Summer temperature drives radial growth of alpine shrub willows on the northeastern Tibetan Plateau

Shrub willows (Salix species) are widespread beyond the latitudinal and altitudinal tree lines. Their ring width has been shown to be a reliable ecological indicator for changes in the harsh cold conditions in the Arctic, but little is known on their growth in alpine conditions. The shrubby Salix oritrepha grows above the treeline on the northeastern Tibetan Plateau (TP), making it an interesting woody species to explore responses of alpine communities to ongoing climate warming in this area. Since precipitation increases with increasing elevation (until 4670 m) in the study area, we hypothesize that the growth of S. oritrepha is mainly constrained by cold summer temperature. We sampled 35 S. oritrepha individuals above the juniper treeline (4200 m), and took basal wood cross sections for dendrochronological analyses. Few missing rings were detected at the shoot base when serial sectioning was applied. Ring width (RW) and basal area increment (BAI) standard chronologies were established. We found that BAI reflected a stronger climatic signal than RW. The radial growth was constrained by low July to August temperatures. We expect that climate warming would enhance the growth of alpine willows, which could alter the services provided by these high-elevation ecosystems.