Estimation of whole-stem respiration, incorporating vertical and seasonal variations in stem CO2 efflux rate, of Chamaecyparis obtusa trees

Abstract

We examined vertical and seasonal variations in stem respiration rates in a 50-year-old plantation of Japanese cypress, Chamaecyparis obtusa (Sieb. et Zucc.) Endl., in central Japan, and discuss a practical and precise method to scale a point-measured stem CO2 efflux rate up to whole-stem respiration. For five selected trees, stem CO2 efflux rates were measured at breast height (1.3 m) and at five or six points above breast height (at approximately 2 m intervals) every 1 or 2 months over two consecutive years. Daily total stem respiration rate (surface area basis) was greater inside the crown than below the crown, especially during the growing season. By incorporating the vertical profile of the respiration rate, annual whole-stem respiration was estimated for each sample tree (Ry). We then compared this estimate (Ry) with another estimate of annual whole-stem respiration (R′y) obtained using a conventional method; it is assumed that the area-based respiration rate at breast height is constant throughout the stem. The ratio of these two estimates (R′y/Ry) was usually less than 1, indicating that the assumptions used to calculate R′y underestimate annual whole-stem respiration. We found that R′y/Ry was negatively correlated with the ratio of crown length to tree height (crown ratio). These results suggest that annual whole-stem respiration in this C. obtusa plantation is substantially affected by the relative proportion of within-crown stem with higher respiratory activity. Methodologically, our results imply that incorporating the crown ratio into the conventional method would improve the accuracy of annual whole-stem respiration estimates.