The effect of habitat type and rock type on individual tree basal area growth response to nitrogen fertilization

Individual tree basal area increment models for nitrogen fertilized stands were developed using data from permanent research plots located throughout the Inland Northwest. Results show that tree size, stand density, habitat type, and rock type significantly interact to affect individual tree basal area growth response to nitrogen fertilization. Suppressed trees growing on moist habitat types and all rock types, except metasedimentary, exhibited greater relative response than did dominant or codominant trees growing in the same stand. However, suppressed trees growing on dry sites or on soils derived from granite rocks did not show different relative response than dominant or codominant trees growing in the same stand. This study quantitatively demonstrates that individual tree competitive relationships are significantly affected by rock type. Rock types proved to be useful in representing broad differences in a siteʹs nutrient environment. Incorporating the new equations into individual tree growth and yield simulators would provide better representation of N fertilization response differences within a stand.