The upper timberline in the Himalayas, Hindu Kush and Karakorum: A review of geographical and ecological aspects

Abstract

Based on comprehensive evaluations and analyses of existing literature and data sources, a review of geographical and ecological aspects of the upper timberline in the Himalayan mountain system is presented. Upper timberline elevations increase along two gradients: a NW-SE gradient corresponds to higher temperature sums at same elevations along the mountain arc. However, mean temperatures of the warmest month are higher at timberlines in the NW, which develop at lower elevations than expected since extreme winter cold, later snow melt, and shorter growing seasons overcompensate the advantage of higher summer temperatures. A second gradient is developed in peripheral-central direction from the Himalayan south slope to the Great Himalayan range and the Tibetan highlands. Increasing timberline elevations along this gradient are related to the combined effects of continentality and mass-elevation, both leading to higher temperature sums. Juniperus tibetica stands in S-Central Tibet even reach 4600-4800 m, the most elevated timberline in the northern hemisphere. Potentially natural timberline elevations are higher at south-facing slopes compared to north-facing slopes. The difference in altitudinal position is up to several hundred meters. The pronounced effects of exposure to solar radiation result in a much higher utilization pressure at sunny slopes, in particular with regard to pastoral use. South-facing slopes have since long been subjected to massive human impacts throughout the mountain arc so that natural conditions are hard to reconstruct. The depression of upper timberline may amount to more than 500 m, largely depending on the local/ regional utilization potential of alpine pasture areas. With regard to physiognomy, high coniferous forests give way to medium-sized broadleaved tree stands and finally to a krummholz belt. This is the dominant timberline pattern on shady slopes, whereas remnant open coniferous forest stands on sunny slopes dissolve into isolated patches or single crippled trees higher up. Along the NW-SE gradient, north-facing slopes show a floristic change from deciduous Betula- to evergreen Rhododendron-dominated upper timberlines, which must be attributed to decreasing winter cold and strongly increasing humidity levels. Juniperus spp. are the principal timberline tree species on south-facing slopes throughout the mountain system. Relationships of Himalayan timberlines to other ecological conditions and processes such as carbon balance, freezing and frost drought, soil temperatures, wind, snow cover, soils, regeneration, etc., are still largely unexplored. The state of the art is summarized in this paper. More systematic, interdisciplinary timberline research in the Himalaya is strongly needed to better understand how complex ecological and socio-economic processes are expressed in present spatial and physiognomic timberline structures. © 2005 Springer-Verlag Berlin Heidelberg.