Prolonged limitation of tree growth due to warmer spring in semi-arid mountain forests of Tianshan, northwest China

Based on radial tree growth measurements in nine plots of area 625 m(2) (369 trees in total) and climate data, we explored the possibly changing effects of climate on regional tree growth in the temperate continental semi-arid mountain forests in the Tianshan Mountains in northwest China during 1933-2005. Tree growth in our study region is generally limited by the soil water content of pre-and early growing season (February-July). Remarkably, moving correlation functions identified a clear temporal change in the relationship between tree growth and mean April temperature. Tree growth showed a significant (p < 0 : 05) and negative relationship to mean April temperature since approximately the beginning of the 1970s, which indicated that the semi-arid mountain forests are suffering a prolonged growth limitation in recent years accompanying spring warming. This prolonged limitation of tree growth was attributed to the effects of soil water limitation in early spring (March-April) caused by the rapid spring warming. Warming-induced prolonged drought stress contributes, to a large part, to the marked reduction of regional basal area increment (BAI) in recent years and a much slower growth rate in young trees. Our results highlight that the increasing water limitation induced by spring warming on tree growth most likely aggravated the marked reduction in tree growth. This work provides a better understanding of the effects of spring warming on tree growth in temperate continental semi-arid forests.