Pre- and post-harvest evapotranspiration, carbon exchange and water use efficiency of a mature peach orchard in semi-arid climate

Better knowledge of the evapotranspiration and carbon exchange of fruit trees is needed to optimize the trade-off between water use and carbon assimilation and to better understand the role of agriculture in the biogeochemical cycles. In this work, we measured water and carbon fluxes with eddy covariance and transpiration with sap flow in a drip-irrigated peach orchard of 70% ground cover located in southern Spain for 2 years. The empirically measured crop coefficient (K-c) under good watering conditions in the summer ranged from 1 to 1.1. The daytime net ecosystem exchange (NEE) flux of the orchard averaged 30 g CO2 m(2) day(-1) during the period of maximum activity in July. The daytime ecosystem water use efficiency (WUE) of the orchard reached a minimum in late June, flattened around 4 g CO2 L-1 throughout the summer, and increased in autumn, but was unaffected by fruit removal or post-harvest irrigation reduction imposed by the farm (30% reduction). The response of instantaneous peach ecosystem WUE to VPD was also investigated. Both K-c, NEE, leaf water potential and stomatal conductance decreased sharply after harvest. Transpiration data from some purposely over-irrigated experimental trees demonstrated that the post-harvest alterations we found were not caused by fruit removal, but are result of mild water stress originated by the irrigation reduction. Hence, the often-observed alterations in water relations after harvest in well-watered trees were not observed in this experiment. This work adds insight on peach irrigation efficiency and on the contribution of orchards to agricultural carbon budgets.