Moisture mediates temperature-growth couplings of high-elevation shrubs in the Tibetan plateau

Radial growth of high-elevation shrubs shows the high sensitivity to climate in dry regions, providing new evidence for drought-mediated dynamics of alpine woody plants. In mountains, shrubs forming their uppermost distributional limits (shrubline) are highly sensitive to increasing cold limitations upwards. However, the constraints imposed by a shorter growing season due to lower temperatures upslope could interact with drought stress in mountainous regions. Here, we aim to investigate that question by comparing radial growth patterns of shrubs in wet (forest region of Nangqian County, BZ hereafter) vs. dry (forest region of Leiwuqi County, LWQ hereafter) regions located on the eastern Tibetan plateau. Considering the climate trend in this area towards warmer and drier conditions, we hypothesize that growth of high-elevation shrubs will be mainly limited by low summer temperatures under sufficient soil moisture. To test this hypothesis, we collected ring-width data from 123 alpine shrubs (Salix oritrepha Schneid.) along an altitudinal gradient from 4200 to 4600 m a.s.l. Dendroecological analyses showed that shrub growth was positively related to mean June or July temperature during the past decades in both wet (BZ) and dry (LWQ) regions regardless of site elevation. By contrast, correlations between shrub growth and seasonal precipitation were not significant. Shrub growth trends were positive before 2010 but turned negative afterwards. Overall, radial growth of shrubs along the altitudinal gradients up to the shrubline could shift from being temperature to moisture limited if warming-induced drought stress intensifies.