table 2) used for calculating floral diversity indices are listed in Table 2. Species diversity index (SDi) starts from 1 when there is only one individual of one species, and the value reaches to the maximum with the increase of species number (Odum 1971). Margalef’s index (R) is high in communities that include a greater number of species and in which the number of individuals of each species decreases relatively slowly on passing from more abundant to less abundant ones (Margalef 1958). Shannon–Wiener diversity index value is maximum when the number of individuals of all species is equal; value is zero if there are only one species (Michael 1990). With Simpson’s diversity index (D), 0 represents infinite diversity, and 1, no diversity. Simpson’s diversity index is neither intuitive nor logical, so to get over the problem, D is often subtracted from 1 to give Dominance of Simpson’s index (D′). The value of this index also ranges between 0 and almost 1, but now, the greater the value, the greater the sample diversity (Magurran 1988). Species Evenness index (E), also known as Shannon’s equitable index, assumes a value between 0 and 1 with 1 being complete evenness (Pielou 1966). Regeneration status of each tree species was evaluated considering the number of seedlings and saplings as ‘good regeneration’, if seedlings > saplings > adults; ‘fair regeneration’, if seedlings > or < saplings > or < adults; ‘poor regeneration’, if the species survives only at sapling stage or only seedlings stage (seedlings and saplings may be less or more or equal to adults); ‘no regeneration’, if a species is present only in adult form, and ‘new regeneration’ if the species has no adults but only seedlings or saplings (Bharathi and Prasad 2015).

Table 1 The list of equations used for calculating phytosociological characteristics of the vegetation
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Table 2 The list of equations used for calculating biodiversity indices of the vegetation
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Result and discussion

In this investigation, 44 tree species belonging to 27 families were recorded from the Wildlife Sanctuary. Fabaceae showed the highest family importance value (FIV: 24.49), followed by Moraceae (FIV: 22.38) and Combretaceae (FIV: 19.73). Maximum species were found to regenerate through seed, and only four species were found to form coppice, namely—Ficus hispida L.f., Cassia saimea Lam., Cassia fistula L., Microcos paniculata L. It was observed that 13.64%, 47.73%, 22.73% and 15.9% of the recorded species showed ‘good’, ‘fair’, ‘poor’ and ‘no’ regeneration status respectively (Fig. 2). But no species showed ‘new regeneration’. Microcos paniculata L. was the most frequent species followed by Cassia saimea Lam. and Artocarpus lakoocha Roxb. (Table 3). Tree density of Rowa Wildlife Sanctuary was recorded to be 599 trees/hectare, and the species Microcos paniculata L. showed the highest density (81 trees/hectare). Microcos paniculata L. was found to be the dominant species of the study site which showed the highest importance value index (IVI: 31.30), followed by Cassia saimea Lam. (IVI: 18.94) and Artocarpus lakoocha Roxb. (IVI: 16.28) (Table 3). The highest number of individuals (34.22%) was found in the girth class 90–120 cm. The lowest number of individuals (2.5%) was found in the girth class 180–210 cm (Fig. 3). Total tree basal area of the Wildlife Sanctuary was found to be 44.77 m2/hectare, and the species Microcos paniculata L. showed the largest basal area (5.45 m2/hectare). The species diversity index (SDI) was calculated to be 0.073. Shannon–Wiener’s diversity index (H) was calculated as 3.488. Shannon’s maximum diversity index (Hmax) was found to be 3.78. Margalef’s species richness index (R) was calculated to be 6.72. Simpson’s diversity index (D) was calculated as 0.04. The dominance of Simpson’s index (D′) was calculated to be 0.96. Species (Pielou’s) evenness index (E) was calculated to be 0.92.

Table 3 Florisctic composition, phytosociological attributes and regeneration status of tree species encountered in Rowa WLS
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Fig. 2
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Regeneration status of Tree species in Rowa WLS

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Fig. 3
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Abundance of trees in different girth classes

