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 ponde rosa was examined within a pine/bunchgrass community. A wire mesh was used to separate bunchgrass culms from pine seedling shoots to determi ne if the bunchgrass canopy influenced the survival of pine seedlings. In addition, two lengths of root exclusion tubes were used to determi ne the effects of bunchgrass root overlap on pine seedlings. The bunch grass canopy did not significantly affect pine seedling survival. Howe ver, root competition, presumably for water, significantly decreased p ine seedling survival. Exclusion of bunchgrass roots from a 0.15-m- an d 0.30-m-deep root zone of pine seedlings resulted in 40% and 80% redu ctions in mortality, respectively. Root exclusion also significantly d elayed mortality 2-3 wk. Pine seedlings developed taproots that reache d below the zone of maximum bunchgrass root density within 4 wk of ger mination. The pine/bunchgrass community is dominated by two species th at utilize similar resource zones in the soil during the establishment of pine germinants in the bunchgrass understory. Interspecific compet ition 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 bunchgras s understory, establishment of pine seedlings may occur successfully o nly if soil moisture is available in deeper soil horizons during the s ummer.