The root zone dynamics of water uptake by a mature apple tree

We report the results from a field experiment in which we examined the spat ial and temporal patterns of water uptake by a mature apple tree (Malus dom estica Borkh., `Splendour') in an orchard. Time Domain Reflectometry (TDR) was used to measure changes in the soil's volumetric water content, and hea t-pulse was used to monitor locally the rates of sap flow in the trunk and roots of the tree. We also measured the tree's distribution of root-length density and obtained supporting data to characterize the soil's hydraulic p roperties. The experimental data were used to examine the output of the WAV E-model (Vanclooster et al, 1995; Ecol. Model. 81, 183-185) in which soil w ater transport is predicted using Richards' equation, and where root uptake is represented by a distributed macroscopic sink term. When the surface soil layers were uniformly wet, 70% of the trees water upt ake occurred in the top 0.4 m of the root zone, in which approximately 70% of the tree's fine roots were located. When a partial irrigation was applie d to just one side of the root zone, the apple tree quickly shifted its pat tern of water uptake with an almost twofold increase in uptake from the wet ter soil parts and a corresponding reduction in uptake from the drier parts . The response of root-sap flow to irrigation was almost immediate (i.e. ro ot flow increased within hours of the irrigation). Following subsequent irr igations over the whole soil surface, TDR measurements revealed a surface-w ard shift in the pattern of water extraction, and root flow measurements re vealed a recovery in the uptake function of seemingly inactive roots locate d in the previously-dry soil. Via our root sap flow measurements, we observ ed two roots on the same tree locally responding quite differently to simil ar events of soil wetting. This observation suggests that there may be cons iderable functional variability across the apple root system. Our measureme nt-model calculations yielded similar results and stress the prime role pla yed by the plant in modifying the root zone balance of water. Following an irrigation or rainfall event, root uptake by apple appears to be more depen dent upon the near-surface availability of water than it is related to the distribution of fine roots.