Figure 8. The Sinharaja region (de Rosayro, 1953)

The lower slopes and valleys comprise the peripheral parts of the Reserve. These areas would have contained Dip - terocarpus zeyanicus (Hora) and D. bispidus (Bu Hora) as the dominant canopy trees reaching 45 meters and forming a clear emergent layer. However, today these areas are covered with secondary scrub and "Kekilla" fernlands of Dichranopteris linearis. The only patches which have escaped destruction are a few along stream banks and valleys.

"The kings of the forest stand aloof" (The jungle tide)  emergent trees of the canopy

The forests which thrive on the middle slopes cover the greatest expanse. These are characterised by Mesua nagassarium (Batu Na), M. ferrea (Diya Na) nad several Dipterocarp apecies belonging to the genus Shorea (Dun). They are popularly referred to as the Mesua-Doona (Na-Dun) community. These forests reach a height of 30 to 45 meters and have an unbroken canopy devoid of emergents.

Shorea congestiflora (Thiniya)

Towards the upper slopes and ridges, the tree size decreases and on exposed summits such as Moulawella and Hinipitigala the trees assume stunted proportions which are typical of montane vegetation.

Vegetation Structure 

The basic framework of the intricate rain forest ecosystem is the complex vegetation structure. At first glance, the forest seems to be a bewildering chaos of vegetation. However, a closer look reveals that the vegetation can be categorized on the basis of several factors such as life-forms (trees, shrubs, herbs and woody climbers), atrata or groups of plants living under similar conditions of light and moisture with each group having its own "life-style" (the canopy, subcanopy or under-storey, shrub layer and ground layer, stranglers, epiphytes, parasites and saprophytes). They could all either stand on their own, like trees and thus be mechanically independent, or be dependent, like climbers, epiphytes and parasites. Stranglers begin their life as dependents, but later establish their own support. The plants could either be photosynthetic, thus nutritionally self sufficient or non-photosynthetic like the saprophytes ; some, on the other hand, are totally parasitic. The plants could also represent several stages of maturity e.g. seedlings, saplings, poles, etc. As expected, the variety of combinations that could exist in the forest can be enormous. These are the factors that form the basis of scientific descriptions of the vegetation structure.

As in all rain forests, in Sinharaja too. the forest is tall with the canopy reaching a height of 30 to 45 meters. The canopy is well packed with tree crowns and is usually devoid of emergent trees i.e. trees which reach over the main layer of packed crowns.

Among the large trees of the canopy, columnar tree boles are typical. These are supported by plank buttress roots a typical characteristic of large rain forest trees. Several theories have been put forward to explain the presence of buttresses, the most accepted one being that they are necessary to stabilize the unusually tall tree trunks, which support a crown at the very top and have a shallow root system. In the Sinharaja however, buttresses are few and not as developed as in other South-East Asian rain forests.

The root systems of rain forest trees are relatively superficial. Only a few trees have deep penetrating tap roots. There is growing interest in the study of the underground structure of trees and the part they play in nutrient cycling. Also of current interest is the symbiotic association between tree tree root and fungireferred to as mycorrhizae which facilitate nutrient absorption.

Below the canopy is a stratum of sub-canopy trees that are usually between 15 and 30 meters in height. Under the deep shade cast by the overstorey are trees of the under-storey which grow to a height of 5 to 15 meters. A vegetation profile of trees in Kanneliya, a forest close to Sinharaja, is presented here to illustrate the vertical stratification of the tree vegetation (Figure 9).

