Potential and limitations of improving olive orchard design and management through modelling

Data from Spanish and Italian olive orchards with different cultivars (‘‘Manzanilla’’, ‘‘Frantoio’’, ‘‘Leccino’’, ‘‘Razzola’’ and ‘‘Taggiasca’’) growing in different environments (southern Spain; north, centre and south of Italy) were used to illustrate how models on water use and photosynthetic behaviour of the olive tree can be useful tools for choosing pruning intensity and canopy shape for optimum water use and carbon assimilation. Both original and published data on olive physiological behaviour were used to illustrate limitations of model performance due to both inadequate input variables and poor description of the processes involved. We observed differences on leaf water status between cultivars and locations, likely due to differences in soil characteristics related to soil matric potential and soil hydraulic conductivity. This suggests that the volumetric soil water content may not be the best variable for characterizing the soil water status when modelling, and highlights the importance of considering specific environmental conditions before extrapolating data from the literature to specific orchards. We evaluated the importance of considering the reduction in olive photosynthetic capacity under stress conditions, driven by a reduction in leaf mass per area and leaf nitrogen content. A modelling exercise was then made for a ‘‘Manzanilla’’ olive tree at the Spanish orchard, by combining a gas exchange model for olive leaves with a model able to simulate the spatial distribution of radiation and leaf – gas exchanges within the olive canopy as a function of canopy structure, canopy microclimate, and physical and physiological leaf properties. Simulated values for two different canopy shapes showed that a top-open spherical canopy, typical in the area, improved water use efficiency, as compared to a spherical canopy closed at the top. The model also showed the value of leaf area density that must be left after pruning for optimizing carbon assimilation relative to water consumption. Abbreviations: A: leaf net CO2 assimilation; Ap: tree photosynthesis; Ci: internal CO2 concentration; Ep: tree transpiration; ETc: crop evapotranspiration; gs: stomatal conductance; h: soil matric potential; LA: leaf area; LAD: leaf area density; LMA: leaf mass per area; n: number of replicates; Na: nitrogen content per unit leaf area; P: probability; PAR: photosynthetic active radiation; r: correlation coefficient; R: actual soil water content; Rmin: minimum soil water content measured during the experiments; Rmax: soil water content at field capacity; REW: relative extractable water; RMSE: root mean-square error; Vcmax: photosynthetic capacity expressed as maximum activity of Rubisco; VPD: vapour pressure deficit of the air; WUE: water use efficiency; y: volumetric soil water content; Cpd: predawn leaf water potential