Climate Trend and Treeline Dynamics in Nepal Himalaya

Abstract

Representing each site and species, six ring-width site chronologies dating back up to AD 1763 were developed from A. spectabilis and B. utilis. Response function analysis revealed that, depending upon the study sites, the radial growth of A. spectabilis was limited by temperature and or moisture stress. In SNP region it was more sensitive with summer temperature, while in MCA and RNP region, the growth was more sensitive with spring season temperature and moisture. The radial growth of B. utilis was limited mainly due to the moisture stress during spring season with positive relation with precipitation and negative relation with the temperature of the same season; however, in some sites it was due to insufficient temperature during growing period. The March–June average temperature and total precipitation of western Nepal was reconstructed for past over 170 years (1840-2013 AD), which showed non-significant long-term trend. The temperature reconstruction identified several periods of warming and cooling. The short cold episodes were observed around 1880s, and 1910s, while warm episodes were centered in 1870s, 1890s, and 1980s. Similarly, the reconstructed precipitation showed several wet and dry periods. The correlation between the reconstructed spring temperatures with indexes of sea surface temperature of the equatorial Pacific and extended multivariate ENSO Index revealed a significant negative correlation with monsoon and post monsoons seasons. The relationship with precipitation was in opposite direction than that with temperature. This relationship demonstrates that the climate of western Nepal has spatio-temporal linkages with global scale climatic variability. However, both the observed and reconstructed temperature from MCA region has weak relation with global climatic phenomenon suggesting some local effect in climatic condition. The different regeneration and migration pattern of studied species indicated that the treelines in Nepal Himalaya were also subject to changes in species composition. Population demography and growth response studies suggested that trees in Nepal Himalaya have a species-specific and site dependent response to climate change. Further studies at treeline ecotone incorporating climatic and non-climatic, biotic and abiotic factors will enhance our understanding on treeline dynamics in Nepal Himalaya in response to the rapid environmental change including climate change.