The Philippines is one of the mega diverse (Mittermeier et al. 1997) and “hot spot” countries, with more than 10,107 described plants (Barcelona et al. 2013) and supporting 1.9% of the world’s endemic plant and vertebrate species (Myers et al. 2000). Over 57% of the major faunal and floral groups in the Philippines occur nowhere else in the world (Oliver and Heaney 1996). Bohol Island, in particular, has significant remaining forested areas within the 93,000 hectares in Rajah Sikatuna Protected Landscape (RSPL), the Loboc Watershed Reserve (LWR) and Alejawan-Cansuhay-Anibongan River Watershed Forest Reserve. Previous studies reported many indigenous timbers, including 16 species of dipterocarps (Fernando et al. 2009), 20 premium trees, one conifer, 35 edible fruit-bearing native trees, 169 ferns and fern allies (Barcelona et al. 2006) and the rest are lesser-used species in Bohol Province (Reyes Jr et al. 2015). Despite these few studies conducted, Bohol exibits a high number of recorded plant species.

The RSPL is one of the last parts in Bohol with relatively-undisturbed forest with a high diversity of plant species (Vegchel 2013). In the past, huge quantities of hardwood species, such as red lauan, Shorea negrosensis and white lauan, Shorea contorta, have been harvested. RSPL is on the southern part of the Island and all of it is mainly on karst limestone (Fernando et al. 2009). Unlike the very scientific and organised conservation programmes being poured into other forest ecosystems in the country, there is no consensus effort that would ensure effective biodiversity conservation and forest rehabilitation where a forest over limestone ecosystems and diverse fragmented forest remains with an imminent threat. Furthermore, despite being declared a protected landscape, its remaining forests have declined to only 4% from their original full-forested area due to anthropogenic and climatic pressures (Bullecer and Socorin 2013).

RSPL was declared a national park in 1987 and a protected landscape in 2000 by the Department of Environment and Natural Resources (DENR) (Ecosystem Research and Development Bureau, DENR 2005). Soil Water Conservation Foundation (SWCF), a non-government organisation, pioneered conservation assessment in 1996, which became the basis for the said declaration. However, SWCF has stopped biodiversity assessment and monitoring activities, with the Bohol Environment Management Office (BEMO) now taking point.

Meanwhile, there is growing emphasis on the importance of conservation planning which identifies priority areas and directs limited conservation resources in a strategic manner to help address challenges to biological diversity. Approaches in conservation planning have evolved over the past few decades from focusing mainly on species to encompassing broader aspects of biodiversity. Gordon et al. 2005 identified six criteria relating to biodiversity value (e.g. the number of endemic species, conservation status, diversity, variety of taxa) to select priority areas for conservation. Thus, the present study aimed to select priority areas for conservation in RSPL using six biological parameters, namely: diversity, richness, abundance, basal area and number of threatened and endemic species.

Study site

Rajah Sikatuna Protected Landscape is located in the six municipalities of Batuan, Bilar, Carmen, Dimiao, Garcia Hernandez, Sierra Bullones and Valencia (Fig. 1), with a total land area of 10,452.6 hectares. RSPL is about 8 km inland from the national road of Bilar, in the low mountain range in the south of Bohol Island. Practically all of the remaining contiguous forest in Bohol is inside this park, with only patches of intermingled plantations and dipterocarp forest elsewhere on the island. RSPL is characterised by rolling hills with remnant natural forest on steep limestone terrain, surrounded by plantation forest, deforested hills and grassland. Limestone molave forest covers 60% of the landscape, grassland 15%, forestry and agro-industrial plantations 5% and permanent agricultural areas 10%. The protected area is a popular site for birdwatchers, with many recent records of threatened and restricted-range species of the Mindanao and Eastern Visayas Endemic Bird Areas. The floral habitats within RSPL support six species of large mammals; Philippine tarsier (Carlito syrichta), long-tailed macaque (Macaca fascicularis), Philippine flying lemur (Cynocephalus volans), Malay civet (Viverra tangalunga), common palm civet (Paradoxurus hermaphroditus) and wild pig (Sus philippinensis) (Mallari et al. 2001). Table 1 shows the sampling location and the description of the plots which were randomly selected using ArcGIS software and were established in each sampling site.

Table 1.

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The general and specific locality of sampling sites in Rajah Sikatuna Protected Landscape (RSPL) with the plot codes (Plot) used in the hierarchical cluster analysis. Geographic coordinates (Lat: latitude, Long: longitude) in each plot are in decimal format.

