Carbon exchange and water use efficiency of a growing, irrigated olive orchard

We measured eddy covariance fluxes of CO₂ and H₂O over a flat irrigated olive orchard during growth, in different periods from Leaf Area Index (LAI) of 0.3-1.9; measurements of soil respiration were also collected. The daily net ecosystem exchange flux (F NEE) was practically zero at LAI around 0.4 or when the orchard intercepted 11% of the incoming daily radiation; at the end of the experiment, with LAI of 1.9 (and the fraction of intercepted daily radiation close to 0.5), F NEE was around 10gCO₂ m⁻² day⁻¹. The night-time ecosystem respiration (R eco), calculated from eddy fluxes in well-mixed night conditions, show a clear but non-linear dependence with LAI; it ranged from 0.05 to 0.15mgCO₂ m⁻² s⁻¹ (in average), being the lower limit ideally close to the heterotrophic soil respiration at the site. The gross primary production flux (F GPP) was linearly related to LAI within the LAI range of this experiment (with 11gCO₂ m⁻² day⁻¹ increments per unit of LAI) and to the fraction of intercepted radiation. The maximum rates of F GPP (0.75mgCO₂ m⁻² s⁻¹) were obtained in the summer mornings of 2002, at LAI close to 1.9. F GPP was strongly modulated by vapour pressure deficit (VPD) through the canopy conductance, even in absence of water stress. Hence, especially in the summer, the maximum rates of carbon assimilation are reached always before noon. The daily course of F GPP shows a two-phase pattern, first related to irradiance and then to canopy conductance. The water use efficiency (WUE) was, in average, 3.8, 6.3 and 7gCO₂ L⁻¹ in 1999, 2001 and 2002, respectively, with maxima always in the early morning. Hourly WUE was strongly related to VPD (WUE=-10.25+22.52xVPD⁻⁰.³⁴). Our results suggest that drip irrigated orchards in general, and olive in particular, deserve specific carbon exchange and carbon budget studies and cannot be easily included in other biomes.