A comparison of overstory density measures for describing understory conifer growth

Abstract

Stand basal area (TBA), hemispherical photographic estimates of canopy coverage (PCC), and visual estimates of canopy coverage (VCC) were used to predict underplanted seedling growth in an experiment on conifer regeneration in two-storied stand management. VCC was the best predictor of western redcedar ( Thuja plicata) seedling volume when evenly and unevenly thinned treatments were combined and when unevenly thinned plots were examined separately, while TBA predicted seedling volume better in evenly thinned plots. VCC also was the best predictor of grand fir ( Abies grandis) seedling volume when thinning treatments were combined or unevenly thinned treatments were examined separately; VCC, PCC, and TBA predicted grand fir volume in evenly thinned plots about equally well. VCC was more strongly correlated with TBA than was PCC, especially at higher levels of overstory retention in the evenly thinned stands. In unevenly thinned plots, the VCC and PCC estimates were correlated with stand basal area about equally. In unevenly thinned plots, both estimators were better at lower than at higher residual densities. With roughly 41% of variance of growth of individual seedlings explained by VCC, it is a relatively poor predictor for individuals. For large groups of seedlings, however, VCC is a good predictor of a general pattern and considerably better than PCC, on average.