Thinning even-aged Douglas-fir stands: effects of density and structure on stand volume growth

Abstract

The usefulness of thinning studies in Douglas-fir stands has been limited by difficulties such as the long time period required, changing study objectives, and the limited scope of these studies. An approach is presented where two baseline models--based on volume estimates for unthinned stands from two Douglas-fir growth and yield models--are compared to the growth of measured thinned stands. The baseline models were similar over most of their common range of age and density, but at lower densities and older ages the models indicated substantially different volumes.Results from the 28-year Delezenne thinning study found volumes from thinned plots were comparable to, and in some cases exceeded, those of the baseline models. These high levels of production in the Delezenne plots suggest: (1) the growth and yield models are underestimating the growth of widely-spaced stands; and/or (2) an unrecognized potential exists for thinning to increase volume growth.Gross volume increment from the Delezenne plots was unaffected by thinning in most of the lightly-thinned plots and reduced in heavily-thinned plots. Net increment was increased in some thinned plots.Dominant and codominant trees produce more volume per tree and a greater portion of stand volume growth in thinned and unthinned stands. Average growth rates per unit of occupied growing space (growing space efficiency) were higher for dominants and codominants in unthinned stands. In thinned stands average growing space efficiency was similar among all represented crown classes. Tall trees and trees with medium-sized crowns were generally the most efficient.The correlations between growing space efficiency and other individual tree characteristics, and between individual tree characteristics, were lower in thinned stands than in unthinned stands, suggesting these relationships do not apply after thinning.Results suggest: (1) stand density measures do not provide similar expressions of competition in thinned stands; (2) density/growth relations are oversimplifications of a complex interaction of many variables; (3) optimal stand growth is obtained with a particular stand structure, not necessarily with a particular stand density; and (4) thinning may have the potential to increase gross stand volume growth.