Plant biogeography and conservation of the south-western hill forests of sri lanka I. A. U. N. G
DISCUSSION Forest types represented. –
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- Sinhagala plots Tibbottagala plots Hinidumkanda plots
- Localized distribution of threatened species.
- Forest degradation and destruction.
DISCUSSION Forest types represented. – In the past many attempts have been made to classify the forests of Sri Lanka using floristic (de Rosayro, 1950; Gaussen et al., 1966), bioclimatic and physiognomic (Greller & Balasubramaniam, 1993; Koelmeyer, 1957) characteristics (Table 4). The floristic analysis of the present study reveals that the Hinidumkanda vegetation represents both the Dipterocarpus and Vitex-Dillenia (Wormia)-Chaetocarpus-Anisophyllea communities of the wet evergreen forest climax formation of de Rosayro’s classification and the lowland mesophyllous evergreen dipterocarp rain forests of Greller & Balasubramaniam. The Sinhagala vegetation, on the other hand, represents the Mesua-Doona (Shorea) community of de Rosayro; but in Greller & Balasubramaniam’s classification, the distinction between these coastal and inland lowland forest communities of de Rosayro has not been made. The presence of several abundant species in Tibbottagala (670 – 900), which are also found in both Hinidumkanda (300–668 m) and Suriyakanda (950–1050 m), suggests that the Tibbottagala vegetation represents an ecotone between lowland and lower-montane forests of this region. In Tibbottagala, Mesua nagassarium is replaced by Calophyllum trapezifolium as one of the most abundant canopy species. Floristically, the Suriyakanda vegetation conforms to the submontane evergreen forests of Koelmeyer (1957), Doona-
Fig. 6. A stand of Loxococcus rupicola (Araceae) on steep rocky slopes near Handapan Ella.
17 THE RAFFLES BULLETIN OF ZOOLOGY 2005 Fig. 7. Classification (Two-Way Indicator Species Analyses) of 62 plots sampled in the hill forests of Hinidumkanda (plots Y1– Y10, Z1– Z10), Sinhagala ( A11 – A21, B11 – B21) and Tibbottagala ( A33 – A42, B33 – B42) in southwest Sri Lanka, using abundance data of each species. The indicator species and the eigen values at each division are also given. the lower montane notophyllous evergreen dipterocarp rain forests of Greller & Balasubramaniam (1993). de Rosayro has not recognised this as a distinct forest type in his classification. The Handapan Ella vegetation, characterized by the absence of Shorea gardneri, corresponds to the lower montane notophyllous evergreen mixed rain forests of Greller & Balasubramaniam (1993). This distinct forest type has been overlooked in other forest classifications. The floristic data in our study support the forest classifications based on distinct floristic assemblages although no single classification 7 4 4 Plots: Sinhagala 22, Hiniduma 20 and Tibottagala 20 17 A41
B41 A42
B42 A37
B37 A38
B38 B35
A36 A17
B17 A18
B18 A12 A13,B13 A14,B14 A15,B15 A16,B16
A19,B19 A11
B11 B12
A20 B20
A21 A21
A39 B39
A40 B40
12 A33
B33 A34
B34 A35
A36 4 2 10 11 6 10 5 7 Y10
Z10 Y7 Z7 Y8 Z8
Y9 Z9 Y5 Z5 Y6 Z6 Z4 Y1
Z1 Y2 Z2 Y3 Z3
Y4 4 2 6 20 Plots 6 15 32 8 42 Plots Upper
slope plots
Valley plots
Sinhagala Summit
plots Lower
slope plots
Hinidumkanda Tibbottagala High plateau
plots Valley
plots Steep
slope plots
Slope plots
Valley plots
Ridge plots
0.302 0.22
Chionanthus albidiflora 1 Dillenia retusa 1 Mesua nagassarium 1 Gyrinops walla 1 0.384
0.322 0.402
0.386 0.290
0.422 0.366
0.37 Cyathocalyx zeylanica 1 Vitex altissima 1 Calophyllum moonii 1 Cullenia ceylanica 1 Bhesa ceylanica 1 Nothopegia beddomei 1 Agrosti- stachys coriacea 1 Humboldtia laurifolia 1 Palaquium laevifolium 1 Shorea worthingtonii 1 0.383
Palaquium 1 canaliculatum Garcinia sp. 1 Axinandra zeylanica 1 Garcinia quaesita 1 Campnosperma zeylanicum 2 Palaquium petiolare 4 Timonius flavescens 1 Less steep slopes & +ive border-line plots
18 Gunatilleke et al.: South-western Hill Forests of Sri Lanka Sinhagala plots Tibbottagala plots Hinidumkanda plots High plateau plots Valley
plots Less steep slope plots Borderline plots -ve + ve Lower slope
plots Upper
Slope Plots
Valley plots
Summit plots
valley plots
Ridge & steep slope plots Steep slope plots
Axis 2 Axis 1
Fig. 8. Ordination (Detrended Correspondence Analysis) on the 62 plots sampled in the hill forests of Hinidumkanda (plots Y1–Y10, Z1– Z10), Sinhagala (A11–A21, B11–B21) and Tibbottagala (A33– A42, B33–B42) in southwest Sri Lanka, using abundance data of each species. The plot distributions in the ordination diagram are indicated by the plot number used in field sampling. Table 3. Numbers of species in the south-western hill forests of Hinidumkanda (Hinidum.), Sinhagala and/or Tibbottagala (Tibbott.) in different categories of the 2003 IUCN Red List. CR = critically endangered, EN= endangered, VU = vulnerable, LR/cd = lower risk: conservation dependent; RIRL, NIRL = number of species recorded and not recorded in the Red List, respectively. Study Sites Sampled CR EN VU LR/cd
