A new population record of the Critically Endangered Dipterocarpus bourdillonii Brandis from the Anamalai Tiger Reserve, India

: Dipterocarpus bourdillonii , a Critically Endangered tree species endemic to the Western Ghats, India, has hitherto been reported mainly from the states of Kerala and Karnataka on the western slopes of the mountain range. In Tamil Nadu, this species has been reported to occur in two locations, but no population details have been documented and the species has neither been listed in state floras nor in a recent compendium of plant species. The present study documents the occurrence of a population of the species, with at least 40 individuals, in the Anamalai Tiger Reserve, Tamil Nadu, extends the known upper limit of its altitudinal range to 733 m, and suggests further surveys and in situ conservation efforts.


INTRODUCTION
The family Dipterocarpaceae includes a diverse group of tropical trees that form dominant stands with some of the tallest standing tree species in southern and southeastern Asian lowland tropical forests (Appanah & Turnbull 1998;Ashton 2014).About 500 species in 17 genera of Dipterocarpaceae are known around the world (Ashton 2003), of which five genera and 34 species, including 10 species in the type genus Dipterocarpus, occur in India (Kundu 2008).Within India, Dipterocarpus is distributed largely in lowland tropical forests of the north-east, the Andaman & Nicobar Islands, and the south-west in the Western Ghats (Brandis 1906).The two species endemic to India, Dipterocarpus indicus and D. bourdillonii, are both restricted to the Western Ghats in southwestern India (Ramesh & Pascal 1997;Sreekumar et al. 2021).
D. bourdillonii has been assessed as a Critically Endangered species by the IUCN Red List, with the global population currently estimated at under 250 mature individuals and the largest known subpopulation having less than 50 mature individuals (Deepu et al. 2021).The species is considered rare and has so far been recorded only in scattered locations in the states of Kerala, Karnataka, and Tamil Nadu, mainly on the western aspect of the Western Ghats mountain range of India (Ramesh & Pascal 1997;Swarupanandan et al. 2013;Sreekumar et al. 2021).It is reported to occur between 175 m and 600 m elevation in valleys along river courses (Jose et al. 2010;Puttaswamy et al. 2010).Within Tamil Nadu, D. bourdillonii occurrence has been reported from the Nilgiris and Megamalai hill ranges (Ramesh & Pascal 1997) but nothing is known of its population and associated species in the state.The present paper describes the occurrence of a population of D. bourdillonii in the Anamalai Tiger Reserve, in the Tamil Nadu Western Ghats.

