BOTANICAL CHARACTER OF THE LEUSER PARK 377 IX. On the special botanical character of the Leuser Park and vicinity, with Emphasis on the High MountainBlang Vegetationof Northern Sumatra W.J.J.O. de Wilde & B.E.E. Duyfjes Nationaal Herbarium Nederland,Universiteit Leiden branch, P.O. Box 9514, 2300 RA Leiden, The Netherlands Summary ‘Blang’ is the Gayo name for the extensive, open, grassy, lowland savanna land of their area in Aceh, N Sumatra. By analogy, the open areas with heath-like vegetation high in the mountains are called ‘mountain blang’. There somewhat and believed are two intergrading types: a wet a dry one. They are to since immemorialtimes have been caused by regularly burning by hunters, but our ex- planation is that they have (at least largely) developed foredaphic reasons, i.e. are due to prevailing climatic conditions on very poor, leached-out and almost impermeable kaoli- nitic soils. Theirextension was possibly enlarged by occasional frost and burning, which may account for the sometimes, but not at all always, sharp demarcation with the sur- rounding stunted forest. The wet-type blang probably is maintained by the absence of which stand water-soaked conditions of the woody species cannot the permanent peat soil under the boggy vegetation. It is advanced here that the mountain blangs, although in total they may cover only a of few square kilometres, merit the status of a high-ranking vegetation formation their own, next to the subalpine ericoid forest, rather than being included in the latter as ac- cepted for the whole of Sumatra. This is supported by: 1. its floristic composition, in- cluding a numberof ‘northern’ as well as ‘eastern’ elements and a remarkable number of very localendemic species on the old, largely non-volcanic N Sumatran mountains, com- pared to those over base rock of volcanic origin as found on the high mountains of the rest of the Barisan Range, possibly in connection with isolation by the relatively recent volcanic Toba eruptions; 2. the singular physiognomy of the high mountain blang veg- restricted etation, located on a number of high mountains in areas in N Sumatra; 3. the edaphic, non-anthropogeneous origin, both of the dry and the wet types of the blang. Introduction While compiling a brief survey of the vegetation of the Gunung Leuser National Park for 'Leuser, a Sumatran sanctuary' (De Wilde & Duyfjes, 1996) and treating the bio- diversity of this natural reserve, our attention was drawn again to the remarkable sub- alpine heath-like vegetations unique to several high mountains of northern Sumatra, the so-called mountain Similar known from high blang (Fig. 1). vegetation types are other in- outside Malesia for instance the of high tropical areas and (see many pictures New Guinea vegetation types in Van Royen, 1979,and theircomposition in Hope, 1976, Chap- ter 8), but how these relate to the Sumatra type is not studied here. 378 FLORA MALESIANA BULLETIN 12(7/8), 2001 important most the of names with (black), occur blangs mountain 1978. high Park, where Leuser altitude Plan m 2500 Management above Strien, Park van Leuser N.J. the from of areas Adapted Approximate summits. 1. Fig. mountain BOTANICAL CHARACTER OF THE LEUSER PARK 379 covered The Leuser Park mainly consists of a complicated rugged mountain complex which be described the forest formations by forest, can by using generally accepted altitudinal climatic zonation. This is of criteria for according to also one the leading defining the vegetations accepted for the vegetation map of Sumatra. On the map for northern Sumatra (Laumonier et al., 1987) the vegetation of the top zone of the high mountains above 2700 m were qualified as "low forest and thicket of the ericaceous zone", leaving out the blangs as a separate vegetation type. The blang vegetation was discovered by Van Steenis during the Leuser expedition of 1937 (Van Steenis, 1938) and was further sampled by us in more recent times during extensive expeditions in the summit areas of several high mountains within the Leuser Park (De Wilde & Duyfjes, 1994). THE BOTANICAL EXPLORATION TOURS BY C.G.G.J. VAN STEENIS IN THE GAYO LANDS, ACEH, 1937 The scientific interestin the flora of northern Sumatra, especially that of the non-volca- of Steenis his nic mountains Aceh, was initiated by Van prompted by study on the origin of the mountain floraof Malesia(Van Steenis, 1934-1936). This suggested that the flora of the high mountains ofAceh might have played a major role. Accordingly, in 1937 Van Steenis explored Mt Leuser as well as some other high mountains in the vicinity, all above 3000 m altitude. In 1934 during a preliminary orientation tour in the area he visited the surroundings of Lake Takengon and climbed some higher mountains nearby. These, although not reaching 3000 m altitude, yielded some most exciting species with distinct Swertiabimaculata a northerly affinity, e.g. Pyrola sumatrana (Pyro.), (Gent.), and