Pinus ponderosa seedling establishment and the influence of competition with the bunchgrass Agropyron spicatum

Interspecific competition between Agropyron spicatum (Pursh.) Scrib. & Smith bunchgrasses and naturally established seedlings of Pinus ponderosa was examined within a pine/bunchgrass community. A wire mesh was used to separate bunchgrass culms from pine seedling shoots to determine if the bunchgrass canopy influenced the survival of pine seedlings. In addition, two lengths of root exclusion tubes were used to determine the effects of bunchgrass root overlap on pine seedlings. The bunchgrass canopy did not significantly affect pine seedling survival. However, root competition, presumably for water, significantly decreased pine seedling survival. Exclusion of bunchgrass roots from a 0.15-m- and 0.30-m-deep root zone of pine seedlings resulted in 40% and 80% reductions in mortality, respectively. Root exclusion also significantly delayed mortality 2-3 wk. Pine seedlings developed taproots that reached below the zone of maximum bunchgrass root density within 4 wk of germination. The pine/bunchgrass community is dominated by two species that utilize similar resource zones in the soil during the establishment of pine germinants in the bunchgrass understory. Interspecific competition tends to be greatest during this establishment stage and becomes reduced as pine seedlings grow and explore the deeper regions of the soil profile. In areas with shallow soils and an established bunchgrass understory, establishment of pine seedlings may occur successfully only if soil moisture is available in deeper soil horizons during the summer.