Based on numerous ecogeographical classifications of the vegetation of the island, the vegetation of the Sinharaja may be described either as a Tropical lowland rain forest or Tropical wet evergreen forest. However, the most resent classifications adopted by the UNESCO - MAB programme identifies it as a Tropical Humid Ceylonese Rain Forest of the Indo - Malayan realm.
The vegetation within the reserve can be broadly categorized into three main types based on topography. viz. the lower slopes and valleys (150 - 600 meters), middle slopes (600 - 1000 meters) and the upper slopes and ridges (above 1,000 meters) (Figure 8).
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 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.
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.