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Endemic Philippine teak (Tectona philippinensis Benth. & Hook. f.) and associated flora in the coastal landscapes of Verde Island Passage, Luzon Island, Philippines

Anacleto M. Caringal, Inocencio E. Buot, Jr* and Elaine Loreen C. Villanueva Batangas State University–Lobo, Lobo, Batangas, Philippines School of Environmental Science and Management, University of the Philippines Los Baños, Laguna, Philippines Institute of Biological Sciences, College of Arts and Sciences, University of the Philippines Los Baños, Laguna, Philippines Faculty of Management and Development Studies, University of the Philippines Open University, Los Baños, Laguna, Philippines

THE tropical forests of Southeast Asia differ in floristic composition, diversity and structural ensembles due to biogeography and environmental heterogeneity [1][2][3][4][5][6][7][8][9] . Across the Philippines, diverse natural forests exist with distinct species composition and structure [10][11][12] . These ecotypes or formations ranging from sea level to over 2954 m altitudes (i.e. mangroves, beaches, freshwater peat swamps, forests over ultramafic soils and limestone karst, semi-deciduous forests, pines, lower and upper montane forests and sub-alpine) based on the dominant floristic elements have been the focus of ecological classification since 1900s (refs 13-16). The Philippine teak forest (PTF), however, has not yet been included in these national classifications.
The forest with endemic Tectona philippinensis Benth. & Hook. f., (APG: Lamiaceae) has long been considered as one of the most important areas of floristic richness 10,17,18 . Until the present study, however, PTF remains to be classified among the major forest ecotypes in the Philippines. Ecological studies for PTF have specially focused on phenological synchrony 19,20 , xerophytic adaptation 21 , genetic diversity 22 , and a pioneering conservation assessment for the International Union for the Conservation of Nature (IUCN) 23 . However, there is less information on landscape-level details about the floristic elements and structure across the Verde Island Passage Marine Corridor (VIPMC), an area of conservation importance within the Southern Luzon Biogeographic Region of the Phillippines 24,25 .
T. philippinensis was first described by British botanists George Bentham and Joseph D. Hooker (1876) based on the specimens deposited at London's Kew Herbarium. The species was collected by the English naturalist Hugh Cuming (1432) from the shores of Batangas, Luzon, Phillippines 17 . Since then, the Philippine teak is of special interest to phytogeographers in the Malesian region.
The Batangas Province has one of the fastest rates of land-use conversion in the Philippines 26 . Massive development of road networks and lateral expansion of urban settlements to outlying rural districts have been ongoing in Batangas during the last 30 years 26 . Madulid et al. 23 reported for the Species Survival Commission of the IUCN that these developments have threatened the natural population of T. philippinensis in the Province. Hence,  this study reveals the species composition and structure of PTF which can be lost or degraded in the course of these anthropogenic expansions. The results can be used as an input in designing and implementing native forest restoration projects, particularly in the locally important biodiversity conservation corridor.
As there are fewer accurate reports on the floristic composition and structure about PTF, this study aims to describe the species composition and vegetation structure of the PTF.

Study area
The study was conducted along Isla Verde-Batangas City-Lobo-San Juan coastal landscapes in Batangas Province (8-321 m asl, 1330-1340N and 12105-12115E), southwestern Luzon, Philippines (Figures 1  and 2). The 24 study plots (marked as black squares in the map) from Isla Verde to the mainland (56.48 km connectivity) have gentle to undulating, rolling to moderately steep slopes (0.10-51.99%), located for some distance (58.5-6225.28 m) along the coastal cliff towards inland ridges and narrow valley facing the sea of Verde Island Passage. The landscape consists of volcanic agglomerates from various ages such as Talahib Andesite (Upper Oligocene-Miocene), Lobo agglomerate (Pleiocene-Pleistocene) and San Juan Quartz Diorite (Lower Miocene) where the underlying geography is shallow, soft and porous reef limestone (Pleistocene), including bushes of corals, shells, algae structure and recent alluvium formation 27-29 .
The mean annual precipitation for 1980-2016 was 151.89 mm in Batangas Province. Highest precipitation (113-331 mm) was observed during the southwest monsoon season, viz. May to July, which decreased (292-101 mm) from August to December. Low rainfall (27-32 mm) was experienced during summer, viz. January to April. The annual mean temperature for the same period at the weather station was 27.36C, ranging from 26C to 29C.