Sub Total: Sub Total: Total RIRL NIRL
Hinidumkanda only 6 4 16 – 26 42 68 Sinhagala only 3 2
– 8 6 14 Tibbottagala only 1 1
– 4 11 15 Hinidumkanda & Sinhagala 3 2
– 15 4 19 Hinidumkanda & Tibbottagala 1 2
– 4 13 17 Sinhagala & Tibbottagala 8
6
14 12
Hinidum., Sinhagala & Tibbot. 9 5 28 4 46 19 65 Total in all three sites 31 16 66 4 117
107 224
Individual Study Sites Total in Hinidumkanda 19 13
4 91 78 169 Total in Sinhagala 23 9
4 83 41 124 Total in Tibbottagala 19 8
4 68 55 123 19 THE RAFFLES BULLETIN OF ZOOLOGY 2005 recognised all the different forest communities encountered in our survey. Interestingly, the six forests quantitatively sampled by us, along this altitudinal gradient some 80 km direct east from the south-west coast represent five different forest types. This study emphasizes the need for a quantitative analysis of a much larger database including those from other forests in the south-western region of Sri Lanka in order to identify distinct floristic communities facilitating a review the existing forest classifications in a more objective manner recognising at the same time micro-scale habitat variations to capture the habitats of localised endemics.
plots of the three hill forests revealed the presence of distinct floristic communities akin to those described in literature along the altitudinal gradient at landscape level. Separation of plots in each of the three hill forests along axes 1 and 2 in the DCA provides quantitative evidence for the presence of distinct forest communities in the coastal lowland (Hinidumkanda), interior mid-elevational (Sinhagala) and lower montane/ ecotone (Tibbottagala) hill forests confirming previous qualitative classifications of the vegetation. The only exception was the overlap of four steep slope plots (A39, B39, A40 and B40) in Tibbottagala with the ridge and steep slope plot cluster in Sinhagala (Fig. 8), owing to the higher proportion of species shared between them. Both analyses indicated a finer scale separation of floristically recognizable local communities in valleys, lower and upper slopes and high plateau or summit plots in each hill forest. However, the plots on the ridges, summits or plateaux, slopes and valleys in each hill forest separated from their counterparts in both analyses suggesting their distinctiveness. This provides evidence for spatial heterogeneity and ecological differentiation of tree species along the finer-level climatic and topographic gradients and consequently, provides the scientific underpinning for conservation management planning of these forests. The presence of distinct species assemblages on the ridges, slopes and valleys, along the altitudinal gradient of the Mesua-
further supported by census data of the 25 ha Forest Dynamics Plot (FDP) in Sinharaja, where all free standing woody plants > 1 cm dbh have been enumerated and plotted on a contour map (Gunatilleke et al., 2004). This FDP plot spans an elevation range of only 150 m; yet it depicts eight different habitats based on topography, convexity and slope. Further, at least for some canopy dominant taxa, the ecophysiological adaptations of their seedlings to micro-climatic gradients of light, moisture and soil nutrients along the topographic gradient during early establishment and recruitment phases have been shown (Ashton et al., 1995; Gunatilleke et al., 1996,1997,1998). All these studies, including the present study, show habitat specialization of plants along small scale gradients in the hill forests of Sri Lanka, which has important Table 4. The south-western hill forests of the present study according to different forest classifications in the literature. Forest classification of Hill forests sampled in de Rosayro Koelmeyer Gaussen et al. Greller & Balasub- present study (1950) (1957)
(1966) ramaniam (1993) Hinidumakanda
Tropical wet Doona-Dipterocarpus Lowland mesophyllous community evergreen Mesua series evergreen dipterocarp rain forests forests
Vitex-Dillenia- Chaetocarpus- Anisopyllea community Sinhagala
Tropical wet Doona-Dipterocarpus Lowland mesophyllous community evergreen Mesua series evergreen dipterocarp rain forests forests
Tibbottagala Mesua-Doona Tropical wet Doona-Dipterocarpus Lowland mesophyllous community evergreen Mesua series evergreen dipterocarp rain forests forests
Suriyakanda Not recognized Submontane
Lower montane mesophyllous evergreen
evergreen dipterocarp rain forests forests
Handapan Ella Not recognized Not Not
