Estimation of compensatory growth of coastal Douglas-fir following pre-commercial thinning across a site quality gradient

Abstract

This study is aimed to address the issue of whether the volume loss in stands with partial cutting such as pre-commercial thinning (PCT) treatments could catch-up with the ones of unthinned stands, employing the lens of compensatory growth (CG) in plant ecology to ask the question of how long one could expect to see the complete CG to happen, if any. CG is the ability of plants or vegetation to vary their growth to offset damage caused by disturbances. Though it has been demonstrated in annual plants such as crops and grasslands, it has not been well studied in trees except Douglas-fir under different PCT regimes in the Swiss Central Plateau, trembling aspen seedlings under different fertilization regimes, and fast-growth willow coppice with clipping, etc. Our data from a long-term silvicultural trial (combined PCT and fertilization) near Shawnigan Lake, Vancouver Island, British Columbia, Canada, demonstrate complete CG in coastal Douglas-fir 40 years after treatments at both the tree and stand levels. We present a method of estimating the length of time required to achieve complete CG in stand volume. Our results also show that different patterns of complete CG could be expected under different site conditions differentiated by levels of fertilization. Without fertilization a long length of time would be required to reach complete CG, and the length of time could be reduced by increased fertilization application. Potential forest management implications are discussed including possible alternative shapes of yield curves for managed stands and hence the forecast of future wood supply, positive economic evaluation of PCT and fertilization, and contribution to regional carbon budget.