Mapping site indices for five Pacific Northwest conifers using a physiologically based model

Questions: How well can we predict tree growth potential (site index) of five, locally dominant tree species in reference to estimates made with a detailed vegetation classification? Location: The forested region of the Pacific Northwest, USA and Canada. Methods: We employed a physiologically based process model (3-PG, Physiological Processes to Predict Growth) to generate estimates of site index under averaged climatic conditions (1971-2000) generated from hundreds of weather stations and extrapolated, with adjustments for topography, across the region at 1-km resolution. The model was parameterized from published information, but we had to assume fixed values of soil water storage capacity at 200 mm and soil fertility at 70% of maximum across the region. Field estimates of site index for the five dominant species were derived from published correlations with detailed mapping of vegetation provided by The British Columbia Ministry of Forests and Range. Results: The site indices projected with the 3-PG model for the five species combined, when compared with those produced by the Ministry of Forests and Range, produced an r(2) averaging similar to 0.5 with a standard error of 2.8 m at 50 yr, equivalent to 10% of the mean. Some of the variation may be attributed to inadequate information on soil properties. Importantly, the relationship between the two estimates was not significantly different from a 1:1 line, with an intercept of zero. Conclusions: The 3-PG modelling approach offers a means of predicting spatial variation in site indices across the Pacific Northwest and provides a basis for predicting future site indices under a changing climate.