© 2007 Heron Publishing—Victoria, Canada
Does growing on a slope affect tree xylem structure and water relations?
Nadia Barij (1), Alexia Stokes (1–3), Thom Bogaard (4) and Rens van Beek (4,5)
1. Laboratoire de Rhéologie du Bois de Bordeaux (Mixed Unit INRA/CNRS/Université Bordeaux I), Domaine de l’Hermitage, 69, rte
d’Arcachon, 33612 Cestas cedex, France / 2. Present address: INRA, LIAMA-CASIA, P.O. Box 2728, Haidian District, 100080 Beijing, China / 3. Corresponding author (stokes@liama.ia.ac.cn; stokes@lrbb3.pierroton.inra.fr) / 4. Faculty of Geosciences, Utrecht University, P.O. Box 80115, 3508TC Utrecht, The Netherlands / 5. IBED - Physical Geography, University of Amsterdam, Nieuwe Achtergracht 166, NL 1018 WV Amsterdam, The Netherlands / Received February 24, 2006; accepted May 12, 2006; published online February 1, 2007
Summary
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 (Wc) and wood density (Dw) were measured in the sapwood. Wood compression strength (δ) in the longitudinal direction was measured with a fractometer.
Thin sections were cut from the transversal face of each core and vessel lumen area (VA) was measured and xylem theoretical hydraulic conductivity (Lth) estimated over the sapwood. Topsoil volumetric water content (θv) 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 θv tends to decrease with increasing altitude. Groundwater levels were lower upslope than downslope, and results from the hydrological
model confirmed these trends. Mean Wc 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 Wc measured at the tree base and θv along the hill slope, but not for Wc measured at BH. Values of Dw and δ measured at both stem positions increased significantly with increasing altitude and decreasing θv. Significant negative relationships were found between δ and θv measured at the stem base and at BH. At both stem positions, δ was closely related to Dw and Lth. Vessel lumen areas at BH and the stem base were significantly regressed with altitude, θv, Dw and δ. Xylem theoretical hydraulic conductivity at both stem positions was negatively related to altitude and soil θv, but only Lth measured at the stem base was negatively regressed with Dw. The results are discussed in the context of how tree position along a hill slope influences water uptake and internal xylem
structure.
Keywords:
hydraulic conductivity, Quercus pubescens, soil hydrology, vessel lumen, wood density.
