Seasonal drought shapes the relationship between stem CO efflux and belowground respiration in an even-age rubber plantation on Hainan Island, China.

Introduction: Stem CO efflux ( ) reflects the amount of photo-assimilated carbon released back into the atmosphere and plays a critical role in the carbon balance of tree and forest ecosystems. Despite previous studies indicating that a portion of stem CO originates from root respiration ( ), the seasonal dynamics of and its relationship with belowground respiration remain poorly understood, particularly in tropical regions characterized by distinct dry and wet seasons.

Methods: To address this gap, we investigated in tapping and non-tapped rubber trees, along with environmental factors and physiological traits (sap flow flux density, root respiration, and leaf area index) from 2018 to 2021.

Results: Our results showed that tapping activity increased the of rubber trees compared to non-tapped trees, with increases ranging from 10.37% to 233.66%. However, the magnitude of this increase varied between the dry and wet seasons. Although tapping enhanced the , it did not alter the seasonal pattern. Consequently, in both tapped and non-tapped rubber trees displayed an overall single-peak pattern, with significantly lower values during the dry season compared to the wet season, suggesting growth phenology primarily regulates seasonal dynamics. Structural equation modeling revealed that root respiration ( ), sap flow flux density ( ), and soil moisture at 50 cm depth as the primary drivers of the variations during the dry season. In contrast, soil moisture at 5 cm depth and air temperature ( ) were identified as dominant factors influencing in the wet season, with belowground respiration having a negligible influence.

Discussion: These results suggest that the relationship between and belowground respiration is environmentally sensitive and exhibits seasonal dependency.