Combining sap flow and trunk diameter measurements to assess water needs in mature olive orchards

Sap flux (Q) and trunk diameter variation (TDV) are among the most useful plant-based measurements to detect water stress and to evaluate plant water consumption. The usefulness of both methods decreases, however, when applied to species that, like olive, have an outstanding tolerance to drought and a remarkable capacity to take up water from drying soils. Evidence shows that this problem is greater in old, big trees with heavy fruit load. Our hypothesis is that the analysis of simultaneous measurements of Q and TDV made in the same trees is more useful for assessing irrigation needs in old olive orchards than the use of any of these two methods alone. To test our hypothesis, we analysed relations between Q, TDV, midday stem water potential (Ψ_stem), relative extractable water and atmospheric demand in an olive orchard of 38-year-old ‘Manzanilla’ trees with heavy fruit load. Measurements were made during one irrigation season (May-October), in fully irrigated trees (FI, 107% of the crop evapotranspiration, ET_c, supplied by irrigation), and in trees under two levels of deficit irrigation (DI60, 61% ET_c; DI30, 29% ET_c). Time courses of Q and TDV measured on days of contrasting weather and soil water conditions were analysed to evaluate the usefulness of both methods to assess the crop water status. We calculated the daily tree water consumption (E_p) from Q measurements. For both DI treatments we calculated a signal intensity by dividing daily E_p values of each DI tree by those of the FI tree (SI−Ep). We did the same with the maximum daily shrinkage (MDS) values (S_I−MDS). Neither SI−Ep nor S_I−MDS rendered useful information for assessing the crop water needs. On the contrary, the daily difference for maximum trunk diameter (MXTD) between each of the DI trees and the FI tree (D_MXTD) clearly indicated the onset and severity of water stress. A similar analysis with the E_p values, from which DEp values were derived, showed the effect of water stress on the water consumption of the trees. We concluded that the simultaneous use of D_MXTD and DEp values provides more detailed information to assess water needs in mature olive orchards than the use of Q or TDV records alone.