Vertical variation in wood CO2 efflux is not uniformly related to height: measurement across various species and sizes of Bornean tropical rainforest trees.

Limited knowledge about vertical variation in wood CO2 efflux (R-wood) is still a cause of uncertainty in R-wood estimates at individual and ecosystem scales. Although previous studies found higher R-wood in the canopy, they examined several tree species of similar size. In contrast, in the present study, we measured vertical variation in R-wood for 18 trees including 13 species, using a canopy crane for a more precise determination of the vertical variation in R-wood, for various species and sizes of trees in order to examine the factors affecting vertical variation in R-wood and thus, to better understand the effect of taking into account the vertical and inter-individual variation on estimates of R-wood at the individual scale. We did not find any clear pattern of vertical variation; R-wood increased significantly with measurement height for only one tree, while it decreased for two more trees, and was not significantly related with measurement height in 15 other trees. Canopy to breast height R-wood ratio was not related to diameter at breast height or crown ratio, which supposedly are factors affecting vertical variation in R-wood. On average, R-wood estimates at individual scale, considering inter-individual variation but ignoring vertical variation, were only 6% higher than estimates considering both forms of variation. However, estimates considering vertical variation, while ignoring inter-individual variation, were 13% higher than estimates considering both forms of variation. These results suggest that individual measurements at breast height are more important for estimating R-wood at the individual scale, and that any error in R-wood estimation at this scale, due to the absence of any more measurements along tree height, is really quite negligible. This study measured various species and sizes of trees, which may be attributed to no clear vertical variation because factors causing vertical variation can differ among species and sizes.