Radial growth of alpine shrubs at wet sites can benefit from climate warming on the southeastern Tibetan Plateau

Key Messages

Radial growth of alpine willow shrubs showed contrasting sensitivity to climate warming depending on local soil moisture availability, being enhanced in wet sites.

Abstract

Secondary growth of alpine shrubs such as willow species (Salix spp.) may be enhanced by climate warming, but there could be also negative impacts due to warming-related drought stress. Willow shrubs appear from alpine treelines to mountain glaciers on the southeast Tibetan Plateau, thus proving an ideal setting to test whether their growth is improved by climate warming along wide elevational gradients. We tested this idea by sampling seven willow shrub (Salix gyamdaensis) sites spanning 4400–4800 m in southeast Tibet. A total of 171 ring-width samples were collected and measured. Detrended basal area increment (BAI) series were built for each site. The resulting BAI indices were correlated with NDVI and monthly climate variables. Shrub growth indices were positively associated with summer NDVI, suggesting the regional greening is associated with increased shrub growth. Willow growth also showed positive responses to summer temperatures at five out of the seven study sites. However, shrub growth in the other two sites showed either a positive response to the minimum temperature of the previous December or a negative response to August temperature. In the last site, climate warming may reduce willow growth because of a decrease in soil moisture availability. Climate warming favors the growth of alpine shrubs and subsequent regional greening in sites where soil moisture availability is not limiting, but this growth enhancement is contingent on site aridity.