Vertical variations in wood CO₂ efflux for live emergent trees in a Bornean tropical rainforest

Difficult access to 40-m-tall emergent trees in tropical rainforests has resulted in a lack of data related to vertical variations in wood CO₂ efflux, even though significant variations in wood CO₂ efflux are an important source of errors when estimating whole-tree total wood CO₂ efflux. This study aimed to clarify vertical variations in wood CO₂ efflux for emergent trees and to document the impact of the variations on the whole-tree estimates of stem and branch CO₂ efflux. First, we measured wood CO₂ efflux and factors related to tree morphology and environment for seven live emergent trees of two dipterocarp species at four to seven heights of up to ~40 m for each tree using ladders and a crane. No systematic tendencies in vertical variations were observed for all the trees. Wood CO₂ efflux was not affected by stem and air temperature, stem diameter, stem height or stem growth. The ratios of wood CO₂ efflux at the treetop to that at breast height were larger in emergent trees with relatively smaller diameters at breast height. Second, we compared whole-tree stem CO₂ efflux estimates using vertical measurements with those based on solely breast height measurements. We found similar whole-tree stem CO₂ efflux estimates regardless of the patterns of vertical variations in CO₂ efflux because the surface area in the canopy, where wood CO₂ efflux often differed from that at breast height, was very small compared with that at low stem heights, resulting in little effect of the vertical variations on the estimate. Additionally, whole-tree branch CO₂ efflux estimates using measured wood CO₂ efflux in the canopy were considerably different from those measured using only breast height measurements. Uncertainties in wood CO₂ efflux in the canopy did not cause any bias in stem CO₂ efflux scaling, but affected branch CO₂ efflux.