Plot	Municipality	Barangay	Total land area (ha)	Forest cover (%)	Topography	Lat / Long
1	Bilar	Zamora	459.87	80	Steep to rolling	9.7192, 124.1362
2	Valencia	Omjon	714.63	60	Sloping	9.7268, 124.2257
3a	Carmen	Montehermoso	350.69	50	Sloping	9.7460, 124.2330
3b	Carmen	Montehermoso			Rolling	9.7531, 124.2234
3c	Carmen	Montehermoso			Sloping	9.7527, 124.2300
4a	Sierra Bullones	Nan-od	812.91	85	Relatively flat to rolling	9.7520, 124.2618
4b	Sierra Bullones	Nan-od			Relatively flat	9.7472, 124.2573
4c	Sierra Bullones	Nan-od			Relatively flat	9.7490, 124.2627
4d	Sierra Bullones	Nan-od			Flat to rolling	9.7452, 124.2560
5a	Garcia Hernandez	Cambuyo	403.12	65	Rolling	9.7088, 124.2561
5b	Garcia Hernandez	Cambuyo			Rolling	9.7094, 124.2543
6a	Garcia Hernandez	Datag	252.55	75	Flat to rolling	9.7291, 124.2641
6b	Garcia Hernandez	Datag			Flat to rolling	9.7262, 124.2645
7	Sierra Bullones	Bugsoc	190.56	50	Steep to rolling	9.7631, 124.2693
8	Batuan	Cabacnitan	306.46	50	Relatively flat	9.7229, 124.1867

Figure 1.  

Elevation map of Rajah Sikatuna Protected Landscape with the locations of the sampling plots.

Field Data Collection

Data Analyses

Conservation status and endemicity

Selecting priority areas

Floristic composition

Rajah Sikatuna Protected Landscape was recorded to have 368 plant species belonging to 223 genera from 101 families. Angiosperms were the most diverse represented by 327 species, while there were 40 species of pteridophytes and a single gymnosperm species (Gnetom gnemon) (Table 3). At family level, the most abundant taxa included Moraceae, Meliaceae, Lauraceae, Dipterocarpaceae, Rubiaceae, Myrtaceae, Phyllantaceae, Annonaceae, Araceae and Lauraceae. The remaining 91 plant families were represented by less than 10 species. With regards to plant genera, the richest recorded were: Aglaia, Asplenium, Canarium, Ficus, Hopea, Neonauclea, Shorea and Syzgium.

In the analysis of similarity of vegetation structure and composition of each plot, three main floristic groups were identified (Fig. 2). Cluster 1 had five plots grouped together, in which four of them were characterised by severe incision, dominated by Trigonostemon philippinensis. On the other hand, cluster 2, grouped by seven plots, was characterised by higher plant composition compared to the other clusters. Moreover, plant species were characterised and dominated by Diplodiscus paniculatus, Phaeanthus ophthalmicus, Shorea squamata, Lithocarpus coopertus, Gomphandra mappioides, Donax canniformis and Shorea contorta. Lastly, three plots were grouped to form cluster 3, in which all of them were characterised by severe incision, all dominated by Symplocos odoratissima and Lepiniopsis ternatensis. Furthermore, our SAC (Fig. 3) implies that more species were discovered with increasing number of sampling plots.

Figure 2.  

Dendogram of 15 sampling plots generated through UPGMA using the Bray-Curtis Similarity Index. Bootstraping was done at n = 1000; cophenetic correlation is 0.75.

Figure 3.  

Species accumulation curve (SAC) of recorded plant species in each plot.

Plant diversity

High diversity was observed in all established plots in RSPL (Fig. 4). All plots have more than 90 species and 240 individuals (Table 7). Nan-od had the highest value of the Shannon index (H' = 3.77), while Cambuyo had the lowest (H' = 3.13). In terms of species, Datag had the highest number with value 134, followed by Nan-od with 133, then the lowest was Montehermoso with 93. Meanwhile, the individuals in Datag had the highest with a value of 337, followed by Nan-od with 332 and lastly Cabacnitan with 248.

Figure 4.  

Species diversity indices (Shannon and Simpson) in each plot.

Species importance value

Based on the computed IV (Table 4), Trigonostemon philippinensis had the highest importance value in plots 1 and 5a. Meanwhile, Symplocos odoratissima had the highest importance value in plots 2, 3a and 7. The highest computed importance value of 93.16 for Symplocos odoratissima was observed in plot 3a. Moreover, dipterocarp species, such as Shorea contorta and Shorea squamata, had the highest importance value in plots 6b, 4a and 4c, respectively.