Methods of floristic inventory and diversity analysis
Vegetation data were collected between October 2016 and February 2017 from 24 20 m  20 m (=0.96 ha) plots, which were randomly established along the 56 km islandmainland agro-ecosystem continuum. Altitudes and coordinates of all study plots were taken using a Garmin 76Csx global positioning system receiver. In each plot, 100% tallying (abundance count) of woody and non-woody species was done. The diameter at breast height of trees (at least 10 cm) was measured with a diameter tape at 1.3 m from the ground, while heights were measured using a tree height meter from the base to the topmost part of the crown. The tree heights were estimated visually 30 . Crown dimensions were measured by a meter tape 31  At least 85-90% of floristic morpho-species (trees, shrubs, vines, herbs, ferns and epiphytes) were enumerated. Species nomenclature and growth forms classification were done according to the literature [32][33][34][35][36] . The families of ferns followed the classification by Smith et al. 37 and that of Pteridophytes Phylogeny Group 38 , while angiosperms and gymnosperms followed the subsequent classification updates (1997-2016) of the Angiosperm Phylogeny Group (APG) 39 .
The species richness (SR) and total abundance or number of individuals (N) were derived from species enumeration and density tallying count, while the Shannon-Weiner diversity index (H) and other diversity indices were generated using the Microsoft Excel (2007)

Floristic composition of the Philippine teak forest
Supplementary Table 1 shows all the observed species (Sobs) other than trees as suggested by Langenberger 42 , with notes about growth habit, habitats, abundance records, occurrences and as well as geographic affinity. Altogether, four subspecies and four varieties belonging to 128 species under 113 genera in 46 families were recorded from the 24 plots. Following the species abundance classification of Colwell 43 , 80 (63%) of the total 128 observed species in this study were considered abundanteach with more than ten individuals, while 29 species (23%) as rareeach with at least three or fewer than ten individuals. Six species (4%) were rare doubletonswith exactly two individuals, while 13 species (10%) were rare singletonswith exactly a single individual.
In terms of phytogeographic affinity (Supplementary Table 1

Vegetation structure
The different growth habits or life-form structures which are the valid primary elements of vegetation type or plant association were observed in different morpho-species in the PTF (Figure 3) [44][45][46] . There were trees (68 species), shrubs (5) and subshrubs (9), bush palm (1) and palm-like tree (1), lianas (7), epiphytic tree (1), semi-woody climbers (3), epiphytic and geophytic orchids (4), epiphytic and terrestrial ferns (5), annuals to erect succulent perennial herbs (9), herbaceous climbers and creepers (6), geophytic herbs and climber (1), low and tall grasses, including bamboo (5) and sedges (2). Physiognomically, such structural diversity comprises some of the unique botanical ensembles in the PTF landscape. Table 1 presents the comparative measures of diversity for the general floristic composition of PTF. The proportion of abundance among species resulted in 'very low to moderate level of diversity', as reflected in abundancedependent Shannon-Wiener index (H), with values ranging between 0.84 and 2.69 across plots (Figure 4). This can be attributed to the disproportion of abundance among species, but if taken collectively, the PTF consists of highly abundant plant population (N) resulting in high abundance to species richness ratio (N/SR; Table 1).

Diversity analysis of the flora
SR as a direct measure of diversity varied across the 24 sites with 15-42 species and was within Chao-1 estimate of 15-50 species. SR also complements the results of Hill's diversity numbers. Chao-1 and Chao-2 SR estimators revealed that there was a minimum of 141 species and maximum of 172 species (Table 2). This is an indication of the possible range of SR in PTF as there are species that might not have been recorded during sampling. Even so, both estimates revealed that the total SR (128 species) in the present study accounts for 74.56-90.78% of the potential range of SR. Moreover, the species accumulation curve for the 24 sites shows gradual elevation and has not attained its asymptote ( Figure 5), suggesting that if sampling efforts continue, there may be additional species.
Considering both the maximum and mean values, Fischer's  and Margalef index gave the highest diversity values for PTF, as these two indices were weighted to SR   regardless of abundance [47][48][49] . The values obtained for Brillouin, Mackintosh and Menhinick indices showed similar tendency with H ( Table 1).
The other measures of diversity such as dominance (Simpson's 1D, Berger-Parker, Simpson's D), evenness (Pielou's E) and equity (Shannon's J) all reflect low to moderate diversity (Table 1), as their values diverge away from complete evenness (1) and decreasing towards zero (0). This shows that PTF is dominated by the most abundant species. The increasing values of Whittaker beta diversity (w), on the other hand, indicates that there are many species not shared in all the quadrats which can be attributed to rare, doubleton and singleton taxa. Both w and the above-mentioned alpha diversity indices together measure the overall diversity or biotic heterogeneity of an area 50,51 .

