The impact of Asian summer monsoon on forest distribution patterns, ecology, and regeneration north of the main Himalayan range (E-Hindukush, Karakorum)
The range and precipitation effectiveness of the Indian summer monsoon in the NW of the subcontinent is subject to contentious discussions for several decades. This topic was mostly treated from a climatological point of view. Only recently, integrated approaches are pursued using the indicator functions of the vegetation for monsoonal humidity influx in more detail. In this paper, new climatological findings are correlated with recent vegetation ecological and phytogeographical studies in order to assess the dependence of closed sub-humid forests on monsoonal precipitation north of the main Himalayan range. General climatic conditions in the region are characterized by a SW-NE-directed gradient of decreasing humidity. They result from the position in a transitional area of two genetically different precipitation regimes: Extratropical westerly circulation and tropical monsoon regime. This paper analyzes the relationships of monsoonal humidity influx and distribution patterns, synecology, synsociology, and regeneration of major forest communities (Pinus wallichiana-, Picea smithiana-communities) towards the dry limits of forests in the region. Small-scale distribution patterns of forest stands correspond to the spatial differentiation of water vapour transport from shallow monsoonal depressions over the crestlines of West Himalaya and Karakorum. This transport is modified by the topography of high mountain chains, acting as barriers against moist labile air masses from the south. The gradual decrease of humidity along the SW-NE-gradient is reflected by synsociological characteristics of communities, i.e. by their differentiations into mesic and xeric subassociations. The forests appear to be relict ecosystems of moister periods of the Holocene. Under presently drier conditions in an extreme subtropical arid high mountain environment, the maintenance of an equable forest climate is indispensable for successful regeneration. Thus, further forest degradation will lead to an irreversible loss of forest habitats.