| Climate change is causing more frequent and intense climatic extremes.Tree mortality associated with extreme droughts has been increasingly documented in arid and semi-arid regions.Drought-induced tree dieback has strong impacts on forest structure,function and ecosystem services,and poses a major challenge to forest sustainability in anticipative warming-drying climate scenarios.Knowledge on the responses of trees to environmental stresses,especially long-term continuous information on physiological adjustments,is not easily acquired but valuable to project tree growth performance and decline risk under future climate scenarios.Mongolian pine(Pinus sylvestris var.Mongolica)is one of the most important tree species used for afforestation in the Three-North regions(North,Northeast and Northwest)of China,but Mongolian pine shelter forests are prone to mortality associated with increased extreme droughts under projected climate change.Understanding the long-term responses to climate extremes is of great relevance for taking appropriate practices for the sustainability-oriented management of Mongolian pine plantations in droughtprone environments.By combining standard dendrochronological methods and quantitative wood anatomy,we studied the radial growth and stem xylem anatomy of Mongolian pine trees planted in a water-limited area of northern China in response to extreme drought events in a long time series,so as to determine the hydraulic strategy of Mongolian pine trees in response to extreme droughts.The main research results are as follows:(1)Water-related climatic variables acted as the main factors regulating the radial growth and xylem tracheid size of the plantation-grown Mongolian pine trees in waterlimited area,with wet conditions improving tracheid size and radial growth while dry conditions decreasing radial growth and increasing tracheid density.The extreme drought in 2003 almost stagnated the trunk radial increment.However,immediately after this climatic extreme,the stem growth sharply increased to a level substantially higher than that preceding the extreme drought,which was probably attributable to the reduction of inter-tree competitions for resources due to the mortality of a large number of trees caused by the extreme drought event and ultimately resulted in a sharp increase in radial growth of the remaining trees after the extreme drought event.(2)In response to an extreme drought event,there was synchronicity between radial growth and stem xylem anatomical traits,extreme drought significantly reduced xylem theoretical hydraulic conductivity and almost stagnated the trunk radial increment.We found that tree ring width of Mongolian pine was significantly proportional to the number of earlywood tracheid cell layers but was uncorrelated with tracheid size metrics.(3)There was a significant negative correlation between anatomical traits of xylem water transport efficiency and safety during an extreme drought event,demonstrating the trade-off between tree hydraulic efficiency and safety.The radial growth of plantation-grown Mongolian pine trees during an extreme drought event can be significantly regulated by adjusting xylem structural characteristics.This study revealed the xylem hydraulic regulation strategy of Mongolian pine plantation under extreme droughts of a water-limited area in northeast China.Based on tree-ring and wood anatomical chronologies,it was determined that the water-related climate variables play important roles in effecting inter-annual fluctuation of xylem tracheid size and radial growth.The response patterns of wood anatomical characteristics obtained from tree-ring series reflect the plasticity adjustment of trees to avoid hydraulic failure during extreme droughts,as well as the trade-off between hydraulic efficiency and safety.This study can provide some theoretical support and scientific reference for formulating reasonable forest management measures in waterlimited area under the background of climate warming and drying. |
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