The effect of monsoon shadow on forest ecology of the western and central Himalayas with focus on conifers

With regard to the exposure to monsoon rain (June to September), the Himalayas are split into two parts: (i) the outer moist ranges, directly exposed to monsoon thrust, generally receiving > 1500 mm annual precipitation, and (ii) the inner, monsoon-shadow dry ranges, generally with < 1000 mm annual precipitation. In the outer moist ranges, broadleaved forests are prominent, while the inner dry ranges are mostly dominated by conifers. The main objective of this study is to identify the monsoon-shadow effect on the (i) distribution of conifer and broadleaved tree species; (ii) structure and composition of forests; and (iii) phenology in relation to climate change, at an indicative level. The study employs a mixed method—mixture of analytical and review approaches. It is based on literature review, analysis of secondary data, and field surveys. The outer and inner ranges differ not only in annual precipitation, but also in the proportion of annual precipitation occurring during monsoon (June–Sep), the proportion being > 70% in outer ranges, and ~ 30% to 70% in inner ranges. The dominant conifers include species of Cedrus, Abies, Pinus, Picea, Tsuga, Larix and others, and they vary in their distribution in relation to outer and inner ranges. The PCA shows a strong climatic structuring of conifer species distribution along precipitation gradients. The first two principal components explain a substantial share of environmental variance (PC1 = 54.3%, PC2 = 20.5%), indicating that species distributions are largely structured by temperature, precipitation, and elevation-related factors. On average, plant species richness is significantly lower in the dry inner ranges (25.5 species per forest type) than moist outer ranges (52 species per forest type). Winter snowfall which is a conspicuous feature in the inner ranges provides melt water for plant growth, and protection to the seeds lying on ground for several months.