Figure 9. Vegetation profile of a rain forest close to Sinharaja in

Kanneliya (Gunatilleke and Gunatilleke, 1983)

1. Diospyros insignis (Ebenaceae)  2. Bhesa ceylanica (Celastraceae)  3. Myristica dactyloides (Myristicaceae)  4. Carallia calycina (Rhizophoraceae)*  5. Shorea congestiflora (Dipterocarpaceae)*  6. Strombosia nana (Olacaceae)  7. Garcinia hermonii (Clusiaceae)  8. Ostodes zeylanica (Euphorbiaceae)  9. Xylopia championii (Annonaceae) 10. Memecylon gardneri (Melastomataceae) 11. Symplocos cuneata (Symplocaceae)   * buttressed tree

Below the under-storey trees the vegetation is sparse, contradicting the popular belief that the rain forest is impenetrable. Treelets, saplings and a few shrubs occupy the vegetation below 5 meters in height, these being mostly the saplings of over-storey trees.

The ground layer of the forest has a few herbaceous plants and an abundance of seedlings. The herbaceous plants are often variegated or coloured in shades of red and purple, while others are iridescent. These are considered to be adaptation for survival under low light intensity.

Large woody climbers and epiphytes are two other characteristic life-forms of the rain forest. Both groups illustrate an alternative strategy for reaching sunlight, a resource which is in low supply within the forest. However some of the epiphytes are also adapted to the dim shady environment of the forest interior.

Among other noteworthy features of the rain forest are drip-tips of the leaves. The presence of elongated leaf tips is probably an adaptation to drain-off the excess moisture which constantly collects on the leaf surface. This feature is therefore conspicuous among life forms and seedlings and saplings which inhabit the forest interior and are also found close to streams where the moisture regime is high. In Sinharaja, this phenomenon was first observed by Jhhn Baker in 1938. Of the 41 species of plants he examined 90% had "drip-tips" (Figure 10).

Other features such as coloured young leaves, and cauliflorous fruits and flowers are also peculiar to rain forests. The prominent colours of varying hues of red, orange ,brown, and purple are nature's way of protecting the young leaves from harmful radiation. Flowers and fruits which arise directly from the main trunk or branches are thought to be beneficial for pollination and dispersal of seeds by animals, in as environment where dispersal by wind is not common.

The total vegetation density of the forest including trees, shrubs and seedlings has been estimated to be around 240,000 individuals per hectare, of which 95% comprises individuals of the ground layer. The density of trees and lianes above 30 centimeters girth, ranges between 600 to 700 individuals per hectare while the number of commercially exploitable trees i.e. trees of girth greater than 150 centimeters, ranges between 45 to 55 individuals per hectare.

Forest gaps occur infrequently and are usually caused by nature phenomena such as the death of over-mature trees or the uprooting of trees by the wind. In these gaps, as well as along  waterways where light reaches the ground, a  vigorous growth of saplings and other vegetation forms impenetrable thickets.  

Rare Species 

Rain forest are known to contain a great diversity of species with low population densities. Having analysed the general structure of the forest and its species composition it is possible to go a step further and study the actual distribution of species in space, and their population sizes i.e. the number of individuals of each species. such studies have in fact been carried out in the Sinhaaja, and show that there is a high degree of species localization with respect to spatial distribution. Among the woody flora above 30 centimeters girth, 26% of the species were restricted to a single site. The majority densities. For instance, among the trees, 50% are represented by less than 10 individuals per hectare while 15% had only a single individual per hectare. Similar trends are exhibited by the smaller life forms such as the treelets, shrubs, and herbs ; their localization is even more marked probably because their dependence on micro-habitats is greater. While 15% were represented by only one individual in an area of 1,000 square centimeters, 87% had less than 100 individuals in an equivalent area.

 

A rear endemic plant "Binara" (Exacum trinervium)

A rare orchid of medicinal value "Wanaraja" (Anoectochilus setaceus)

The Endemic Flora 

The endemic flora of the sinharaja occupies a unique status it accounts for 64% t o75% of the total number of species recorded among the trees and lianes over 30 centimeters in girth. The contribution of the endemics to the forest stand ranges from 75% to 92%. In a single site studied for the vegetation below 30 centimeters in girth, 60% of the total number of species were endemic, and their contribution to the density was 85%.

