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

Abstract

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 (~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 (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. Results 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. 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.

Climate-Driven Differences in Growth Performance of Cohabitant Fir and Birch in a Subalpine Forest in Dhorpatan Nepal

“…Several studies across the Nepal Himalaya have reported lower value of Rbar. For example, Rbar values of A. spectabilis in Sagarmatha [10,11,17], Rolwaling [12], and B. utilis chronologies in Sagarmatha [10,11,27], Mustang [14], Langtang and Manasalu [27], all had similar correlation coefficients compared to our results. Pandey et al [11,51] in eastern Nepal, and Dawadi et al [26] in central Nepal, reported much lower Rbar values for both species compared to our results.…”

“…For example, Harsch et al [9] reported 57% of worldwide treelines (including latitudinal) experiencing warming have advanced upslope and seen increases in recruitment. The Himalayas are experiencing greater warming than the global mean [7,8,[10][11][12]. Forecasted warming is expected to bring dynamic changes to temperature sensitive subalpine plant communities [13,14].…”

“…While there have been varying site-and species-specific growth responses to climate, a more consistent finding is that A. spectabilis growth and treeline shift rate is higher than B. utilis [20]. Warm growing season temperature has often been found to favor the growth of A. spectabilis [10,11,13,17] in the eastern part of the Nepal Himalayas. However, radial growth of the same species from Rolwaling [12] and Mustang [19] was limited by spring moisture deficit.…”

“…However, radial growth of the same species from Rolwaling [12] and Mustang [19] was limited by spring moisture deficit. There is a general consensus that warming induced moisture stress during the spring disrupts growth in B. utilis [10,11,14,26,27]. Although mixed stands of A. spectabilis and B. utilis are often found in the subalpine zone, very few studies have compared both species [10,11,13,18,20], and their dynamics of climate sensitivity have not been sufficiently addressed.…”

“…There is a general consensus that warming induced moisture stress during the spring disrupts growth in B. utilis [10,11,14,26,27]. Although mixed stands of A. spectabilis and B. utilis are often found in the subalpine zone, very few studies have compared both species [10,11,13,18,20], and their dynamics of climate sensitivity have not been sufficiently addressed. We hypothesize that current climate trends have favored A. spectabilis compared to B. utilis in mixed stands.…”