CAPTURE AND ALLOCATION OF NITROGEN BY QUERCUS-DOUGLASII SEEDLINGS IN COMPETITION WITH ANNUAL AND PERENNIAL GRASSES

The spatial overlap of woody plant root systems and that of annual or perennial grasses promotes competition for soil-derived resources. In this study we examined competition for soil nitrogen between blue oak seedlings and either the annual grass Bromus mollis or the perennial grass Stipa pulchra under cotrolled out-door conditions. Short-term nitrogen competition was quantified by injecting N-15 at 30 cm depth in a plane horizontal to oak seedling roots and that of their neighbors, and calculating N-15 uptake rates, pool sizes and N-15 allocation patterns 24 h after labelling. Simultaneously, integrative nitrogen competition was quantified by examining total nitrogen capture, total nitrogen pools and total nitrogen allocation. Stipa neighbors reduced inorganic soil nitrogen content to a greater extent than did Bromus plants. Blue oak seedlings responded to lower soil nitrogen content by allocating lower amounts of nitrogen per unit of biomass producing higher root length densities and reducing the nitrogen content of root tissue. In addition, blue oak seedlings growing with the perennial grass exhibited greater rates of N-15 uptake, on a root mass basis, compensating for higher soil nitrogen competition in Stipa neighborhoods. Our findings suggest that while oak seedlings have lower rates of nitrogen capture than herbaceous neighbors, oak seedlings exhibit significant changes in nitrogen allocation and nitrogen uptake rates which may offset the competitive effect annual or perennial grasses have on soil nitrogen content.