Early establishment of Douglas-fir and ponderosa pine in grassland seedbeds

Abstract

Grassland of interior British Columbia are being encroached upon by Douglas-fir (Pseudotsuga menziesii var. glauca (Beissn.) Franco) and ponderosa pine (Pinus ponderosa Dougl.). A pot experiment placed in the field was conducted to determine the effect of forest and grassland seedbeds on seedling emergence and early establishment of the 2 species with 2 seed collections each. For these seedbeds, structural characteristics were evaluated and the effect of seedbeds water extracts on seed germination was determined. Seedling emergence of both species was significantly reduced by Douglas-fir needles and enhanced by fescue litter and cattle manure compared to mineral soil. The rate of emergence was reduced by Douglas-fir needles and sagebrush litter, and for some collections, by ponderosa pine needles, but was always enhanced by manure compared to mineral soil. Seedling survival was generally not affected by seedbeds. Douglas-fir seedlings emerging earlier in the season survived better, and both Douglas-fir and ponderosa pine seedlings emerging earlier lived longer than these emerging later. Seed germination of ponderosa pine was not affected by the water extract while that of Douglas-fir was reduced by the water extract from sagebrush litter. Therefore, differences in seedling emergence of the 2 species among seedbeds were related more to structural than to chemical characteristics of seedbeds. Successful establishment of the 2 species in grasslands within this region likely relies on the ability of seeds to germinate early in the growing season on seedbeds in which soil moisture is conserved, as summer droughts are severe.