Combined tree-ring width and wood anatomy chronologies provide insights into the radial growth and hydraulic strategies in response to an extreme drought in plantation-grown Mongolian pine trees

Mongolian pine (Pinus sylvestris var. Mongolica) is among the most commonly used coniferous afforestation tree species for building shelter forests in vast areas of northern China, yet it is at a high risk of mortality in the context of increased frequency of extreme drought events associated with anticipative climate change. Understanding the long-term ecophysiological response to climate extremes is of great relevance for taking appropriate practices for the sustainability-oriented management of Mongolian pine plantations in drought-prone environments. Here, the hydraulic strategy behind the response of the radial growth of Mongolian pine trees planted in a water-limited area of northern China to the extreme drought event from 2001 to 2004 was retrospectively investigated by combining standard dendrochronological methods and quantitative wood anatomy. We found that water-related climatic variables acted as the main factors regulating the radial growth and xylem tracheid size of the Mongolian pine trees at the study site, and the extreme drought significantly reduced xylem theoretical hydraulic conductivity and almost stagnated the trunk radial increment. 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. Around the extreme drought event, tree ring width was significantly proportional to the number of earlywood tracheid cell layers but was uncorrelated with tracheid size metrics, alongside a clear hydraulic safety-efficiency trade-off among the stem xylem traits obtained from tree ring series. This study stresses the role that xylem hydraulics play in modulating the radial growth of plantation-grown Mongolian pine trees during extreme droughts by adjusting xylem structural characteristics and hence hydraulic transport functions.