Risk to Long-term Site Productivity Due to Whole-tree Harvesting in The Coastal Pacific Northwest

Abstract

The growth of 68 intensively managed, mid-rotation, Douglas-fir stands in western Oregon, Washington, and British Columbia was projected to 50-55 years of age using the SMC variant of the ORGANON growth and yield simulator. From the ORGANON output, component biomass removal was estimated for stem-only harvest and a more intense whole-tree harvest. Utilizing published equations which estimate tree component N content based on biomass and total site nitrogen from the 68 sites, nitrogen removal under the two harvest intensities is expressed as a proportion of total site nitrogen store. Based on the proportion of N removed to the total site store, the 68 sites were assigned a risk rating, and regional patterns were assessed. Based on the simulation results, nearly half of the stands in the study were at risk of N depletion or site productivity loss under whole-tree harvest, while most stand ranked in the lowest risk category under stem-only harvesting. The highest concentration of stands at risk of long term site productivity loss from N depletion is on young glacial soils in Vancouver Canada and the Puget Sound region of Washington. This simulation also suggests that stands of similar planting density and age on sites with less than approximately 9000 and 4000 kg/ha of total site N will be at an elevated risk for long term site productivity loss under whole-tree and stem-only harvests respectively.