Extrapolating base-line trunk shrinkage reference equations across olive orchards

Abstract

Maximum daily trunk shrinkage is a common measurement in irrigation scheduling of fruit trees. But the strong relationship between these measurements and the environment severely limit field applications. Reference baselines are the solution for understanding the influence of environmental conditions. Nevertheless, the extrapolation out of the original conditions is not clear. The aim of this study was to compare several approaches to estimate a reference baseline in an olive orchard where there were no previous data from other seasons. Two orchards, separated 60m, with different tree density were used. Orchard 1 had greater tree density than orchard 2, though the age and the cultivar were the same. Trunk diameters of both orchards were similar but the crown volume of orchard 2 was slightly lower than orchard 1. The current reference baselines of maximum daily trunk diameter in both orchards were not significantly different between them (p<0.05). In orchard 1, the previous reference baseline was calculated in a 5-year study (the so called multi-seasons approach). The multi-seasons approach was not significantly different in slope but it was in the y-interception to the current reference baselines in both orchards (p<0.05). This approach over-estimated the values in both orchards. Two additional approaches were tested. These latter approaches used data before massive pit hardening to estimate the current reference baseline. One of them used the early data to estimate a complete reference baseline (the so-called early approach). The other (the so-called y-early approach) used the same data only to estimate the y-interception and assumed that the slope was the same as in the multi-seasons approach. The early approach under-estimated the value of maximum daily trunk shrinkage. The early-y approach provided a satisfactory estimation of the reference baseline and improved those obtained with the multi-seasons approach. The limitations and uses in irrigation scheduling are also discussed.