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

Abstract

Interannual variations in CO2 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 (Reco). Here, we analyse observations of the components of Reco (leaves, live and dead woody tissue, and soil) to provide first estimates of changes in Reco 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 0.5m2m-2yr-1 in leaf area index, we estimate a reduction in respiratory CO2 efflux from leaves and live woody tissue of 1.0 (+/-0.4) tCha-1yr-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 CO2 efflux of 0.13-0.18tCha-1yr-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 CO2 efflux of 2.0 (+/-0.5) tCha-1yr-1. Thus, the majority of short-term respiratory responses to drought point to a decline in Reco, an outcome that contradicts recent regional-scale modelling of NEP. NEP varies with both GPP and Reco but robust moisture response functions are clearly needed to improve quantification of the role of Reco in influencing regional-scale CO2 emissions from Amazonia.

Figure 1

Variation with leaf area index (LAI) in CO₂ efflux from above-ground woody tissue (R_wc). Graph redrawn from Meir & Grace (2002), using new data for site 7 (R_wc, Cavaleri et al. 2006; LAI, D. Clark 2007, personal communication) and data for two additional tropical rainforests, sites 8 and 9, near Manaus (C. Amazonia) and at Reserva Jaru (SW Amazonia), respectively (McWilliam et al. 1993; Kruijt et al. 1996; Meir 1996; Chambers et al. 2004). LAI was obtained directly for sites 1 and 5–8 and estimated indirectly for sites 2–4 and 9; the LAI for site 8 is taken from a nearby destructive harvest (McWilliam et al. 1993) because this estimate includes all tree sizes. R_wc was obtained from measurements of woody tissue CO₂ efflux at all sites. Original $R_{\text{adj}}^2=0.85$, p=0.003, n=7 (Meir & Grace 2002); revised fit $R_{\text{adj}}^2=0.93$, p<0.0001, n=9 (R_wc=29.1e^(0.45.LAI)). TRF is tropical rainforest. Solid line, model; open square, site 1: Sahelian shrub, Niger; filled triangle, site 2: Ponderosa pine, USA; open diamond, site 3: Black spruce, Canada; plus sign, site 4: TRF, Cameroon; filled circle, site 5: deciduous broadleaf, USA; filled diamond, site 6: TRF, Brazil (Manaus); filled square, site 7: TRF, Costa Rica; open circle, site 8: TRF, Brazil (Manaus); open triangle, site 9: TRF, Brazil (Jaru).

Figure 2

Moisture constraints on R_s and R_eco. (a) The relationship between soil water potential and soil respiration (R_s, μmol m⁻² s⁻¹; R²=0.43, p<0.001); data are from a soil drought experiment at an eastern Amazon rainforest, Caxiuana (Sotta et al. 2007). (b) Net ecosystem exchange (NEE) in a transitional forest, Sinop, Mato Grosso (Vourlitis et al. 2005). During the four-month dry season (May–August), R_eco is more sensitive to moisture stress than is maximum photosynthesis, A_max (a decline of 28% in R_eco versus 5% in A_max; Vourlitis et al. 2005). Data show changes in diel and daily NEE of CO₂. Positive values indicate CO₂ release (mean NEE (±s.d.) of 30-min data (filled circle) and average daily NEE by month (open circles); horizontal solid line is diel NEE=0 and horizontal dashed line is daily NEE=0).