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Gunung Leuser National Park

The Gunung Leuser National Park (GNLP) covers approximately 1’092’692 ha and straddles the borders of the two provinces of Nanggroe Aceh Darussalam and North Sumatra. It lies within the Leuser Ecosystem and takes its name from the towering Mount Leuser, whose peak stretches to 3’404m. The region is predominantly mountainous. It covers most of the West Barisan, West Alas and East Barisan ranges and is nearly divided by the Alas valley. 40% of the Park, mainly in the north, is steep and over 1,500 m; 12% of the Park only, in the lower southern half, is below 600 m (but it has been largely destroyed in the recent decades). Rainfalls are among the highest in the world with an annual average of 3000 to 4500mm depending on the region. Temperatures average between 21 °C to 28 °C and the humidity is always above 60%.

The GLNP is a complex, amazingly rich and fragile environment. Thanks to the soil diversity and altitudinal range, approximately 8’500 plants species grow in the Leuser Ecosystem, with no less than 4’000 of these growing in the GLNP itself. It is core of many endangered species remaining habitat; these unique flora and fauna are in critical need of conservation and protection. 

The region represents one of the last remaining expanses of lowland dipterocarp forest across Indonesia. These luxuriant forests 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. A specificity of these forests is that trees do not typically fall down or get blown over as it seen in neo-tropical forests. The trees rather die standing, gradually losing their branches. As a consequence the dipterocarps forests are darker and more stable than Amazon or African forests.

What are Dipterocarps?

The Dipterocarpaceae are a family of 17 genera and approximately 500 species of mainly tropical lowland rainforest trees. They often emerge from the forest canopy and are typically reaching heights of 40–70 m tall. They are recognizable to their smooth, straight trunks rising to great height without side branch and to their characteristic cauliflower-shaped crown. Their fruits consist of a hard and oily seed with one or two 'wings', which lends this family its name (from the Greek di = two; ptero = wing; carpos = seed). 

The emergent trees, which can reach heights of 80 m poke up above the majority of other trees and appear like “islands” popping out of the forest. They are dominated by evergreen dipterocarps such as Damar (Agatis sp.), Meranti (Shorea subspecies), Anisoptera costata, Dipterocarpus crinitus and Parashorea lucida. It takes ~100 years to a dipterocarp tree to attain a canopy height of 30 m; as a consequence the regeneration of the forest after logging is extremely slow. Another specificity of Sumatran dipterocarp trees is that’s every two to seven years they produce exceptionally large amounts of fruits at the same time. This phenomenon, known as mass fruiting, is related to the El Nino Southern Oscillation. Other

important upper canopy trees are Dyera costulata, Koompassia malaccensis, Mangifera rigida, Myristica gigantea, and the endemic Heritiera sumatrana (Sterculiaceae) and Polyalthia sumatrana (Annonaceae).

In continuation to the emergent trees is the main canopy. It normally sits 25-40 meters above the ground level, and is a proper sea of leaves, with the bulk of the forest trees spreading their crowns at this level, and competing with each other for the sun. Among the many trees of this layer are Dacryodes rugosa, Diospyros buxifolia, Diplospora singularis, Drypetes longifolia, and Xanthophyllum elmeri.

Below the canopy lies the subcanopy.  It comprises smaller trees and climbers on their way up into the canopy; this layer normally lies between 5 – 20 meters above the ground. Small trees of up to 15 m include an abundance of Aporosa frutiscens, Baccaurea javonica, Croton oblongus. 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. Lianas and climbers are also common especially climbing palms mostly represented by rattans like Calamus flabellatus, Daemonorops depressiuscula and Korthalsia rigida, while other climbing species include Agelaea macrophylla, Sarcostigma paniculata, Smilax leucophylla, Uncaria calophylla, and endemic Aristolochia singalangensis  (Aristolochiaceae) and Erycibe ramiflora  (Convolvulaceae).

Dammar trees such as Hopea dryobalanoides are part of the Dipterocarp family. They are large tree which can grow up to 60m and are harvested for their resin. The well-known resin can be burned and used for starting fire and as pleasant smelling incense. The wood is also very valuable and these trees play an important role in the local ecosystem, supporting the growth of a certain type of strangling fig. (Left image)

Dyera costulata or Jelutong is a tall tree found in undisturbed forest up to 400m. It yields latex that has long been used to make chewing-gum, while its wood is used for furniture, boarding, matches… It has been traditionally over-harvested and is now a threatened species in many areas.  (Right image)

From above ground level to several meters above it, sits the understory or shrub layer. shrubs such as Agrostistachys longifolia, Gonocaryum gracile and Rinorea anguifera are common. Endemic shrubs include Arthrophyllum papyraceum  (Araliaceae), Clethra sumatrana (Clethraceae) and Saraca tubiflora (Fabaceae). This layer is also the abode of other shade tolerant understory flora, such as as fern like Cyathea moluccana and palms like Licuala ferruginea and Pinanga malaiana

The last layer is the ground layer, apart from the canopy this is the richest zone in terms of biodiversity. Covered in dense fallen leaves, broken branches, fallen trunks, and seedlings, many thousands of ants, termites, worms, larvae, and various types of bugs and creepy crawlies can be found within every square meter of soil. It is estimated that some parts of the rainforest floor receives as little as 2% of the sun’s rays, most of which would have been intercepted by the many layers above. Some of the most common plants species are Hanguana malayana, Labisia pumila, several orchids such as Cystorchis saceosepala and ferns like Lindsaea doryphora. It is also home to many very rare species that can only be found in scattered locations. These forests are for example famously known as habitat for the plant with the world largest flower, Rafflesia arnoldii and for the plant with the world’s tallest flowers, the endemic Amorphophallus titanium (Araceae), a magnificent arum that has flowers up to 2 m tall.

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