AN ASSESSMENT OF ECOLOGICAL THINNING FOR STRUCTURAL DIVERSITY IN COASTAL FORESTS OF THE PACIFIC NORTHWEST, USA

Abstract

Restoration for increased structural integrity is a relatively new strategy that is being applied to protected areas in the Pacific Northwest. Not enough data has been collected to determine how best to approach restoration, so the current projects have made educated guesses about what the best approach might be to increase structural diversity. My analysis seeks to determine the most effective thinning regime to increase structural diversity. The current stands which are undergoing restoration are young even aged plantations which have been acquired by conservation organizations and government agencies. Ecologically, these forests differ from the historical forests both in structure and species composition. Restoration seeks to remove a portion of the overstory trees to allow the recruitment of a new cohort of trees and increase the growth rate of the remaining overstory trees. This approach essentially involves tunneling through the classic development curve laid out by Odum (Odum, 1969). I used a version of FVS ported into the R language to model the background forest growth in the face of windthrow against 25 possible thinning types. I compared thinnings across a range of intensity of tree removal (10-50% of trees removed per cycle) and cycle length (10 to 50 years). I compared these results to a background scenario consisting of windthrow disturbance with a 1% probability of occurring each year and lognormally distributed impacts with low intensity damage more common than high intensity damage. My work has shown that removing 30 – 50 % of basal area every 30 to 50 years can help forests achieve structural complexity before it would develop on its own accord. Without restoration, natural forests achieve structure similar to old growth between 100 and 200 years of age depending on the frequency and intensity of windthrow. After 100 years of restoration, forests will achieve higher levels of structural complexity and tree growth.