Comparing site quality indices and productivity in ponderosa pine stands of western Montana

For Pinus ponderosa, indices based on quantifying the biophysical factors or physiological processes that control productivity (available water index and a relative index of seasonal photosynthesis from computer simulations) worked as well as those based on tree or stand measurements (site index and leaf area index). The following correlations of mean annual stem volume increment were found: with leaf area index, R² = 0.93; with available water index, R² = 0.95; with site index, R² = 0.98; with gross photosynthesis R² = 0.96. Available water and photosynthesis indices were also highly correlated to site index (R² >0.95). However, the tree-dependent site quality indices varied by stand density. Leaf area index and volume growth increased with stand density while site index decreased. Depletion of soil water effectively halted transpiration and photosynthesis by midsummer. Even with adequate water, cold spring and fall temperatures ultimately defined the length of the growing season and hence site quality. -from Authors