More tree growth reduction due to consecutive drought and its legacy effect for a semiarid larch plantation in Northwest China

Extreme climate has increasingly led to negative impacts on forest ecosystems globally,especially in semiarid areas where forest ecosystems are more vulnerable.However,it is poorly understood how tree growth is affected by different drought events.In 2006–2009,the larch plantations in the semiarid areas of Northwest China were negatively affected by four consecutive dry years,which was a very rare phenomenon that may occur frequently under future climate warming.In this study,we analyzed the effect of these consecutive dry years on tree growth based on the data of the tree rings in the dominant layer of the forest canopy on a larch plantation.We found that the tree-ring width index(RWI)in dry years was lower than that in normal years,and it experienced a rapidly decreasing trend from 2006 to 2009(slope=-0.139 year^(-1),r=-0.94)due to water supply deficits in those dry years.Drought induced legacy effects of tree growth reduction,and consecutive dry years corresponded with greater growth reductions and legacy effects.Growth reductions and legacy effects were significantly stronger in the third and fourth consecutive dry years than that of single dry year(p<0.05),which might have been due to the cumulative stress caused by consecutive dry years.Our results showed that larch trees experienced greater tree growth reduction due to consecutive dry years and their legacy effect,and the trees had lower recovery rates after consecutive dry years.Our results highlight that consecutive dry years pose a new threat to plantations under climate warming,and thus,the effect of climate extremes on tree growth should be considered in growth models in semiarid areas.

High-altitude tree growth responses to climate change across the Hindu Kush Himalaya

Aims Rapid warming at high altitudes may lead to a higher sensitivity in tree growth to temperature.The key factors constraining tree radial growth and to what extent regional tree growth has suffered from climatic changes are unclear.Methods Tree-ring width data were collected from 73 sites across the Hindu Kush Himalaya(HKH),including three dominant genera(Abies,Juniperus and Picea)at high altitudes over 3000 m.Dynamic time warping was introduced to develop subregional chronologies by considering the synchrony of annual tree growth among different sites.We quantified the contribution of the climate variables,and analyzed the spatiotemporal variation of the growth–climate relationship.Important Findings The site chronologies were grouped into three clusters,corresponding to the three distinct bioclimatic zones,i.e.the western HKH,central-eastern HKH and southeastern Tibetan Plateau(TP).Tree growth was positively correlated to winter and spring precipitation in the drier western HKH,and to winter temperature and spring precipitation in the humid southeastern TP.Tree growth was markedly constrained by the minimum temperature,especially in winter,with its importance increasing from the west toward the east.As shown by moving correlation analysis,the signal of winter temperature in tree growth was weakened in the western and central-eastern HKH,while it was enhanced in the southeastern TP following rapid warming since the 1980s.Our results highlight that continuous warming may cause forest recession due to warming-induced moisture deficit in the western HKH,but forest expansion in the southeastern TP.