| Juvenile Douglas-fir, ponderosa pine, and whiteleaf manzanita growth in southwest Oregon varied with density of co-developing manzanita and presence of herbaceous cover. Plant xylem pressure potential and stomatal conductance of each species was responsive to competition-induced depletion of soil water. Rates varied among species. The best correlations with growth usually accompanied a two-year lag between stress and observed response. The densities of manzanita observed ranged from 0 to 27000 seedlings per hectare.;Six levels of manzanita density were provided by thinning and planting to manipulate biomass, leaf area index, and canopy cover. These parameters were used as inter-specific indices of competition for Douglas-fir and ponderosa pine. Competitive influence of shrubs on conifers was slight at age three, and increased progressively through the fifth year. Stem volume in 1985 was most highly correlated with manzanita canopy cover in 1983. Conifers grown with both manzanita and herbaceous competition had the smallest stem volumes, and those kept herb-free during the first and second years of the study had smaller stem volumes than those that were herb-free during the entire three years.;Species showed different strategies in water use. Manzanita maintained high levels of xylem pressure potential with high levels of stomatal conductance. Douglas-fir had intermediate level of xylem pressure potential, and pine was lowest. Douglas-fir and pine had similar stomatal conductance. This suggests the pine had access to soil water that the Douglas-fir and manzanita could not exploit.;Intra-specific competition between individual manzanita seedlings began at age three and became more accentuated, reducing growth by age five. The competitive indices used were basal diameter, canopy volume, above-ground biomass, and leaf area. Growth was always least when herbaceous plants were present. Soil moisture depletion was negatively correlated to amount of seedling growth. The community parameters leaf area index, stand biomass, and stand basal area, increased most rapidly at the highest densities, suggesting full site occupancy did not occur by age five. |
