The fate of assimilated carbon during drought: impacts on respiration in Amazon rainforests

Abstract

Interannual variations in CO ₂ exchange across Amazonia, as deduced from atmospheric inversions, correlate with El Niño occurrence. They are thought to result from changes in net ecosystem exchange and fire incidence that are both related to drought intensity. Alterations to net ecosystem production (NEP) are caused by changes in gross primary production (GPP) and ecosystem respiration ( R _eco ). Here, we analyse observations of the components of R _eco (leaves, live and dead woody tissue, and soil) to provide first estimates of changes in R _eco during short-term (seasonal to interannual) moisture limitation. Although photosynthesis declines if moisture availability is limiting, leaf dark respiration is generally maintained, potentially acclimating upwards in the longer term. If leaf area is lost, then short-term canopy-scale respiratory effluxes from wood and leaves are likely to decline. Using a moderate short-term drying scenario where soil moisture limitation leads to a loss of 0.5 m ² m ⁻² yr ⁻¹ in leaf area index, we estimate a reduction in respiratory CO ₂ efflux from leaves and live woody tissue of 1.0 (±0.4) t C ha ⁻¹ yr ⁻¹ . Necromass decomposition declines during drought, but mortality increases; the median mortality increase following a strong El Niño is 1.1% ( n =46 tropical rainforest plots) and yields an estimated net short-term increase in necromass CO ₂ efflux of 0.13–0.18 t C ha ⁻¹ yr ⁻¹ . Soil respiration is strongly sensitive to moisture limitation over the short term, but not to associated temperature increases. This effect is underestimated in many models but can lead to estimated reductions in CO ₂ efflux of 2.0 (±0.5) t C ha ⁻¹ yr ⁻¹ . Thus, the majority of short-term respiratory responses to drought point to a decline in R _eco , an outcome that contradicts recent regional-scale modelling of NEP. NEP varies with both GPP and R _eco but robust moisture response functions are clearly needed to improve quantification of the role of R _eco in influencing regional-scale CO ₂ emissions from Amazonia.