Abstract:

Objective: This study aims to clarify the short-term characteristics of sap flow and transpiration dynamics in Quercus variabilis stands under light thinning, as well as its impact on tree growth, so as to provide a theoretical basis for water management and sustainable management of Q. variabilis forests. Method: The 30-year-old Q. variabilis stands located on the southern foothills of the Taihang Mountains in northwestern Henan Province were subjected to two treatments of thinning and no thinning (control). From April to September 2023, sap flux density of trees under both treatments was continuously monitored using thermal dissipation probes. Tree-level biological factors (such as diameter at breast height, sapwood area, and stand density) and meteorological variables [including air temperature, relative humidity, solar radiation, and vapor pressure deficit (VPD)] were simultaneously recorded. With the above measurements, the effects of thinning on individual tree sap flux density and stand-scale transpiration were evaluated, and their relationships with environmental drivers were analyzed. Result: 1) The diurnal course of sap flux density in Q. variabilis exhibited a unimodal pattern. Except in July, thinning did not significantly alter the onset time or peak timing of the diurnal sap flux density dynamics. Sap flux density in thinned trees was consistently higher than that in unthinned trees throughout the growing season, and this difference first increased and then decreased with seasonal progression, indicating that the thinning effect was seasonally dependent. 2) After thinning, the explanatory power of radiation (R²=0.52) and VPD (R²=0.42) for sap flux density variation increased, indicating an increased sensitivity of sap flow to abiotic environmental drivers. The explanatory power of radiation on changes in for sap flux density was higher than that of VPD, suggesting that sap flux density in this region may be more strongly influenced by light-related processes rather than primarily constrained by atmospheric dryness. 3) There was no significant correlation between sap flux density and tree diameter (P>0.05), indicating that individual tree size was not the primary driver of sap flow variation under the current stand structure and soil moisture conditions. 4) Although the total sapwood area of the thinned stand was approximately 27% lower than that of the control, the seasonal total transpiration was still about 9% higher than that of the control, which may be associated with the increased sap flux density at the individual tree level. Conclusion: Under the light thinning intensity condition applied in this study, Q. variabilis stands exhibit enhanced sap flow and transpiration responses in the short term, indicating a potential regulatory effect on stand water use processes. These findings provide a scientific basis for the coordinated optimization of water regulation and structural adjustment in Q. variabilis and other deciduous broadleaf forests.

Key words: thinning, sap flux density, Quercus variabilis, transpiration, sustainable management