Lower montane mesophyllous recognized recognized evergreen mixed rain forests 20 Gunatilleke et al.: South-western Hill Forests of Sri Lanka conservation implications considering the fact that many of these species are relict endemics. Localized distribution of threatened species. – Comparison of the species in the IUCN Red List (Table 3) in the three quantitatively sampled forests, shows that 43% of the 224 species identified were found only in one of the three sites, depicting very localized distribution of these species, particularly in Hinidumkanda with as many as 68 species. With 169 tree species it is also the richest among the study sites sampled. Among the endangered species in the IUCN Red list that are highly restricted in their distribution and enumerated in Hinidumkanda are Shorea dyeri, S. lissophylla and Diospyros oppositifolia. The results of this study also show that tree species richness decreases with altitude, Hinidumkanda ranking highest and Handapan Ella the lowest, the latter probably being also due to the smaller extent of forest studied. On the other hand most, if not all, studies in Sri Lanka, including our own, have so far failed to investigate the diversity of the herbaceous and lower plant floras, components of the vegetation known to increase in diversity with altitude. The remaining forests in southwest Sri Lanka represent only a small fraction of less than 9% of their original extent. Today even these are also highly fragmented, most small in extent, with receding boundaries due to a multitude of human impacts. Hinidumkanda is one of the few protected areas that represent the Dipterocarpus community. High point-endemism and high habitat specialization in Sri Lanka’s rain forest species demands that the remaining forests in its south-western quarter, however disturbed or fragmented they are, need to be conserved with a sense of urgency. Among these are the representatives of the Dipterocarpus community in the coastal belt and hill forests and high plateaux of the Rakwana- Deniyaya hill range representing the lower montane notophyllous evergreen mixed/Dipterocarp rain forests. Compared to these forests types, the Mesua-Doona community is better protected in the Sinharaja and the Kanneliya-Dediyagala-Nakiyadeniya groups of forests. Conservation issues. – In response to criticism mounted against the first Forestry Master Plan for Sri Lanka (Anon., 1986) and subsequent assessment of the environmental implications of the Forestry Sector Development Project, an Environmental Management component was included in this project (Anon., 1997). This component was mandated to evaluate all remaining natural forests in the country with respect to their importance for biodiversity, in terms of ecosystem and species diversity, and their value for soil and water conservation. This study, commonly referred to as the National Conservation Review (NCR), inventoried and analysed the data of selected groups of plant and animal species in 204 forests in the island. Among the recommendations of the NCR, those related to the present study are, (a) all montane forests should be strictly protected by upgrading the status of proposed reserves to conservation forests, or even national heritage wilderness areas; and (b) the largest remaining forests in the wet zone should be designated in their entirety as conservation forests, inter alia, Sinharaja complex (a group of 13 forests including Handapan Ella Plains, Diyadawa and Dellawa) and broader Kanneliya, Nakiyadeniya and Dediyagala (KDN) complex so that the long-term conservation of biodiversity could be ensured. The biological value of the forests in close proximity to the northern, western and southern borders of the Sinharaja World Heritage Site (WHS) has been recognised and identified as conservation forests, based on recommendations of the NCR. However, forests along the eastern perimeter of the Sinharaja WHS, where part of this study has focused (Suriyakanda and Handapan Ella), have yet to receive the attention they merit. These areas were not adequately surveyed for their biological wealth even during the NCR. With respect to plants, they harbour an exceptionally diverse herbaceous and epiphytic flora including orchids, balsams, ferns, bryophytes, lichens, fungi & algae, a component of the vegetation in all ecosystems, which was not studied by the NCR and for that matter, has almost entirely been neglected in Sri Lankan biodiversity surveys. Therefore, relying entirely on the NCR as the source of quantitative scientific information for purposes of conservation planning could lead to serious omissions of areas rich in herbaceous and epiphytic plants which are at greater threat of extinction considering their sensitivity to habitat alteration and habitat size decrease. Consequently, due to the dearth of adequate scientific information, particularly with respect to the rich flora of herbaceous and lower-plant taxa, some areas of the Rakwana-Deniyaya hills are not included in the current protected area system. Hence, they are in imminent danger of being degraded further through poaching, timber and firewood harvesting, cardamom cultivation, illicit gem prospecting and other activities inimical to biodiversity.
