Effects of different thinning intensities on Pinus tabuliformis transpiration and forest-water relations in the Loess Plateau of China

Thinning can improve the soil water conditions of forested land, decrease potential water stress, and enhance the drought resistance of trees by adjusting the stand density. However, the effect of thinning intensity on water consumption of plantations in semiarid regions remains unclear, interfering with the identification of forest-water relationships under thinning. In this study, we diagnosed the effects of five thinning intensities (0%, 15%, 30%, 45%, and 60%) on tree canopy transpiration ( E c ) and growth in a 44-year-old Pinus tabuliformis Carr. plantation on the semiarid Loess Plateau, China. Our findings demonstrated that increasing thinning intensities improved soil water. Stand E c was reduced with increasing thinning intensity and reached the minimum at 45% thinning. Regarding the environmental controls, relative extractable water ( REW ) was the dominant factor affecting E c from 0% to 45% thinning, while vapor pressure deficit ( VPD ) dominated the variation of E c under 60% thinning. Increased REW caused by increasing thinning intensity promoted the driving effect of VPD on E c and reduced the effects of REW . Canopy stomata activity was mainly controlled by VPD , and the stomatal sensitivity to VPD decreased significantly after heavy thinning (60%). Additionally, the mean diameter at breast height of individual trees in the stand under the thinning treatment was significantly larger than the control group (i.e., 0%). Notably, the basal area was the largest, and the proportion of E c to precipitation after removing canopy interception was the lowest at 45% thinning. These findings suggest that thinning can efficiently improve soil water conditions, and 45% thinning is an appropriate management that promotes tree growth at the stand level. This study elucidates how thinning intensity affects tree water use in Pinus tabuliformis plantations on the semiarid Loess Plateau, providing a reference for the selection of thinning schemes for plantations in similar dryland areas.