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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.


Forest Floor

  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).


Figure 10. "Drip-tips" on leaves (Baker, 1937)

 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.