© 2004 Heron Publishing—Victoria, Canada
Converging patterns of uptake and hydraulic redistribution of soil water in contrasting woody vegetation types
F. C. Meinzer (1, 2), J. R. Brooks (3), S. Bucci (4), G. Goldstein (4), F. G. Scholz (5) and J. M. Warren (1)
1. USDA Forest Service, Forestry Sciences Laboratory, 3200 SW Jefferson Way, Corvallis, OR 97331-4401, USA / 2. Corresponding author (fmeinzer@fs.fed.us) / 3. US EPA/NHEERL, Western Ecology Division, Corvallis, OR 97333, USA / 4. Department of Biology, University of Miami, Coral Gables, FL 33124, USA / 5. Laboratorio de Ecología Funcional, Departamento de Ciencias Biológicas, Facultad de Ciencias Exactas y Naturales, Universidad
de Buenos Aires, Ciudad Universitaria, Nuñez, Buenos Aires, Argentina / Received April 1, 2003; accepted September 13, 2003; published online June 1, 2004
Summary
We used concurrent measurements of soil water content and soil water potential (Ψsoil) to assess the effects of Ψsoil on uptake and hydraulic redistribution (HR) of soil water by roots during seasonal drought cycles at six sites characterized
by differences in the types and amounts of woody vegetation and in climate. The six sites included a semi-arid old-growth
ponderosa pine (Pinus ponderosa Dougl. ex P. Laws & C. Laws) forest, a moist old-growth Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) forest, a 24-year-old Douglas-fir forest and three Brazilian savanna sites differing in tree density. At
all of the sites, HR was confined largely to the upper 60 cm of soil. There was a common threshold relationship between the
relative magnitude of HR and Ψsoil among the six study sites. Below a threshold Ψsoil of about –0.4 MPa, overnight recharge of soil water storage increased sharply, and reached a maximum value of 80–90% over
a range of Ψsoil from ~ –1.2 to –1.5 MPa. Although amounts of water hydraulically redistributed to the upper 60 cm of soil were relatively
small (0 to 0.4 mm day–1), they greatly reduced the rates of seasonal decline in Ψsoil. The effectiveness of HR in delaying soil drying diminished with increasing sapwood area per ground area. The relationship
between soil water utilization and Ψsoil in the 20–60-cm layer was nearly identical for all six sites. Soil water utilization varied with a surrogate measure of rhizosphere
conductance in a similar manner at all six sites. The similarities in relationships between Ψsoil and HR, soil water utilization and relative rhizosphere conductance among the six sites, suggests that, despite probable
differences in maximum rooting depth and density, there was a convergence in biophysical controls on soil water utilization
and redistribution in the upper soil layers where the density of finer roots is greatest.
Keywords:
coniferous forest, Pinus ponderosa, Pseudotsuga menziesii, rhizosphere, roots, soil water potential, tropical savanna.
