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

Question 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. Location Altitudinal treeline (similar to 4000 m a.s.l.) in the Sagarmatha (Mt. Everest) National Park (Central Himalaya, Eastern Nepal). Methods We assessed the relationships between ring widths and monthly precipitations and mean temperatures, and analysed cellulose stable isotopes (delta 13C and delta 18O) and their derived C discrimination (Delta 13C), and intrinsic water use efficiency (iWUE) inAbies spectabilisandBetula utilisat the Himalayan treeline. Results Growth ofA. spectabilisstrongly depended on summer temperatures, whereas that ofB. utilison spring precipitation. delta 13C and iWUE increased with time in both species, especially inA. spectabilis. The long-term decrease in Delta 13C was accompanied by an increase in delta 18O in both species, thus suggesting an increase in photosynthetic efficiency rather than a stronger stomatal control of transpiration. Conclusions 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.