Application of eddy covariance to determine ecosystem-scale carbon balance and evapotranspiration in an agroforestry system

The inclusion of belts of trees in the agricultural areas of south-western Australia is gaining popularity, through perceived benefits in water use, biodiversity and carbon sequestration. However, water use and carbon assimilation are difficult to quantify at the ecosystem scale. In this research, we investigate the application of eddy covariance in a ‘belt and alley’ system. Footprint modelling indicated that the proportion of the signal from the tree belts was similar to the proportion of tree belts in the ecosystem. Eddy covariance units were installed in two adjacent fields: one containing belts of four-year-old oil mallee Trees 10 m wide planted 60 m apart; and one containing agricultural crops. Tree belts had little impact on wind direction or its standard deviation for any measurement height, and stationarity, integral turbulence and energy balance closure were similar for the two fields. Persistent upward wind flows were observed for wind directions aligned with the tree belts, suggesting the possibility of advection. For our experimental site, removal of data for winds aligned with the tree belts had a negligible impact on fluxes of carbon dioxide and water. During summer and autumn, the field containing oil mallees used 27 mm more water, and assimilated 0.93 Mg CO2/ha more than the field without trees. Both daytime and night time CO2 fluxes were greater in magnitude for the oil mallee field than the control field during summer. Water use by the trees was lower than other fields in the region where herbaceous perennials had been grown, and also lower than other estimates based on sap flow measurements. Further research will be necessary to determine the impact of spatial variability of water use on groundwater recharge at the catchment scale. Extra carbon storage associated with tree belts may increase their economic attractiveness to farmers in the region, but the impact of respiration after summer rainfall requires further investigation.