Rainforests in Sri Lanka

By, Sri Lankan Traveller

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The lowland rainforests of Sri Lanka occur in the southwest of the island in the low country wet zone. They are found at largely below 1,000m in elevation, where the warm, wet conditions and evenly distributed rainfall provide the climate for a luxuriant growth of plants. Physical factors, such as soil, slope and rainfall, are key in determining which plants become the climax species, ie: those that predominate when the rainforest is left undisturbed. The tallest trees average a height of 30m or more. Most belong to the family Dipterocarpaceae, an Asian group extending from India to China and Indonesia. Around 50 species of this family are found in Sri Lanka, all but one endemic. The growth of Dipterocarps requires fungi called ecto-mycorrhiza — though these are not specific to individual species, unlike the more famous association between mycorrhiza and orchids. Rainforest trees often engage in mass flowering and fruiting, which may trigger seasonal movements of birds from higher elevations to take advantage of this rich food source. Contrary to popular belief, rainforests are often found on poor soil. This is why the soil becomes totally impoverished after just one or two harvests when they are cleared for agriculture. Roots of rainforest trees are often shallow and run along the surface where the nutrients are concentrated. Almost all of the organic mass of the rainforest is above the ground. In fact, nutrients shed in the form of fallen leaves and dead animals are rapidly recycled by a vast array of organisms, from microbial bacteria and fungi to earthworms and other invertebrates. Many rainforest trees have buttress roots for support on the thin soils. Unusually, the Dipterocarps do not, and their large, straight trunks are prone to toppling.

This the Dipterocarps do not, and their large, straight trunks are prone to toppling. This creates open spaces with abundant light, allowing new trees to take their place. Saplings can lie in a dormant state of growth for many years. Those that look less than a year old may actually be several decades old. Unless a tree fall creates the right conditions of light and humidity, the saplings can’t easily reach the light at the top of the canopy Water is also a key factor in determining the growth and species density of rainforest plants, with wetter areas having a higher diversity of species. Amongst the Diptcrocarps, for example, the genus Vatic(‘ occurs in the dry zone in gallery forests, but only along riverbanks. Sri Lanka’s rainforests arc vital for conserving its biodiversity. Some 24% of Sri Lankan plants, comprising more than 830 species in 25 genera, are endemic to the island. Of these, 60% arc found in the rainforest and 40% are confined to it. The Sri Lankan rainforest provides a link between the rainforests of Africa and southeast Asia, because of its origins in the ancient continent of Gondwanaland. Thus it has an ancient floral affinity with the African continent. The subsequent link with the Asian mainland allowed mainland species to colonise the island during periods when a land bridge was present and climatic conditions were suitable. As climates changed, species were left marooned on the island, resulting in relict species and causing new species to evolve. Sadly, only an estimated 2% of Sri Lanka has rainforests left, a tiny fraction of what there was just a few centuries ago. Much of what remains, estimated in total at a mere 750km2, consists of heavily disturbed fragments. The largest stretch is the Kanneliya-Nakiyadeniya-Dediyagala forest reserve complex in the Galle district , but this has been badly affected by human intervention and is poor in its animal life. The Sinharaja Man and Biosphere Reserve is smaller, but represents the largest single undisturbed rainforest remnant. Sri Lanka’s rainforests arc also of vital importance to people. They conserve the hydrological cycle on which agriculture is dependent: without rainforests, the rivers would literally run dry and temperatures would rise. They are also an important carbon sink, and a genetic reservoir of plants and animals whose genes may be commercially valuable for medicines and other products.