The respiratory ecophysiology of woody stems and branches in temperate forest trees

Fundamental questions regarding woody tissue respiration are unresolved despite the importance of woody tissue respiration to whole-tree carbon budgets. In this study, I sought to improve the understanding of woody tissue respiration in forest trees by investigating the effects of stem hydraulics, specifically xylem sap flow rates and the dissolved CO2 concentration of sap, on CO2 efflux from woody stems and branches. Ecophysiological measurements for this study were conducted in Quercus rubra (Northern red oak) in an aggrading deciduous forest in southeastern New York, United States and Dacrydium cupressinum (rimu) in a lowland podocarp rainforest in South Westland, New Zealand.; Respiration in the woody stems of forest trees was an important component of the carbon budgets in both these forests. For instance, stem respiration in D. cupressinum accounted for an annual carbon flux to the atmosphere of 1.22-1.29 Mg C ha-1 and consumed 6.7-7.2% of GPP in the New Zealand podocarp forest; whereas, estimates of woody tissue respiration in three Quercus stands ranged from 1.14-1.72 Mg C ha-1 and accounted for 10.3-17.0 % of GPP. Concurrent measurements of stem CO2 efflux and sap velocity in D. cupressinum indicated that xylem transport of respiratory CO 2 results in declines in stem CO2 efflux during periods of sap flow. This xylem transport of CO2 was estimated to account for 10.6% of woody tissue respiration over a 24 h period and 13-38% during periods of peak transpiration. Some evidence for interactions between stem hydraulics and stem CO2 efflux was also observed in Q. rubra but was not as significant as that observed in D. cupressinum . Instead, stem CO2 efflux in Q. rubra was strongly related to local respiratory activity during the dormant season, when CO2 efflux was correlated with inner bark respiratory potential, and the growing season, when CO2 efflux was correlated with stem diameter growth. These findings suggest that further research aimed at integrating tree water relations and the respiratory ecophysiology of woody tissues will enhance our understanding of intra- and inter-specific variation in stem CO 2 efflux and improve our ability to scale stem CO2 efflux to the tree- and stand-level.