A Leaf Based Classification for Himalayan Forests

Abstract

The topographic heterogeneity of the Himalayas leads to a mosaic of growing conditions which limits the utility of traditional systems of forest classification. A leaf phenology based classification is suggested as an alternate to help understand patterns of Himalayan forests. Leaf traits such as longevity, time of new flush and shedding and type of leaf, are linked with ecosystem processes and show more consistent patterns across Himalayan landscapes. While low temperatures and a short growing season limit growth at extremely high altitudes and near the timberline, drought stress is the variable that most influences forest structure in the majority of Himalayan forests. Precipitation occurs through two major weather systems: the monsoon which is the dominant climate system for much of the Himalayas, and the westerlies, which are active in the Western part and northern fringes of the Himalayas. The pre-monsoon dry season is the most important in determining forest type and leaf phenology. Based on these differing weather systems and rainfall deficits, the Himalayas can be broadly divided into five categories based on type of leaf and when it is shed. Trees which leaf out in the pre-monsoon period, with approximately one year leaf life span and almost simultaneous leaf fall (e1 type), can best benefit from the monsoonal climate.  This type lies on a continuum between pre-monsoon deciduous (dp) and evergreen species with multiple leaf flush (e2). At higher altitudes and dryer sites conifers (e3 type) dominate while winter deciduous (dw) are most abundant near the treeline. The Himalayas lie at the confluence of the Palearctic and Indo-malayan biogeographic zones and the influence of these different vegetation types is evident. Despite the presence of a floristic base and diversity of deciduous species Himalayan forests are largely evergreen as climatic conditions favour this evergreen strategy.