Large net CO2 loss from a grass-dominated tropical savanna in south-central Brazil in response to seasonal and interannual drought

Abstract:

The savanna vegetation of Brazil (Cerrado), accounts for 20-25% of the total land cover of Brazil; however, little is still known about the spatial and temporal variations in mass and energy exchange of Cerrado. Our goal was to evaluate the seasonal and interannual variations in CO₂ exchange for a grass-dominated Cerrado stand over a three-year period and to elucidate the biophysical controls on CO₂ exchange. We hypothesized that soil water availability (SWA) would be a key control over the direction and magnitude of CO₂ exchange. Field measurements were conducted Mar 2011- Dec 2013 at the Fazenda Miranda, located 15 km SSE of Cuiabá, Mato Grosso, Brazil. Net CO₂ (NEE), latent, and sensible heat fluxes were quantified using eddy covariance. NEE was strongly affected by SWA; however, other biophysical variables were also important in controlling temporal patterns of CO₂ exchange. Step-wise regression revealed four variables that explained 69% of the seasonal and interannual variation in gross primary production (GPP), and GPP increased as a function of the MODIS enhanced vegetation index (EVI), solar radiation (R_s), and soil water content (θ_s) but declined as a function of the vapor pressure deficit (VPD). Similarly, stepwise regression identified six variables that explained 78% of the variance in ecosystem respiration (R_eco), and R_eco was positively related to air temperature, the EVI, R_s, and θ_s and negatively correlated with VPD and precipitation. Consistent net CO₂ losses during the dry season caused a cumulative net CO₂ loss of 840 gC/m² over the 3 year study period. Periods of accumulated net loss occurred when the difference between precipitation and evapotranspiration declined, which typically occurred between March and November of each year. The annual net CO₂ losses are in contrast to rates of NEE reported for other cerrado ecosystems, and the differences between our results and those reported for other cerrado ecosystems may reflect the low average annual rainfall that occurred during the study period and/or the low water-holding capacity of the shallow, coarse-grained soils typical of the Cuiaba Basin. These data indicate that seasonal and interannual drought can lead to high rates of CO₂ emission from Brazilian savanna ecosystems, especially those with low soil water holding capacity