Effects of climate change on treeline trees in Sagarmatha (Mt. Everest, Central Himalaya)

Tree growth at high altitude in the Himalayan region is limited by cold temperatures and also strongly influenced by the seasonality of the Asian monsoon. Understanding whether the ongoing increase in temperatures and changes in precipitation regimes in the Himalayan region can stimulate or limit tree growth is of particular importance to predict the local treeline dynamics. We assessed the relationships between ring widths and monthly precipitations and mean temperatures, and analysed cellulose stable isotopes (δ13C and δ18O) and their derived C discrimination (Δ13C), and intrinsic water use efficiency (iWUE) in Abies spectabilis and Betula utilis at the Himalayan treeline. Growth of A. spectabilis strongly depended on summer temperatures, whereas that of B. utilis on spring precipitation. δ13C and iWUE increased with time in both species, especially in A. spectabilis. The long-term decrease in Δ13C was accompanied by an increase in δ18O in both species, thus suggesting an increase in photosynthetic efficiency rather than a stronger stomatal control of transpiration. Climate change is progressively reducing the physiological limitations due to low temperatures and low spring precipitations at the Central Himalayan treeline, thus potentially facilitating a further altitudinal forest advance.