Does growing on a slope affect tree xylem structure and water relations?

Variations in slope, exposure, relief and substrate over a short distance and their influences on plant function are poorly understood. We investigated the influences of soil hydrological characteristics on internal stem structure and hydraulic properties of downy oak (Quercus pubescens Willd.) growing along a hill slope. Increment wood cores were extracted from the base and at breast height (BH) of tree stems. Relative wood water content (W(c)) and wood density (D(w)) were measured in the sapwood. Wood compression strength (delta) in the longitudinal direction was measured with a fractometer. Thin sections were cut from the transversal face of each core and vessel lumen area (V(A)) was measured and xylem theoretical hydraulic conductivity (L(th)) estimated over the sapwood. Topsoil volumetric water content (theta(v)) was determined around trees and the hydrological behavior of the slope was studied through field surveys. Data were used as input to a hydrological model to simulate topsoil water distribution along the slope. Results showed that theta(v) tends to decrease with increasing altitude. Groundwater levels were lower upslope than downslope, and results from the hydrological model confirmed these trends. Mean W(c) at the base of each tree decreased significantly with increasing altitude, whereas at BH, no differences were found along the slope. There was a significant positive relationship between W(c) measured at the tree base and theta(v) along the hill slope, but not for W(c) measured at BH. Values of D(w) and delta measured at both stem positions increased significantly with increasing altitude and decreasing theta(v). Significant negative relationships were found between delta and theta(v) measured at the stem base and at BH. At both stem positions, delta was closely related to D(w) and L(th). Vessel lumen areas at BH and the stem base were significantly regressed with altitude, theta(v), D(w) and delta. Xylem theoretical hydraulic conductivity at both stem positions was negatively related to altitude and soil theta(v), but only L(th) measured at the stem base was negatively regressed with D(w). The results are discussed in the context of how tree position along a hill slope influences water uptake and internal xylem structure.