Greater risk of hydraulic failure due to increased drought threatens pine plantations in Horqin Sandy Land of northern China

Abstract

Widespread decline and mortality of shelterbelt forests in water-limited regions of northern China have been occurring over the past few decades and may become more severe under the influence of climate change; however, our understanding on how droughts affect tree performance in a long-term context and the underlying water-related physiological mechanisms are still limited. Here, using a combination of tree-ring analysis and measurement of water-related functional traits, we investigated patterns of long-term radial growth and the underlying physiology in Pinus sylvestris var. mongolica (Mongolian Pine) and Pinus tabuliformis (Chinese Pine) in relation to variation in environmental water availability in Horqin Sandy Land (HSL), i.e. a key area of land desertification control in northern China where the two species are commonly used for afforestation. Particularly, we studied the association between tree-ring growth responses to drought and xylem hydraulics of the two species at the two sites of HSL that differ in climate regimes, examining the potential impacts of climate warming-related aridification on tree performances using a space-for-time substitution approach. Tree-ring analyses revealed greater degrees of recent growth decline, lower resilience to drought, and more severe tree hydraulic limitations in both pine species at the semiarid site compared to the semi-humid site. Tree-ring width indices of both species in the drier site showed significant positive correlations with May-July total precipitation of the current year and negative correlations with mean temperature of the same period, whereas such correlations were not significant at the wetter site. Lower stem hydraulic conductivity and higher degrees of xylem embolism in the drier site suggest that both pine species suffer from stronger hydraulic limitation when the environment gets drier. Moreover, Mongolian pine with a significantly higher hydraulic conductivity consistently showed higher basal area increments than Chinese pine at both sites. Our results suggest that Mongolian pine, the currently most widely used trees to create shelterbelt plantations in desertified areas of northern China, is more susceptible to decline than Chinese pine as the climate becomes hotter and drier, although the former species has a higher growth rate than the latter one. Our investigation provides a mechanistic explanation for the commonly observed decline and dieback of pine trees in water-limited environments of northern China and suggests that Chinese pine may be a better choice to replace Mongolian pine as the main tree species in plantations of the study area with a changed climate regime in the future.