From tree to plot: investigating stem respiration and its drivers along a logging gradient in Sabah, Malaysian Borneo

Respiration by trees stems constitutes a substantial proportion of autotrophic respiration in forested ecosystems and has been estimated to contribute 12 – 25 % of total ecosystem respiration, yet little is known about its associated drivers at different spatial scales. Stems are the largest contributor to forest biomass and so the respiratory consumption of stems has the potential to considerably affect carbon budgets in forest communities. As logged and degraded forests are fast becoming the most dominant land-use type throughout the tropics, it is also important to contextualise stem respiration over land use gradients. In this study we quantified stem respiration at individual tree and plot scales in nine 1-ha plots over a gradient of heavily logged to old-growth forest in Malaysian Borneo. We investigated how logging intensity, forest structure, plant functional traits, and soil chemistry influence stem respiration in logged and old-growth forest plots at both scales. We found that, at individual tree level, stem respiration rate per unit stem area was significantly higher in logged than old-growth plots, and this was consistent within most diameter classes. At the 1-ha plot scale, however, total stem respiration did not differ between forest types: the higher stem respiration rate in logged plots was offset by the higher stem area in old-growth plots. At plot level, stem respiration was driven by forest structure and soil chemistry. We found that basal area was a strong predictor of stem respiration within both forest types at plot scale; for a similar basal area, logged plots exhibited a higher stem respiration rate. Partitioning stem respiration into its growth and maintenance components at plot scale highlighted how logged plots prioritise growth in response to intense light competition, as logged plots had significantly higher allocation to growth respiration, whereas old-growth plots prioritised maintenance and cell structure. Our analysis at individual tree scale reinforced these differing priorities, as stem respiration in logged plots was driven by plant traits associated with growth and wood anatomy, as opposed to within old-growth plots where stem respiration was driven by traits associated cell structure and maintenance. These results reflect the different strategies of resource allocation for trees growing in logged and old-growth plots and adds to the growing body of research on autotrophic respiration, the least studied component of forest carbon dynamics within a very understudied yet expanding land use.