Growth and development of ponderosa pine on sites of contrasting productivities: relative importance of stand density and shrub competition effects

Effects of stand density and shrub competition on growth and development were compared across a gradient of study sites. Challenge, the most productive site, is located in the foothills of the Sierra Nevada, northern California. Pringle Falls is of intermediate productivity in the rain shadow of the central Oregon Cascades. Trough Springs Ridge is the poorest site with minimally developed soils in California's North Coast Range. Treatments included a minimum of four stand densities, from 150 to 2700 trees·ha–1, in combination with at least no or full shrub removal. Challenge produced almost twice as much tree volume as Pringle Falls, and about three times the volume of Trough Springs Ridge. Regardless of site quality, growth was significantly greater in full shrub removal plots for stand densities <2000 trees·ha–1. After 26–36 years, stand volumes were 25–67 m3·ha–1 (11%–38%) greater at Challenge, 30–33 m3·ha–1 (25%–52%) greater at Pringle Falls, and 27–41 m3·ha–1 (115%–326%) greater at Trough Springs Ridge when shrubs were removed. Periodic volume growth declined substantially during the last 10 years at Challenge and Pringle Falls, regardless of treatment, because of confounding effects of mortality, drought, inter-tree competition, and insect defoliation. Further, the importance of shrub control on growth increment was not evident during the last 10 years at both sites, as tree–shrub competition likely switched to tree–tree competition. On the low quality site, shrub control is critical for stand development.