Schoepfia fragrans (Olac.), suggesting even more spectacular records to be discov- ered on the other still higher summits in the area. Van Steenis (1938) extensively related his explorations in a paper with many beauti- ful, clearand illustrativephotographs ofthe vegetation and species, which unfortunately has not become generally known. He was much perturbed when Merrill (1940) preemp- tively, so he felt, published several of 'his' new species and awaiting identificationsof his collections he full and detailed of the own by true specialists never came to a report scientific results of the 1937 expedition. In this brief analysis of the flora and vegetation we have, on his own urgings, in- cluded his observations which modifiedhis ideas of 1936 on the cause of the pathways along which mountain plants moved into Malesia. ON THE CAUSE AND CHARACTER OF THE HIGH MOUNTAIN BLANG There has been some dispute regarding the reason for the presence of the high mountain blang, that is, the ratherextensive low heather-like vegetation on flat or slightly sloping plains in the summit areas of the larger mountain complexes in the Gayo Lands. Van Steenis was the first botanist to see and study it during his 1937 expedition, although the blangs will have been known long before by local people, and later by the military and the geographers. According to his report (Van Steenis, 1938) blangs were caused by the regular or incidental burning by local hunters during dry spells. Indeed, in 1937, an extensive, recently completely burnt forested slope and ridge of some 15 kilometres 380 FLORA MALESIANA BULLETIN 12 (7/8), 2001 side of because soil taken from length at the East Mt Leuser, was seen. Also, samples underneaththe blang appeared to be similar to those taken from adjoining scrub forest, Van Steenis argued that in the blang areas the humus-rich layer of top soil had been and washed rains after the of the scrub that burnt away by burning original forest, so conditions for regrowth of taller vegetation had become unfavourable. This is the now generally accepted explanation of most of similarextensive low vegetation areas of high elevation in New Guinea and elsewhere. Hoogerwerf (1939), in his preliminary zoological report on the same expedition of 1937, doubted Van Steenis' assumptions and explanation, reasoning that the washing of the of the almost horizontal terrains away topsoil layer usually blang was unlikely, and he rather believed that the blang originated on areas with naturally poor soils. This latterexplanation was corroboratedby the extensive research by the pedologist Van Beek (1982), who found that the soil of the blang is extremely infertile and that its special vegetation should be attributed to edaphic properties, particularly to the lack of plant nutrients. These soils, largely over quartzitic base rock, appeared to be very old. They lay generally more than 2.5 m thick, and consist of deeply, intensely, weathered-out almost impermeable kaolinitic clay, sometimes mixed with some quartzitic sands, largely formed already before the geological upheaval of the mountains. The thick peaty soil layer under the wet blang started developing over these poor, impermeable, soils more recently, apparently some 8000 years ago, after the last Pleistocene ice covering. Whitten et al. (1984) comprehensively reported on a visit to the Mt Kemiri summit area and commented on the observed slow rate of decay of plant materials and flora attributedthe of and regeneration. They blang ("a complex grassy, heathy, boggy areas"), for soil the held Van at least the wet parts, to factors, alluding to more recent notion, by Steenis (personal communication to Whitten and the authors), that in the Malesian re- which stand inundation. Whitten gion no (sub)alpine tree species occur can permanent beautiful colour of Other et al. (1984, opposite p. 396) gave a photograph peaty blang. of mountain habitat Van Steenis photographs the N Sumatran blang were published by (1938), Hoogerwerf (1939, f. 89), Kern (1976) in his treatment of Cyperaceae for Male- sia, Van Beek (1982), and De Wilde & Duyfjes (1994, 1996). From the foregoing we can conclude that the cause and character of the high moun- in the N is tain blang areas, including the wet and the dry facies, Gayo Lands, Sumatra, complex, and largely set by the following circumstances: for of flattish 1. The presence in the Gayo Lands (unique Sumatra) old partially glaciated land forms in the cold of the old non-volcanic mountain top zones high complexes, with soils mainly over quartzitic rock and shales. strata flat lands 2. The equally unique presence at high altitudes of old, thick over these of kaolinitic soil of low which low heath- impermeable putty-like very fertility, on like thick of soil could vegetation, partly forming water-soaked layers peat develop; the be and/or account of from peat may ombrogeneous topogeneous on seepage higher slopes.