Soil Moisture, Native Revegetation, and Pinus lambertiana Seedling Survival, Growth, and Mycorrhiza Formation Following Wildfire and Grass Seeding

Grass seeding is widely used for erosion control, but its consequences for soil and regeneration following fire have been measured only infrequently. This study investigates the effect of grass seeding on the type and extent of plant cover; soil moisture percentage; and moisture stress, survival, growth, and root-tip and mycorrhiza formation of Pinus lambertiana (sugar pine) seedlings in a clearcut intensely burned by wildfire. One-year-old containerized sugar pine seedlings were planted in seeded and nonseeded areas in Spring 1988 and 1989 in the Longwood Fire area of southwest Oregon. In 1988, tree seedlings in grass-seeded plots experienced intense competition from the grass, reduced root-tip and mycorrhiza formation, low levels of soil moisture to meet evapotranspirational demand, high levels of mortality, and reduced growth. In 1989, however, the opposite was true: tree seedlings in nonseeded plots experienced competition from invading native annuals and perennials, low levels of soil moisture in summer, and higher levels of mortality. The studies we report here further indicate that, in an area characterized by extended summer drought, annual ryegrass impeded regeneration of sugar pine during the first season following the fire. Native species cover and richness have been significantly reduced in the seeded area and may affect long-term soil stability, productivity, and conifer restoration. Seeding of annual ryegrass at high rates under these conditions would seem ill advised.