Direct Measurements of Evapotranspiration and Carbon Fluxes in a Grazed Eucalypt Savanna Using Flux Tower and Sap Flow Sensors

Abstract

Direct measurements of evapotranspiration (ET) and carbon fluxes in grazed tropical savannas are less common than other ecosystems, despite their extensive geographical extent, socioeconomic value and significant contribution to global water and carbon budgets. Concurrent measurements of vegetation transpiration and ET are rare, and questions remain on ET partitioning across multiple vegetation types that coexist in a savanna. Knowledge of the water and carbon balance is needed to enable sustainable management of extensively grazed savannas today and into the future. To address these knowledge gaps, we measured water and carbon fluxes in a Eucalypt savanna woodland on a cattle station in northeastern Australia.The grazed savanna proved to be a strong carbon sink, with a two-year net ecosystem productivity of 4655 kgC ha-1. Average water use efficiency, a key ecosystem property linking carbon and water, was 2.4 gC m-2 mm-1 and peaked during the dry season (8 gC m-2 mm-1) when grasses were largely absent and woody vegetation maintained high rates of gross primary production per unit of ET, demonstrating substantial carbon assimilation for little water use. Evapotranspiration tracked seasonal vegetation dynamics closely, with average wet and dry season rates of 2.4 mm day-1 and 0.7 mm day-1, respectively. Trees and shrubs used very little water, with average dry season transpiration rates of 0.2 mm day1, increasing to 0.4 mm day-1 in the wet season. Critically, this study presents the first evidence that managed dry tropical savanna ecosystems can act as carbon sinks under moderate grazing pressure. Findings from this study contribute significantly towards improving the representation of tropical savannas in modelling and remote sensing products, through increasing our understanding of tree-shrub-grass interactions in the water and carbon balance.

Keywords: ecohydrology, Evapotranspiration, Eddy covariance, sap flow, tropical, savanna