Tree stucture and density

Tree species with diameter class (DBH) of 10-20 cm had the highest proportion of individuals across all size classes (57%), followed by 21-30 cm (24%), 31-40 cm (11%) and 40+ cm (7%) (Table 5). High percentage of 10-20 cm and 21-30 cm diameter classes were observed in plot 3b (Monthermoso), while 31-40 cm was in plot 4a (Nan-od) and 40+ cm was in plot 2 (Omjon). Moreover, the computed basal area for 10-20 cm, 21-30 cm, 31-40 cm and 40+ cm diameter classes were 1.76, 0.89, 0.37 and 0.35 m²/ha which totalled 12.05 m²/ha in all plots. A trend on decreasing number of trees with increasing diameter class was also observed (Fig. 5).

Figure 5.  

Structure and density of tree species recorded in each plot of Rajah Sikatuna Protected Landscape. Diameter class per plot is reflected in centimetres.

Threatened and endemic species

Forty-six (46) species were recorded as threatened (critically endangered, endangered and vulnerable) under either the Philippine Red List (DENR 2017) or the IUCN (2019) (Table 6). Ninety three Philippine endemic species were recorded, of which 37 species were accounted in the plot established in Nan-od. The lowest was 22 accounted in the plot established in Bilar. Of these 46 threatened species, 13 were critically endangered, nine were endangered and 24 were vulnerable. The highest number of threatened species (37) was recorded in the plot established in Nan-od, followed closely by the plots in Datag and Omjon, both with 36 species. The plot with the least number of threatened species of 20 was the plot in Cambuyo. Noteworthy threatened species were White lauan (Shorea contorta) (Fig. 5K), Mayapis (Shorea squamata) (Fig. 5D) and Manggachapui (Hopea acuminata) (Fig. 6E) which are listed as critically-endangered species, but were found to be quite common in the area. Other critically-endangered species, including Dipterocarpus grandiflorus (Fig. 5A), Hopea philippinensis (Fig. 5B), Shorea guiso (Fig. 5C), Vatica mangachapoi (Fig. 5F), Pterocarpus indicus (Fig. 5G), Anisoptera thurifera (Fig. 5H), Hopea quisumbingiana (Fig. 5I), Shorea polysperma (Fig. 5J) and Shorea astylosa (Fig. 5L), were also observed in the area.

Figure 6.  

Some threatened tree species of Rajah Sikatuna Protected Landscape. Dipterocarpus grandiflorus (A), Hopea philippinensis (B), Shorea guiso (C), Hopea acuminata (D), Shorea squamata (E), Vatica mangachapoi (F), Pterocarpus indicus (G), Anisoptera thurifera (H), Hopea quisumbingiana (I), Shorea polysperma (J), Shorea contorta (K) and Shorea astylosa (L).

Biodiversity value

The study provides baseline data on plant composition in the forest over limestone of the Rajah Sikatuna Protected Landscape. A total of 368 plant species, belonging to 223 genera from 101 families, were recorded. This result is higher compared to similar studies conducted in forest over limestone of Dinagat Island which accounted for 144 plant species, belonging to 50 families and 88 genera (Lillo et al. 2018) and 192 species with159 genera belonging to 62 plant families from Canbantug forest in Cebu (Replan and Malaki 2017). Aside from the family Dipterocarpaceae, we observed similar dominant plant families of Moraceae, Meliaceae, Lauraceae, Rubiaceae, Myrtaceae, Phyllantaceae, Annonaceae, Araceae and Lauraceae with the latter studies. Species belonging to the family Dipterocarpaceae, which were commonly found in this study, are known as good sources of timber and are considered economically important. It is worth noting that most of the dipterocarp species reported by Fernando et al. 2009 were recorded in this study, with the addition of Shorea negrosensis, Shorea astylosa, Shorea polysperma and Hopea foxworthyi. The observation of these four dipterocarp species is attributed to the expanded sampling area which was not surveyed by the previous study. On the other hand, Shorea malibato and Shorea polita were not observed in this particular study because these species are considered rare and are mostly found in non-limestone forest of Bohol (Fernando et al. 2009). Meanwhile, the number of pteridophyte species (40 species) recorded in this study was comparably lower than the number in Barcelona et al. 2006 of 169 species. This may be due to the higher sampling effort and larger area (18 localities) covered compared to the eight localities surveyed in this study. Results of SAC and the cluster analysis showed that species are not homogeneously distributed which implies that different management and conservation strategies should be implemented across the RSPL.

