||Van Steenis conducted the research in 1937 (de Wilde W.J.J.O and B.E.E. Duyfjes, 1996), dividing the plant areas in Gunung Leuser National Park in several zones, namely:
- Tropical Zone (including Colline zone, located 500 – 1000 mdpl). Tropical Zone is a heavily forested area covered by various types of wooden stands with large diameter and height up to 40 meters. The tree or wooden stand is used as a tree of liana plants and interesting epiphytes, such as orchids, and others.
- The transition zone from the Tropica Zone to the Colline Zone and the Sub-Montane Zone is characterized by an increasing number of beautiful and distinct flowering plants due to altitude differences. The higher a place the more trees are reduced, the liana type begins to disappear and more and more rattan species are found.
- Montane Zone (including sub montane zone, located 1000 – 1500 mdpl). The montane zone is a montane forest. Wood stands are no longer too high only ranging from 10 to 20 meters. There is no more liana plant species. Moss covered many wooden or tree stands. The humidity of the air is very high and almost every time it is covered in fog.
- Sub Alphine Zone (2900 – 4200 mdpl), is an Ercacoid forest zone and no longer trees. This forest is a thick layer of mixture of dwarf trees and shrubs with some towering umbrella trees (families Ericacae) as well as some types of tundra, orchids and moss.
Almost the entire area of Gunung Leuser National Park is covered or dominated by Dipterocarpaceae forest in Tropical Zone.
Orchid: Species of Orchid Forest in Telagah Village Gunung Leuser National Park Langkat Regency is one type of lowland rainforest in North Sumatra which based on field observations has high macroepifit diversity. This forest has tall trees and moist air that is a suitable habitat for epiphytic growth. Yulinda (2004) reported that in Tangkahan National Park, Gunung Leuser National Park, Langkat regency, there are 47 types of macroepifit belonging to 4 classes, 10 orders, 20 families, and 32 genera.
The family of Orchidaceae is the most numerous family of species found in 9 species, followed by 6 species of Polypodiaceae family, 4 species Aspleniaceae family, Davalliaceae family of 2 species, Lomariopsidaceae family, Nephrolepidaceae, Zingiberaceae, Melastomataceae, and Lindsaeaceae 1 each type.
The high number of species of the Orchidaceae family is probably due to abiotic factors that are appropriate for its growth and development, with temperatures of 21.3′ C still good enough for the types of orchids to grow.
Pewarta (1981) states that this condition is still within the appropriate temperature range for orchid growth that is between 21-35′ C. Anwar et al. (1984) states that the orchid seeds are usually easily dispersed by squirrels or birds, quite resistant against direct sunlight, and rapid growth of seedlings. This causes the species of orchids have a wide spread. Furthermore Comber (2001) added that the family Orchidaceae in Sumatra including the most types of which there are 1111 species. According to Rifai (1993), that the number of species of orchids that live as epiphytes in mountainous wilderness trees is very large, especially from the types of Bulbophyllum. Monk et al., (2000), states that in the lower mountain forests found many epiphytic orchids, especially Corybas orchids, Corymborkis, and Malaxis. Ruhana (2003), reported that there are 25 genus (70 species) of epiphytic orchids at the Ketambe Ekosistem Leuser Research Station.
It is estimated there are about 3,500 species of flora.
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