Sinharaja is a lowland tropical rainforest of global importance showing certain affinities with the rainforests of South and North-East India, Indonesia and Malaysia. As a result of long isolation in the shelter of the central mountains of Sri Lanka, as well as being separated by oceans from other regional rainforests, Sinharaja displays high level of endemism in the composition of both its flora and fauna. As such it warrants special protection. Indeed, the bulk of Sri Lanka's remarkable bio-diversity is concentrated in rainforests such as Sinharaja, along with those of the Peak Wilderness and the Knuckles Range. 22,000 acres in extent, the Sinharaja Rainforest amounts to only 10 percent of the remaining forest cover of the wet zone of Sri Lanka. Saved from a government sponsored logging operation in the mid-1970s, the rainforest was recognized as an international Man and Biosphere Reserve by UNESCO in 1978 and declared a national wilderness area in 1988. The protection of Sinharaja was further strengthened by the subsequent inclusion of the rainforest in UNESCO's list of World Heritage Sites.
Sinharaja displays a unique floristic composition. By 1990, 211 species belonging to 119 genera and 43 families had been described. Nimal and Savitri Gunatilaka's pioneering work in the Sinharaja continues to discover new species, however. Around 64% - 75% of the total number of tree and liana species in the Sinharaja are known to be endemics. Additionally, of the 23 genera of plants endemic to Sri Lanka, 13 are represented in the Sinharaja. The vegetation of this rainforest can be classified from top to bottom into canopy species, sub-canopy species, under-story, treelets and shrubs, root climbers and ground herbs.
The Dipterocarpus family dominates the canopy species of the rainforest. These species of the genus Shorea are the giants of the Sinharaja. Another abundant canopy species is Keena (Calopyhllum trapezifolium), found mainly in the areas disturbed by logging in the past. A unique species of the sub-canopy is Diya-na (Mesua ferrea), notable during the time of its spectacular pink flush of new leaves, which is a phenomenon common to many rainforest trees.
A profusion of treelets and shrubs grow throughout the forest. Prominent among these are the lowland, giant tree ferns (Cyathea sp.) often found growing in dense stands near water. A noteworthy feature of the vegetation is the ubiquitous drip tips of most of the leaves. These pointed tips allow the copious amounts of moisture which collects on the surface of the leaves to drain off.
Many of the canopy giants bloom far above an observer on the forest floor. However, it is possible to see these trees in bloom from vantage points such as Sinhagala that overlook the forest. Another significant feature of the rainforest is that only a little direct sunlight reaches the forest floor in many areas. This creates a perpetual gloom, in which birds and other animals below the canopy are frequently heard long before they are seen.
The forest also has a profusion of large woody climbers, rattans, and epiphytes. Among the latter are many colourful orchids most of which are endemic to the forest.