PREDICTING TREE GROWTH FROM CROWN VARIABLES IN MANAGED NORTHERN HARDWOOD STANDS

The predictability of individual-tree growth rates for Acer saccharum, Fraxinus americana, and Tilia americana in northern hardwood stands in Wisconsin was studied in relation to crown dimension and crown competition variables measured in the field. Data were collected from 221 destructively sampled trees in eight second-growth stands on above-average sites. The single best independent variable for predicting basal area growth was the projection area of the exposed portion of the crown, which had higher correlations with observed growth than initial stem diameter, diameter-based competition indices, and other crown variables. Basal area growth equations containing total crown projection area, percent exposed crown area, and relative height had R2 values ranging from 0.77 to 0.88, which represent increases of 13 to 47 percentage points compared with similar equations with only diameter-based competition variables. The most accurate height growth models were functions of total tree height and percent exposed crown area, with R2 values of 0.74 for the non-linear maple equation and 0.44 for the linear white ash equation. The results appear to support the hypothesis that significant competitive stress on individual trees is induced only by the ring of competitor trees immediately surrounding the subject tree crown. By using percent exposed crown area as a crown competition variable, this competitive effect can be estimated without direct measurements of any of the competitors, potentially saving much field measurement time during the model calibration phase and eliminating the statistical lack of independence generally associated with plot competition measures.