Seasonal variation of night-time sap flow of a young olive orchard: the unconsidered process for evapotranspiration estimations

Night-time sap flow (S_n), with transpiration as an important proportion of it at moderate to high vapor pressure deficits (VPD), is an important unconsidered factor that contributes significantly to total evapotranspiration (ET) of horticultural and fruit tree crops. This nocturnal process will be likely increased in a climate change scenario, with increases in night-time temperatures at higher rates compared to diurnal temperatures. The aim of this study was to characterise night-time water consumption over a commercial drip-irrigated young olive orchard (Olea europaea L. 'Arbequina') located in Pencahue valley, Maule Region, Chile (35°23'LS; 71°44'LW; 96 m a.s.l.) and its dynamics within the 2010/11 season. Four olive trees were selected for sap flow measurements using the Compensated Heat Pulse Velocity technique (CHPV). The canopy conductance (G_c) was calculated by inverting a modified Penman-Monteith equation. The aerodynamic conductance (g_a) was calculated using an algorithm of the two-layer model proposed by Shuttleworth and Wallace (1985). An eddy covariance system was installed in the orchard to measure real ET. Results showed that S_n varied between 1.79 and 3.09 L tree^-1 night^-1 depending mainly of the atmospheric demand. The diurnal sap flow (S_d) measured was from 7.1 to 18.2 L tree^-1 day^-1. Parabolic shape curves described the relationship between S_n and G_c. Furthermore, the S_n/S_d ratio changed between 16 and 25% depending on the weather conditions, which it is not currently considered in ET models. It is clear that there is a nocturnal flow of water from soil to plant and water movement within the plant, but it is not yet clear the partition between the transpiration process and hydraulic redistribution. However, the former could be more likely due to the highly significant correlations found between VPD and S_n.