© 2007 Heron Publishing—Victoria, Canada
Species-specific partitioning of soil water resources in an old-growth Douglas-fir–western hemlock forest
Frederick C. Meinzer (1,2), Jeffrey M. Warren (1) and J. Renée Brooks (3)
1. USDA Forest Service, Forestry Sciences Laboratory, 3200 SW Jefferson Way, Corvallis, OR 97331, USA / 2. Corresponding author (fmeinzer@fs.fed.us) / 3. Western Ecology Division, US EPA/NHEERL, 200 SW 35th St., Corvallis, OR 97333, USA / Received August 28, 2006; accepted September 22, 2006; published online March 1, 2007
Summary
Although tree- and stand-level estimates of forest water use are increasingly common, relatively little is known about partitioning
of soil water resources among co-occurring tree species. We studied seasonal courses of soil water utilization in a 450-year-old
Pseudotsuga menziesii (Mirb.) Franco–Tsuga heterophylla (Raf.) Sarg. forest in southwestern Washington State. Soil volumetric water content (θ) was continuously monitored with frequency
domain capacitance sensors installed at eight depths from 0.2 to 2 m at four locations in the vicinity of each species. Vertical
profiles of root distribution and seasonal and daily courses of hydraulic redistribution (HR), sap flow and tree water status
were also measured. Mean root area in the upper 60 cm of soil was significantly greater in the vicinity of T. heterophylla trees. However, seasonal water extraction on a root area basis was significantly greater near P. menziesii trees at all depths between 15 and 65 cm, leading to significantly lower water storage in the upper 65 cm of soil near P. menziesii trees at the end of the summer dry season. Greater apparent efficiency of P. menziesii roots at extracting soil water was attributable to a greater driving force for water uptake rather than to differences in
root hydraulic properties between the species. The dependence of HR on θ was similar in soil near individuals of both species,
but seasonal maximum rates of HR were greater in soil near P. menziesii because minimum values of θ were lower, implying a steeper water potential gradient between the upper and lower soil that
acted as a driving force for water efflux from shallow roots. The results provide information on functional traits relevant
for understanding the ecological distributions of these species and have implications for spatial variability of processes
such as soil respiration and nutrient cycling.
Keywords:
Pseudotsuga menziesii, root distribution, sap flow, transpiration, Tsuga heterophylla
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