many fragile ecosystems of the Handapan Ella plains appear to be extensive, particularly in the grasslands and stream banks, where thousands of gem pits (most ~1 m deep) have been dug over the years. These illegal encroachers have caused extensive damage to the forest by cutting wood and poaching animals in this fragile ecosystem. In early 2004 there was an even more sinister threat to the relict forest fragments adjoining the eastern boundary of the Sinharaja WHS, despite repeated requests and recommendations made to annex them to the Sinharaja WHS to increase its conservation value. Instead, these state-owned forest lands were blocked out and sold for tea and cardamom cultivation. This irresponsible act, amidst public protests, including those of scientists familiar with the biological wealth of the region and its conservation value, has irreparably damaged this fragile ecosystem, critically endangering some of the threatened animal and plant taxa exclusive to the Rakwana-Deniyaya hills. The forests near the south-eastern boundary of the Sinharaja WHS abutting the Sinharaja Division of the
21 THE RAFFLES BULLETIN OF ZOOLOGY 2005 Ensalwatta Plantation and also Beverley and Manikkawatta Estates are on long-term lease to plantation companies. These appear to be relatively free of any encroachments for gem prospecting and wood-cutting. The forests surrounding the Sinharaja Division of the Ensalwatta Plantation represent Shorea gardneri (Sinhala: Rath Dun) and Shorea trapezifolia (S: Yakahalu Dun) tree formations at their very best (Figs. 9, 10). These magnificent forest stands, traversed by streams and spectacular waterfalls, harbour abundant wildlife, including a small herd of elephants, and are an important watershed for the downstream communities. Similarly, forests in the highest peaks of the Rakwana- Deniyaya hills, viz., Abbey Rock (1300 m) and Gongala (1358 m), those near Panilkanda and Aninkanda estates, Naigala, Kabaragala, Beralagala, Suriyakanda and Kurulugala deserve to be conserved. Apart from a few explorations for taxonomic and preliminary ecological study purposes, their conservation value is yet to be evaluated and documented (Jayasuriya et al., 1993). Several animal species endemic to the Rakwana- Deniyaya hills and new to science have been collected from this region (Bahir & Ng, 2005; Manamendra-Arachchi & Pethiyagoda, 2005; Pethiyagoda & Manamendra-Arachchi, 1998). Extension of the current eastern boundary of the Sinharaja WHS to include these forests within an appropriate protected- area category is proposed once again (Gunatilleke & Ashton, 1987), as an urgent measure to conserve their rich biodiversity. The presence of a small herd of elephants in the Rakwana- Deniyaya hills is an important criterion for establishing a wildlife reserve in this region. Their migratory routes need to be studied in order to demarcate and establish habitat corridors connecting the surrounding forests, particularly to the north of the Rakwana-Deniyaya hills. As a dwindling population of elephants similar to that inhabiting the Peak Wilderness area, a conservation management plan for wet zone elephants should also be considered. The role of the elephant in the functioning of these complex rain forests is virtually unknown. Such a study would especially benefit nature-based tourism in this region. The issues highlighted in this paper should be addressed coherently with a sense of urgency, before the next wave of ‘development’ overwhelms Sri Lanka. Conservation planning for Sri Lanka’s south-western region should not be done piece-meal or on a short-term basis, but at landscape level that takes long-term impacts into consideration. Appropriate corridors and buffer zones including multi-species home gardens, where appropriate, could be used to link the different ecosystems and forest types in this region. The present study provides sound evidence that the south- western hills and, the Rakwana-Deniyaya range of lower montane forests in particular, indeed possess high conservation value. However, they have hitherto remained relatively unprotected owing to their biological and ecological value not having been adequately assessed in previous biological surveys. We hope that this study will serve as a stimulus for demarcating such high-priority conservation landscapes (both protected and yet unprotected) along the Gin Ganga basin from its headwaters in the Gongala area through southern Sinharaja, KDN complex and Hinidumkanda and remaining mangroves, together with the Hikkaduwa Marine sanctuary seascape beyond its confluence with the Indian Ocean at Gintota as an integrated model river basin landscape-seascape conservation area. Where feasible, linking of neighbouring forest areas through restoration corridors with already available research experience for this region Gunatilleke (1999) is also strongly recommended. The stratigraphic profile accompanying the map of the south-west Sri Lanka in Fig. 5 provides some directions for the initial identification of the relevant areas for this exercise.
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