Stem and Branch Respiration in Trees:

Monitoring CO₂ Efflux on Organ Surfaces to Understand the Internal Process for Respiration

Abstract

Respiration of woody organs, such as stem, branch and coarse root (woody respiration), is an important component in the carbon cycle of forest ecosystems due to their huge biomass. CO₂ efflux on the surface of a woody organ (E_A) has historically been used as a direct measure of woody respiration. However, there are large environmental variations within woody organs different from other organs, such as leaves and fine roots. Not only the respiration of living tissues but also a series of physical processes, such as CO₂ diffusion and CO₂ dissolution into xylem sap, are included under E_A. Recent studies have shown that there is a substantial portion of CO₂ dissolved in xylem sap and transported away from the site of origin. Consequently, it is of necessity to clarify the internal processes of the organs, such as tissue respiration, anatomical and biochemical characteristics, and CO₂ dynamics, for evaluating woody respiration, besides the traditional approaches focusing on the relationship between E_A and external factors, e.g., temperature and organ size. In this review, I compile the previous knowledge for stem and branch respiration to understand the importance of the internal process, and to organize future issues.