Discussion
Floristic composition, species diversity and interface with human agroscape SR (= 128 species) in the present study is higher than the number of species reported from a previous study 52 for PTF in Batangas, which accounted for 47 species over the range 11-30 species per plot. According to Heck et al. 53 , the recording of at least 50-75% of the total species of a given plant community is sufficient. Apart from trees, the present study has incorporated other possible morphospecies, including 'unseen' or rare species (e.g. palm-like Cycas, sedges, undershrubs, climbers, ferns, grasses, epi-phytes and geophytes). It was found that 80 species (63%) were abundant, while 48 (37%) were rare. Rare taxa (rare, doubleton and singleton) are those having low abundance or small ranges 54 , and a combination of biophysical factors could restrict either species abundance or geographical range 55 . The unexpected encounter with the rare Cycas suggests that the PTF landscape is of primitive origin. This remnant of Cycadales has flourished since the Cretaceous and Mesozoic era and is one of the most ancient coned plants found to this day in the Indo-Malayan Archipelago 56 .
Fabaceae consisting of vines, shrubs and trees is the most species-rich plant group, indicating that PTF has interfaced with the agroecosystem landuse. This is evident with the occurrence of locally important cultivated trees such as Gliricidia sepium and Leucaena leucocephala as well as naturalized stands of Tamarindus indicaa commercially valuable fruit tree in southern Batangas. Gliricidia and Leucaena are used as biofence, fodder and fuelwood, especially in managed hilly farms. The dominance of Fabaceae is an indication that the PTF is within the agro-forest landscape of Lobo Batangas Watershed 52 .
The species composition of PTF is a representation from ridge to coast having very low to moderate species diversity, some less disturbed while other sites are more altered by human presence. SR is a clear biological measure of alpha () diversity and is expressed simply as the number of recorded species in the sampled habitat 50 . Generally, the results imply that SR and abundance can be used as helpful diversity measures of regenerative potential of PTF. This forest superficially allows regeneration or turnover of many other species, as evident from the low to very high number of individuals. Hence, it can be considered as not anti-succession, which means that regeneration of future floristic composition can be sustained naturally.
Regardless of growth forms, dominance in the plant communities of the PTF landscape is expected among individual taxa with greater or high abundance (N), resulting in low mean evenness (0.32) to moderate equity (0.61). Human-propelled landscape alterations such as farming, deforestation for exotic plantation establishment and influx of settlement have all contributed to fragmentation of native tropical forests. They create gaps among fragments, isolating the native plant populations and spreading the alien species over the forest gaps and edges resulting in a deleterious, eliminating impact on native plant diversity 62,63 . This could be the reason for the low to moderate species heterogeneity (H) in the PTF, resulting not only from the shares of native plant population where T. philippinensis was among the most abundant tree element, but by the contribution of abundant generalist, non-native ruderal flora with Asiatic or pantropic distributions.

Conclusion
The PTF is composed of a large number of non-woody and woody species. High SR, diverse morpho-species and very low to moderate level of heterogeneity characterize the PTF in the predominantly agro-ecosystem landscape. High abundance is shared by both endemic-indigenous and disturbance-indicator species, while low abundance has been recorded in rare, doubleton and singleton taxa collectively resulting in low evenness and equity. The proportion of trees is higher than other floristic growths.
The presence of PTF in a provincial scale suggests the narrow distribution of the Philippine teak resulting in endemicity. This is typical of many tropical forests which have been reduced to small, island-like regions called 'refugia' following significant climatic changes during the Pleistocene, which involved decreased precipitation and changes in sea level. These tropical forests often show particularly moderate to high levels of plant diversity and endemism 64 . Overall, the floristics of the PTF is comparable with other Philippine ecotypes as classified according to dominant flora, altitude and local geographic distribution. The PTF along the coastal landscape of Verde Island Passage needs to be added to the list of forest formations of the Philippines on account of its uniqueness with abundance of an endangered, endemic T. philippinensis.