PLANT-SOIL WATER RELATIONS IN FORESTRY AND SILVOPASTORAL SYSTEMS IN OREGON

Plant-soil-water relations of a silvopastoral system composed of a Douglas-fir (Pseudotsuga menziesii) timber crop, subterranean clover (Trifolium subterraneum) as a nitrogen-fixing forage, and tall fescue (Festuca arundinacea) as a forage crop were investigated near Corvallis, Oregon, during 1983-1986. Treatments included all possible combinations of two tree-planting patterns (trees planted 2.4 m apart in a grid, and groups of five trees space 7.6 m between clusters) and two grazing/understory management systems (agroforests were seeded to subclover and grazed by sheep; forests were unseeded and ungrazed). Mean twig xylem water potential (XWP) for Douglas-fir trees ranged from -0.3 to -1.5 MPa on forest plots and from -0.3 to -1.2 MPa on agroforest sites. Pre-dawn and sunset XWP were more negative for forest than for agroforest plots during dry summer periods. Midday XWP was similar for both agroforest and forest plots on all dates. Soil water content at 50-100 cm depth was greater under agroforest plots as compared to forest plots in 1984, but not in 1985 (unusually dry spring). Average foliage nitrogen content of tree needles was 1.54% vs. 1.43% for agroforests vs. forests, respectively. Our data are consistent with the hypotheses that: (1) grazing of understory vegetation may reduce water stress of trees during dry periods by reducing transpirational water use by the forage plants; and (2) nitrogen-fixing vegetation combined with grazing increases nitrogen uptake of associated trees. However, neither mean foliar N nor average XWP differences experienced by trees in agroforest versus forest plantations were sufficient to have an effect on tree growth. Our data demonstrate that it is possible to produce a second crop (i.e. forage grazed by sheep) in timber plantations without reducing the growth of the main tree crop.