Disentangling critical drivers of stem CO2 efflux from Pinus elliottii trees in Subtropical China

Abstract Stem CO 2 efflux ( E s ) plays a critical role in forest carbon budgets and net ecosystem CO 2 exchanges, but there is still a significant knowledge gap on E s and its controlling factors in subtropical forests where forest productivity and transpiration are both very high. In this study, E s and the possible controlling factors such as stem temperature ( T s ), sap velocity ( v s ), and other climatic variables were simultaneously measured in a Pinus elliottii plantation of Subtropical China from January 2014 to July 2015. Temporal dynamics of E s followed similar trends as T s at a 1-cm depth with bell-shaped curves. The monthly E s values were significantly higher during the fast-growing season (April to October) than in the slow-growing season (November to next March). However, temperature sensitivity ( Q 10 , the relative increase of E s with a 10 °C rise in temperature) fluctuated throughout the entire year without a clear pattern. Significant and exponential relationships were found between E s and T s , with correlation factors higher during the slow-growing season than in the fast-growing season. Additionally, the coefficients of determination of E s to stem temperature were highly divergent with respect to tree size during the fast-growing season but not in the slow-growing season. The residuals (Δ E s ), calculated as the difference between the modeled fluxes ( E p ) based on night-time data at zero sap flow and the measured fluxes ( E m ) during the daytime when sap flow occurred, became more prominent during the fast-growing season. Thus, significant and positive correlations were observed between the ratio of Δ E s to E p and v s during the fast-growing season ( r 2  = 0.59, p v s , sap flows could potentially reduce the measured CO 2 efflux to up to 25% of those predicted values on temperature alone during the daytime. Our results clearly demonstrated that temperature was not sufficient to quantify E s , and thus, the effect of sap flow on E s must be integrated into any models simulating stem respiration and carbon budgets in forest ecosystems.