These luxuriant forests found on Sumatra and surrounding islands such as Nias, Siberut, Bangka and Belitungare are some of the most diverse on earth with comparable levels of species diversity to the richest forests in Borneo and New Guinea and comprise over 100 species of dipterocarps, six of which are endemic. Some of the mature parts can have up to four distinct tree stories as well as shrub and herbaceous layers. The emergent trees, which can reach heights of 60 m, are mainly dominated by dipterocarps such as Anisoptera costata, Dipterocarpus crinitus, Hopea dryobalanoides, Parashorea lucida and Shorea macrocarpa. Other important upper canopy trees are Dyera costulata, Fragraea gigantea, Koompassia malaccensis, Mangifera rigida, Myristica gigantea, Polyathia hypoleuca and the endemic Heritiera sumatrana (Sterculiaceae), Horsfieldia triandra (Myristicaceae) and Polyalthia sumatrana (Annonaceae). Among the many trees of the sub-canopy are Aporusa elmeri, Dacryodes rugosa, Diospyros buxifolia, Diplospora singularis, Drypetes longifolia, Gironniera hirta, Nephelium cuspidatum, Timonius hydrangefolius and Xanthophyllum elmeri, while smaller trees of up to 15 m include an abundance of Aglaia tomentosa, Aporosa frutiscens, Baccaurea javonica, Croton oblongus, Fordia johorensis and Koilodepas glanduligerum. The endemic small tree Baccaurea sumatrana (Euphorbiaceae) like many other trees of these forests produces flowers and fruit at the base of its trunk - a phenomenon known as cauliflory. Other endemic small trees are Casearia cuspidata (Flacourtiaceae), Daphniphyllum sumatraense (Daphniphyllaceae), Diplycosia apiculifera (Ericaceae) and Gomphandra fusiformis (Icacinaceae).

In the undergrowth shrubs such as Agrostistachys longifolia, Gonocaryum gracile, Psychotria robusta and Rinorea anguifera are common as is the small tree fern Cyathea moluccana and palms like Licuala ferruginea and Pinanga malaiana. Endemic shrubs include Arthrophyllum papyraceum (Araliaceae), Clethra sumatrana (Clethraceae), Saraca tubiflora (Fabaceae) and Thottea reniloba (Aristolochiaceae). Lianas and climbers are common especially climbing palms mostly represented by rattans like Calamus flabellatus, Daemonorops depressiuscula and Korthalsia rigida, while other climbing species include Agelaea macrophylla, Fissistigma kentii, Hugonia costata, Salacia macrophylla, Sarcostigma paniculata, Smilax leucophylla, Uncaria calophylla, Willughbeia firma and endemic Aristolochia singalangensis (Aristolochiaceae) and Erycibe ramiflora (Convolvulaceae). On the ground some of the most common species are Hanguana malayana, Labisia pumila, several orchids such as Cystorchis saceosepala and ferns like Lindsaea doryphora. These forests are also famously know as habitat for the plant with the world’s largest flower (Rafflesia arnoldii), which is a parasite of the vine Tetrastigma, and the plant with the world’s tallest flowers, the endemic Amorphophallus titanium (Araceae). This magnificent arum has flowers up to 2 m tall and produces a fetid odor that attracts small bees that act as pollinators.


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