 It is also interesting to note that of the 25 genera endemic to Sri Lanka, 13 are represented in the Sinharaja. Several of these genera are monotypic i.e. they are represented by one single species (Table 6).

Endemic tree species considered to be rare have been identified in the Sinharaja as well as in other lowland rain forests. Studies show that 98% of al sub canopy species and 85% of all understorey species fall into the categories of "Rare", "Vulnerable" and "Endangered" as described by the International Union for Nature and Natural Resource Conservation (IUCN) Red Data Book. Of these endemic tree species, almost 25 species were restricted to a single forest site.

Similarly, of the 217 endemic trees and woody climbers of the rain forest region, 65% i.e. 140 species have so far been recorded in the Sinharaja. This percentage probably does not represent the actual value which could be greater. Although the smaller life-forms have been studied, their percentage values have not been computed as yet. 

Forest Regeneration 

Natural forests ate constantly disturbed by man. In the Sinharaja the two main forms of interference have been shifting cultivation and mechanized logging.

Regeneration studies carried out in modified areas at Sinharaja such as those recuperating from selective mechanized logging and shifting cultivation have helped to identify those pioneer species which colonise disturbed sites (Table 7). These are usually fast growing, short-lived species that are  able to establish themselves and grow under unfavourable conditions of high light intensity, low humidity and poor soil conditions. There are other species which have pioneer qualities but are not truly pioneers. These either follow in the wake of pioneers or respond to slight disturbance. 

Although mechanized logging drastically altered the structure of the forest, the altered structure of the forest, the forest shows considerable recovery after 10 years of rest. Nevertheless, long term changes in species composition seem inevitable with light-loving plants such as Shores trapezifolia dominating the new growth. The under-storey species appear to be the group most adversely affected  by logging as they are only adapted to living under specialized ecological conditions created by the presence of an undisturbed canopy. Judging by the present recovery, it is clear that logging based on a polycyclic system of 20 years which was planned for the Sinharaja world no doubt have been highly unsuitable, particularly as mechanized logging methods were to be used.

Macaranga peltata (Kenda)

Shorea trapezifolia (Thiniya)

Artocarpus nobilis (Wal del)

The Broad leaved Honduran Mahogany (Swietenia macrohylly) has been planted along abandoned skid trails within the forest as an enrichment species during the logging period. Its growth has been phenomenal and it now comprises 75% of all individuals of 10 centimeters in girth or more along the skid trails. This has caused some anxiety among scientists that it may displace the nature species of the forest particularly since it is a prolific seed producer, and several attempts have been made to eradicate these trees. Fortunately, however instances of weed trees invading natural forest are unknown.

The abandoned shifting cultivation areas in Sinharaja, appear to recover very slowly. Studies of a site abandoned around 25 to 30 years ago reveal that despite a plentiful source of seed from the surrounding natural forests, the site was still dominated by pioneer species. Species which re-established through seed such as Shorea have had very little chance of success, while those that are able to regenerate as coppice-shoots such as Anisophyllea cinnamomoides have had grater success. 

Current Research 

At present, well co-ordinated research programmes spearheaded by the Department of Botany of the Peradeniya University are being carried out using basic phytosociological studies as a framework. A dozen or so species have been selected for special study such as Caryota urens (Kitul), Vateria coppalifera (Hal), Coscinium frnestratum (Weniwel), and several Shorea app. (Thiniya, Dun, Beraliya). Some of these plants are used as food, spice or medicine by the village communities living around the forest. In addition, the reproductive biology of some commercially valuable plywood species is being studied for the purpose of domestication. An illustrated handbook of the trees of the Sinharaja is currently under preparation. A large scale programme for the propagation of the giant rattans viz. Calamus ovoideus (Thuda rena), Calamus zeylanicus Ochlandra stridula (Bata) has been initiated by the Forest Department, while the National Herbarium is carrying out a taxonomic survey of shrubs, herbs and ferns. 

Read THE FAUNA OF SINHARAJA