Investigating the accuracy of tropical woody stem <scp> CO ₂ </scp> efflux estimates: scaling methods, and vertical and diel variation

Stem CO₂ efflux (EA) significantly contributes to autotrophic and ecosystem respiration in tropical forests, but field methodologies often introduce biases and uncertainty. This study evaluates these biases and their impact on scaling EA at the stand-level. Diel and vertical patterns of EA were investigated, along with the accuracy of estimating stem surface area from allometric equations vs terrestrial light dection and ranging (LiDAR) scanning (TLS) in Maliau Basin Conservation Area, Sabah, Malaysian Borneo. Diel EA exhibited no uniform pattern due to inter-tree variability, but results suggest measuring EA before 15:00 h. EA was significantly higher on buttresses and above the first major branching point, but vertical variations in EA did not impact stand-level EA when stem surface area was accurately estimated. Allometric equations underestimated total stem surface area by c. 40% compared with TLS, but applying a site-specific correction factor yielded a similar stand-level EA and total stem surface area to TLS. This study provides guidance for measuring EA in the field and suggests that measuring at one time point and one height along the stem can produce accurate results if conducted using the correct time frame and if stem surface area is accurately estimated.