Seasonal pattern of water depletion from soil-rock profiles in a Mediterranean climate southwestern Oregon

Neutron probe measurements from aluminum access tubes (3.2 m deep) show that conifers and sclerophyllous shrubs deplete water from soil-rock profiles in distinctly different patterns. Measurements were taken during two growing seasons (1993 and 1994) in southwestern Oregon (U.S.A.) under 14-year-old stands of Douglas-fir (Pseudotsuga menziesii (Mirb.) France), ponderosa pine (Pinus ponderosa Dougl. ex Laws.), Pacific madrone (Arbutus menziesii Pursh), and whiteleaf manzanita (Arctostaphylos viscida Parry) covers. Both conifer species showed very limited abilities to utilize water from bedrock, even during very dry periods. Ericaceous plants, on the other hand, were very efficient in taking water from the deep rock layers, using nearly all available water and removing about twice as much water as conifers of the same age. Patterns of water depletion suggest that the water resources used by these two types of plants overlap considerably, and competition will be very strong in mixed stands. The ability of ericaceous plants to utilize bedrock water that is unavailable to conifers in late summer will enhance their potential for dominance.