METHODS
The field survey was carried out in the Anamalai Tiger Reserve (ATR), Tamil Nadu, India (core zone: 958 km², 10.216 0 N, 76.816 0 E -10.566 0 N, 77.416 0 E) and the adjoining Valparai Plateau (220 km², 10.25 0 N, 76.866 0 E -10.366 0 N, 76.983 0 E) in the Anamalai Hills.The Valparai Plateau is a landscape dominated by tea and coffee plantations with about 45 embedded rainforest fragments ranging in area from 1 ha to over 300 ha (Muthuramkumar et al. 2006;Mudappa & Raman 2007).As the focus of this study was on threatened and endangered tree species found in the mid-elevation tropical wet evergreen forest (tropical rainforest), the fieldwork was confined to the western parts of the Reserve in Valparai, Manamboli, and Ulandy Ranges that contain most of the remaining rainforests.The natural vegetation type falls mainly within the mid-elevation (700-1,400 m) tropical wet evergreen forest of the Cullenia exarillata -Mesua ferrea -Palaquium ellipticum type (Pascal 1988).
Between October 2020 and March 2022, 64 routes (29 sites) of 119.2 km total length were surveyed on foot, spanning an elevation range of 580 m to 2,000 m in the rainforests of the Anamalai Tiger Reserve and rainforest fragments in the Valparai Plateau.After two D. bourdillonii trees were first observed along one of the survey routes (11 km, walked on 30 January 2021) passing through the Ayyankulam area (Figure 1), the same area was subsequently explored covering 0.81 km and 2.63 km (in March-April 2021) and in four trails covering 3.31 km, 3.89 km, 3.0 km, and 4.1 km (in March 2022) recording additional individuals.The total length of 28.74 km of trails were tracked using a hand-held GPS (Garmin GPSMAP 64sc) and a checklist of all tree species encountered along the trail (10 m on either side) was recorded.Plant species were identified using available floras and field guides (Gamble & Fischer 1935;Pascal & Ramesh 1997;Page 2017) and based on the prior experience of the authors with floristic and ecological research in the region (Muthuramkumar et al. 2006;Page et al. 2010;Osuri et al. 2017Osuri et al. , 2019;;Page & Shanker 2018, 2020).Species names were updated with reference to Plants of the World Online, http://www.plantsoftheworldonline.org/ (POWO 2022).
At each of the 40 D. bourdillonii trees found during the survey, the following data centred on the tree were recorded: GPS coordinates and elevation (using GPS unit), girth at breast height (GBH, at 1.3 m, or higher in case of presence of buttresses), and tree height in metres measured with a rangefinder.For measurement of additional variables, a subset of 23 D. bourdillonii trees was chosen after excluding individuals that were less than about 30 m from previously-measured conspecifics (to ensure independence of samples).For these 23 trees, the following additional variables were measured keeping the focal tree as the centre: slope (flat, gentle, moderate, steep), canopy height (average height of trees in the immediate vicinity of focal tree measured with a rangefinder in metres), and canopy cover (0%, 1-25%, 26-50%, 51-75%, 76-100%).The number of D. bourdillonii seedlings (GBH <10 cm) and J TT saplings (GBH 10-30 cm) in a 5 m radius around each focal tree were also recorded.Observations on canopy shape, buttresses, and phenophase (leafing, flowering, fruiting) were noted.To record nearby tree species, point-centred quarter (PCQ) plots were placed keeping focal trees at the centre and the nearest individual tree >30 cm GBH in each of the four quarters was recorded, noting the species and GBH.As these were trees in the immediate vicinity of the focal trees, the frequency of different species was considered as indicative of species association with D. bourdillonii.Together, the survey trails enabled rapid coverage across wider areas to document species occurrence, while the PCQs around focal trees helped document tree species associated with D. bourdillonii.
Herbarium specimens were examined at the Madras Herbarium (MH), Botanical Survey of India, at Coimbatore, the Herbarium at the French Institute in Pondicherry (HIFP), the Herbarium of the Kerala Forest Research Institute (Sreekumar et al. 2021), Peechi (KFRI), and at the Forest Research Institute, Dehradun (DD).No specimens were available at the herbaria of the Botanical Survey of India in Pune (BSI).From select trees observed during this study, leaves, flowers, and maturing fruit were collected and photographed.These were subsequently used for preparing herbarium specimens and deposited into Herbarium JCB (Accession No.: JCB-1337) at the Centre for Ecological Sciences, Indian Institute of Science, Bengaluru.A sample of 11 maturing fruits, fallen on the ground in the vicinity of the trees, were individually measured for weight using a digital Ohaus scale, and nut length (along main longitudinal axis) and width (along two axes perpendicular to the longitudinal and to each other) were measured using Vernier calipers.The length and width of the two wings (enlarged sepals) of each of the fruits were also measured using Vernier calipers (except in the case of 1 fruit where 1 wing was broken, for which only width was measured and not the length).Data from the study are available on Zenodo (Page et al. 2022).

RESULTS
Tropical wet evergreen forest areas between 580 m and 700 m elevation were present only along six survey routes within the Manamboli Range in ATR.These routes were in the Ayyankulam-Manamboli area located along the Parayankadavu Ār (Ār = river) that flows into the Parambikulam Reservoir in neighbouring Kerala State (Figure 1).In 2021, 20 Dipterocarpus bourdillonii trees were recorded along three trails passing through two main locations along the Parayankadavu Ār within ATR: 13 trees at Ayyankulam (10.381 0 N, 76.910 0 E) and 7 trees at Ayyankulam Parai (10.386 0 N, 76.926 0 E), the latter about 2.4 km (1.8 km straight line distance) upstream from the former location.In March 2022, another 20 trees were recorded, including two trees at Ayyankulam The 40 D. bourdillonii trees were located at elevations between 627 m and 733 m and from the edge of the river to less than 100 m away from the river banks.Two tall trees of the species, when first noted on 30 January 2021 along the river banks at Ayyankulam, were flowering (Image 1).On two subsequent visits to the area, on 26 March 2021 (Ayyankulam) and 10 April 2021 (Ayyankulam Parai), fruiting trees were observed with different stages of fruit developments, a sample of which were measured (Table 1) and photographed (Image 2).In March 2022, subsequently, flowering trees and trees with immature fruits were also observed.
The 40 D. bourdillonii trees recorded averaged 375.4 cm in girth at breast height (range 90-622 cm) and 40.0 m in height (range 12-51.3m, Table 1).In the PCQ plots centred on 23 individual D. bourdillonii trees, a total of 37 tree species (92 individual trees >30 cm GBH) were recorded, with the most frequently associated species being Paracroton pendulus (13 individuals), Monoon fragrans (8), Cullenia exarillata (8), and Reinwardtiodendron anamalaiense (5).In the vicinity of these D. bourdillonii trees, the average density of conspecific seedlings was higher than that of saplings, which were in turn higher than the density of D. bourdillonii trees (Table 1).Twelve (52%) of 23 trees were noted to be emergent and the remainder were canopy trees.Canopy shape was oval in 19/23 trees (remainder had spreading canopies) and most (20/23) were located at spots with 75-100% canopy cover (2 trees in spots with 51-75% canopy cover, 1 at <25% cover).While four trees were on flat terrain, the remainder were on gentle (7), moderate (6), or steep (6) slopes.Nine trees had buttresses.

