Effect of aspect on ponderosa pine height and diameter growth

Abstract

Most forest site-quality models assume the optimal aspect for forest growth in the northern hemisphere to be on northeast-facing slopes. However, the optimal aspect for forest growth may vary with elevation and/or season of growth. We measured Pinus ponderosa Laws. initial 10-year diameter increment and site index (base age 100) on two cinder cones near the species' upper elevational limit. The highest-elevation site had significantly greater mean site index and mean diameter growth on southerly aspects. This is likely to be the result of warmer air and soil temperatures, and a prolonged growing season occurring on southerly slopes. The second site, at a slightly lower elevation, had significantly greater diameter growth on southerly slopes, but mean site index was not significantly different between northerly and southerly slopes. Because ponderosa pine diameter growth occurs later in the summer than does height growth, greater diameter growth on southerly slopes may result from differences in radiation interception between northerly and southerly slopes as the diameter-growth season progresses.