Long-term response of spruce-fir stands to herbicide and precommercial thinning: observed and projected growth, yield, and financial returns in central Maine, USA.

摘要

Herbicide application and precommercial thinning (PCT) are common silvicultural treatments used across North America and Europe. Despite this widespread use, long-term growth and yield responses from controlled experiments that include both of these treatments are relatively rare. We used 40-year growth and yield responses of spruce-fir stands to various combinations of early herbicide and PCT in a long-term silvicultural experiment in central Maine, USA, to calibrate the Forest Vegetation Simulator (Northeast Variant). Using the calibrated model, we projected rotation-length outcomes for stand development, merchantable wood volumes, and stumpage-based financial returns. Projections indicated gains in total yield (17%-31%) from herbicide treatments at the end of the rotation (~60 years postharvest) relative to untreated stands. Substantial increases in merchantable wood volume also were achieved with PCT. Twenty-four years after PCT, stand stumpage value averaged $907 USD.ha-1 higher than that for unthinned stands. Total yield and stumpage gains from PCT were projected to continue through the end of rotation. Highest stumpage values resulted from combined herbicide and PCT treatments, followed by PCT-only and then herbicide-only. At end of rotation, highest net present value (NPV) resulted from PCT, whether alone or in combination with herbicides. PCT and herbicide investments substantially improved the NPV relative to untreated stands when using discount rates of 2% and 4%, but not when using a 6% rate. Our results documented that good financial returns are possible over the long-term from early investments in herbicide and PCT treatments in Maine spruce-fir stands.