DISCUSSION
The present report is also significant as it confirms the presence of a significant population (at least 40 mature trees) of D. bourdillonii in the Anamalai Hills.As in earlier studies, D. bourdillonii trees were confined to areas close to rivers on relatively flat to moderate slope.The trees were located mainly along the river between Ayyankulam and Ayyankulam Parai, both within the core area of the Anamalai Tiger Reserve, but the occurrence of one individual further downstream along the Parayankadavu Ār indicates there may be more individuals in the intervening area.Given that the species has an estimated global population of under 250 mature individuals, with less than 50 mature individuals in the largest known sub-population (Deepu et al. 2021), the Anamalai Hills population of least 40 mature individuals gains significance as an important site for in situ conservation of this Critically Endangered species.
Most (37) of the 40 trees observed were of large girth (>200 cm) and only few seedlings and saplings were recorded in their vicinity.The species has been reported to have intrinsically poor reproduction besides probably being affected by past selective logging (Swarupanandan et al. 2013).Future studies on population structure and regeneration of D. bourdillonii are required to assess the regeneration status in the study area.
In other parts of its distributional range, D. bourdillonii is reported to occur with species such as Vateria indica, Turpinia malabarica, Dipterocarpus indicus, Humboldtia brunonis, and Nothopegia beddomei (Pascal 1988;Puttaswamy et al. 2010;Swarupanandan et al. 2013).In The newly-discovered Anamalai population also showed some morphological peculiarities.The shape of the leaf apex of D. bourdillonii is described in the literature as shortly acuminate (Brandis 1906).On mature individuals in the Anamalai Hills, the leaves, particularly those at the top of the canopy exhibited an abruptly acuminate leaf apex, which may represent minor intra-specific variation.
Future surveys for D. bourdillonii should cover a wider altitudinal range (0-800 m) in evergreen forest areas along rivers.As the species can be clearly identified from flowers and fruits, carrying out surveys between January and April during the flowering and fruiting season is suggested.The existing population in the Ayyankulam area should continue to be protected, and in situ conservation efforts should focus on areas within the known ranges of this Critically Endangered species.

Figure 1 .
Figure 1.Map of Anamalai Tiger Reserve showing the main locations along the Parayankadavu River where Dipterocarpus bourdillonii was recorded.

d
Dipterocarpus bourdillonii flowers, fruits, and seedling: a-b-fresh fallen flowers | c-view of flower with sepal and petal partially removed | d-maturing fruits | e-seedlings showing tawny stellate pubescence on young parts.© NCF, CC-BY 4.0 Journal of Threatened Taxa | www.threatenedtaxa.org| 26 August 2022 | 14(8): 21651while Paracroton pendulus and Monoon fragrans were most frequent near D. bourdillonii trees, other lower elevation rainforest species such as Vateria indica and Reinwardtiodendron anamalaiense were also recorded in plots, besides species such as Strombosia ceylanica and Anacolosa densiflora in the Ayyankulam area.The expected natural vegetation types for this region include lower elevation (<700 m) tropical wet evergreen forest of the Dipterocarpus indicus -Dipterocarpus bourdillonii -Strombosia ceylanica type and medium elevation (700-1,400 m) tropical wet evergreen forest of the Cullenia exarillata -Mesua ferrea -Palaquium ellipticum type (Pascal 1988).While Dipterocarpus indicus was not recorded in the Ayyankulam Area during the present survey, it is the fifth most common tree species in the Varagaliar area (c.6 km straight line distance) within ATR (Ayyappan & Parthasarathy 1999) and was also recorded there during the present survey.