Stem respiration and growth in a central Amazon rainforest

Key message Annual stem CO 2 efflux increases with stem wood production rates and are inhibited by daily moisture stress. Abstract Tropical forests cycle a large amount of CO 2 between the land and atmosphere, with a substantial portion of the return flux due tree respiratory processes. However, in situ estimates of woody tissue respiratory fluxes and carbon use efficiencies (CUE W ) and their dependencies on physiological processes including stem wood production ( P w ) and transpiration in tropical forests remain scarce. Here, we synthesize monthly P w and daytime stem CO 2 efflux ( E S ) measurements over 1 year from 80 trees with variable biomass accumulation rates in the central Amazon. On average, carbon flux to woody tissues, expressed in the same stem area normalized units as E S , averaged 0.90 ± 1.2 µmol m −2 s −1 for P w , and 0.55 ± 0.33 µmol m −2 s −1 for daytime E S . A positive linear correlation was found between stem growth rates and stem CO 2 efflux, with respiratory carbon loss equivalent to 15 ± 3% of stem carbon accrual. CUE W of stems was non-linearly correlated with growth and was as high as 77–87% for a fast-growing tree. Diurnal measurements of stem CO 2 efflux for three individuals showed a daytime reduction of E S by 15–50% during periods of high sap flow and transpiration. The results demonstrate that high daytime E S fluxes are associated with high CUE W during fast tree growth, reaching higher values than previously observed in the Amazon Basin (e.g., maximum CUE W up to 77–87%, versus 30–56%). The observations are consistent with the emerging view that diurnal dynamics of stem water status influences growth processes and associated respiratory metabolism.