Relationships between MDS, Soil, and Weather Variables for Topaz Apple Tree Cultivated in Coarse-Textured Soils

Abstract

The purpose of this paper is to test the hypothesis of the existence and strength of relationships between maximum trunk daily shrinkage (MDS), soil matric potential (SMP) at various soil depths, and some weather variables in grown apple trees and coarse-textured soils under continental-temperate climate conditions. If found, the relationships could be used in identification of early and mild water stress for irrigation scheduling. The experiment was carried out in a 7–8-year-old apple tree orchard plot with the Topaz cultivar grafted on M9 rootstock. Micrometric trunk diameter fluctuations of three trees were automatically measured by dendrometers, and SMP was measured continuously with resistance blocks installed in three profiles at four depths: 0.2, 0.4, 0.6, and 0.8 m. Many wetting-drying cycles were monitored during two growing seasons in 2016 and 2017. The strongest and highly significant relationships are obtained between MDS and maximum daily vapor pressure deficit (VPD) and daily reference evapotranspiration (ETo). There are also significant relationships between MDS and SMP for all depths studied; however, these relationships are not very strong. Multiple linear regression equations between MDS, SMP at all the four depths, and either maximum VPD or ETo generally improve r2 from the simple regression equations obtained with each of the same variables. The MDS threshold of 0.4 mm approximately corresponds to a SMP range of about −60 to −40 kPa. Relationships found between MDS and SMP should be used with caution for irrigation purposes, combined with SMP observations and after rigorous field experiments.