Species were also observed to form their own phytosociological group, distinct from each other. Symplocos odoratissima, in particular, was found common on top of the mountain hills though was also present in relatively flat terrain. Trigonostemon philippinensis and Pterocymbium tinctorium, on the other hand, formed thickets along valleys and flat rocky substrates in between mountain hills which may occasionally be submerged in flowing water during rainy season, while Shorea contorta and Shorea squamata extended their dominance from hilly areas to relatively rolling undisturbed closed canopy forests. These observations conformed to the findings of Reyes Jr et al. 2015.

All plots showed high diversity in both Shannon (H’) and Simpson’s (D) Diversity Indices, with the plot in Brgy. Nan-od exhibited the highest value of 3.77, followed by Plot 6b in Datag, while the lowest diversity was recorded in Plot 5a in Cambuyo. It is important to note that, in terms of the Shannon Diversity Index, the ordering of the plots was likely affected by the topography, dense forest cover and maturity of the forest (Replan and Malaki 2017) where the plots were established. Barangays, Nan-od and Datag are relatively-young secondary forests characterised by smaller-sized and stunted trees as evidenced by the high basal area of diameter class (10-20). In addition, the topography in these areas is relatively flat to rolling. Jiao-jun et al. 2007 stated that increasing level of disturbance in secondary forest increases the number of species which likely explains the high diversity in these areas. On the other hand, Montehermoso and Omjon are evidently older second-growth forests comprised of few larger trees and less understorey species with sloping topography.

Of the 368 plant species encountered, 93 species were found endemic to the Philippines and 46 were listed as threatened, based on IUCN 2019 and DENR 2017. Of these endemic species, 91 were flowering seed plants and two species were pteridophytes. The highest number (37) of endemic species was recorded in Nan-od while the highest number (21) of threatened species was recorded in Omjon. The number of endemic species found in this study (93) is higher compared to the 23 species recorded by Lillo et al. 2018) in Dinagat Island and 19 recorded by Replan and Malaki 2017) in Canbantug forest, Cebu. In terms of threatened species, the number recorded here (46) is higher than the 18 species in Canbantug forest (Replan and Malaki 2017) and the 25 species in Dinagat Island (Lillo et al. 2018).

Priority areas for conservation

RSPL has a very high diversity value and is a habitat to at least 368 flora species. A total of 93 plants are exclusively found in the Philippines and a significant number of forty six (46) threatened species were observed in the area. Many of these threatened species like White lauan (Shorea contorta), Mayapis (Shorea squamata) and Mangachapui (Hopea acuminata), although listed as critically endangered, were commonly found within its natural forest. Therefore, it is recommended that immediate conservation and management activities should be conducted to parts of RSPL with high diversity and number of threatened and endemic species like Barangays Bugsoc, Datag, Nan-od and Omjon to save these plants from extirpation. However, for areas with low diversity and number of threatened and endemic plants, we suggest that reforestation initiatives, like assisted natural regeneration (ANR) and rain-forestation, be implemented. Additionally, species with high importance value in each area are also recommended to be used in the said reforestation programme. Lastly, future studies should also consider plant associations and environment interactions.

The authors wish to thank Philippine Council for Agriculture, Aquatic and Natural Resources Research and Development (PCAARRD), for the financial assistance of the project. The Department of Environment and Natural Resources (DENR) and the Protected Area Management Board (PAMB) members of Rajah Sikaituna Protected Landscape are also acknowledged for allowing and supporting the researchers in the conduct of their study. We would like to also convey deep gratefulness and heartfelt appreciation to the Central Visayas biodiversity survey team, Rochelyn Parba, Jessie Josol, Jessica Josol, Arianne Pacarat and Oscar Ido and the National Museum of the Philippines - Herbarium and Botany Section consultants: Danilo Tandang, John Rey Callado, and Edwin Tadiosa and Fulgent P. Coritico of CMU-CEBREM for their help identifying the seed plants and fern species. Special thanks to BS Forestry students who joined the survey and utilised study data for their coursework.

Biodiversity Assessment for Sustainable Management in Selected Key Biodiversity Areas of Central Visayas

Bohol Island State University (BISU)

Specimen Collection Gratuitous Permit (GP) Number VII-2019-04 (DENR Region VII) and Prior Informed Consent (PIC) were secured before conducting the research. The research proposal was also endorsed by the Central Visayas Agriculture, Aquatic and Natural Resources Research and Development Consortium (CVAARRDEC) to PCAARRD for funding after passing ethics and security standards of the university.

The authors declare that there is no conflict of interest.