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The tree species diversity documented in the present study from the Rowa WLS is in agreement with the result in Sepahijala Wildlife Sanctuary of West Tripura, where 50 woody species belonging to 28 families were reported (Ali et al. 2015). According to Whitmore (1984), in tropical rain forests, the tree species ranges from 20 to a maximum of 223 ha−1, and our study result (44 tree species ha−1) for species richness lies in between this range. Tree density in tropical forests varies from 245 to 859 for trees of ≥ 30 cm gbh (Richards 1966; Ashton 1964; Campbell et al. 1992) and the recorded values of the present study lie within this range. Family Fabaceae had the highest number of species and had the highest FIV (24.49) followed by family Moraceae (Table 3). In a research work performed in the tropical forest of Central America, family Fabaceae was reported as dominant family (Gillespie et al. 2000). According to Carauta (1989) and Oliveira-Neto et al. (2017), fig-trees (Moraceae) are one of the key components of many tropical forests, as a wide variety of animals and birds feed on their fruits which are responsible for the dispersion of seeds and help in regeneration of the forest. The Shannon–Weiner (H') index (3.488) of the present study site was in between the range (0.67–4.86) reported for tropical forests of the Indian subcontinent (Kumar et al. 2010; Panda et al. 2013). Evenness index (0.92) was comparable with the tropical evergreen region of Mizoram (0.89) and Meghalaya (0.81) (Tynsong and Tiwari 2011), which lies in the similar geographic region of India (Noth East India). The Margalef richness index of the sampled forest is 6.72 and was within the range of 4.54—23.41, reported for other tropical forests (Kumar et al. 2010; Sathish et al. 2013). Simpson’s index values of different Indian tropical forests ranged from 0.03 to 0.92 (Bhuyan et al. 2003; Nath et al. 2005; Devi and Yadava 2006; Deb and Sundriyal 2011; Kushwaha and Nandy 2012), and the value of Simpson’s index for the present study is calculated to be 0.04 and hence lies between the range. Dominance of Simpson’s index value for the present study (0.96) is in accordance with the previous research work performed in (0.97) biodiversity conservation areas of Northeastern Bangladesh (Rahman et al. 2011). It was observed that nearby forest path and waterbody where there was abundant light availability, seedling abundance was significantly higher. So, low light availability caused by dense undergrowth (shrubs and saplings) can be one of the major reasons for poor or low or no regeneration of some tree species. No regeneration was observed for some important tree species like Aquilaria malaccensis Lam. Swietenia mahogoni (L.) Jacq. Neolamarckia cadamba (Roxb.) Bosser etc. and the probable cause of this is the unavailability of proper light due to dense undergrowth, clearing of ground in some places and grazing; however from the gbh observation, Aquilaria malaccensis Lam. was found immature and which can be the possible reason for the unavailability of its regeneration. To enhance the regeneration status of those economically important tree species, selection felling, crop tree release (CTR), controlled grazing, etc. can be adopted. From the biodiversity indices value, it is clear that the forest of Rowa Wildlife Sanctuary is highly diverse; a large number of birds of different species are observed during the survey; and this is a good sign towards the regeneration of tree species. Fruit trees for supporting animals and birds are observed. These include Artocarpus chaplasha Roxb., Ficus religiosa L., Ficus benghalensis L. Ficus hispida L.f, Artocarpus lakoocha Roxb., Terminalia chebula Retz., Terminalia bellirica(Gaertn.) Roxb., Syzygium cumini (L.) Skeels., Lannea coromandelica (Houtt.) Merr. Phyllanthus emblica L., Elaeocarpus floribundus, etc. which characterize the forest as a suitable habitat for wild animals, birds, and insects.

Conclusion

Understanding of tree diversity is essential for the conservation and utilization of tropical biodiversity. Quantitative assessment and analysis of tree diversity, community structure, and regeneration status of Rowa WLSmay provide baseline information for formulating management and conservation strategies of the study site. This investigation has come across 44 species of trees from the smallest Wildlife Sanctuary of Tripura; this species richness alone with the higher values of other quantitative indices reflects that this tropical semi-evergreen forest patch harbours a rich diversity of tree species. This sanctuary possesses many valuable plant species like Artocarpus chaplasha Roxb., Terminalia chebula Retz., Albizia lebbeck (L.) Benth., Syzygium cumini (L.) Skeels., Elaeocarpus floribundus Bl., Phyllanthus emblica L., Aquilaria malaccensis Lam. which are commercially and medicinally important and viable. Presence of endangered species Swietenia mahogoni (L.) Jacq. and critically endangered species—Aquilaria malaccensis Lam., adds to the floristic significance of the sanctuary. Higher species diversity signifies a complex and healthier community, as greater variety of species facilitates more species interactions, leading to greater stability of the system and ensures good environmental conditions. The overall regeneration status of the study site is fair. Discontinuous regeneration of Swietenia mahogoni (L.) Jacq. and Aquilaria malaccensis Lam., listed as ‘endangered’ and ‘critically endangered’ species respectively in IUCN red list, indicates their vulnerable status and may lead to local extinction of the species in the near future. Dominance of Microcos peniculata L. indicates the sensitivity of the species on the site. The tree species growth and reproductive potential of the sanctuary may be at risk if the present anthropogenic activity is continued. The study finally concludes that Rowa WLS is a small but ecologically rich site, and effective protection from human interferences along with a scientific management of natural regeneration of the study area will ensure a sustained biodiversity richness of this protected site.