Thinning red pine plantations and the Langsaeter hypothesis: A northern Minnesota case study

The Langsaeter hypothesis states, "The total production of cubic volume by a stand of given age and composition on a given site is, for all practical purposes, constant and optimum for a wide range of density of stocking. It can be decreased, but not increased, by altering the amount of growing stock to levels outside this range." We used a multistep approach to test this hypothesis in a 46-year-old red pine plantation growing on a site of moderate quality in northern Minnesota. First, we used stem analysis data to construct tree-level red pine volume equations and compared them to existing equations. We then calculated stand-level volume. Doing this allowed us to evaluate the performance of two stand-level volume prediction equations, a growth and yield spreadsheet package, and a computer simulation model using 10-year pre- and post-thinning measurements. Tree volume prediction equations were similar to existing equations. For 10-year projections, the stand-level volume prediction equations and growth projection models provided volume estimates within 10% of actual volumes on average. Ten-year post-thinning measurements showed that a geometric, strip thinning to 100 ft2 of basal area resulted in a 40% volume gain, a crown thinning to 125 ft2 of basal area a 35% volume gain, and a low thinning to 140 ft2 of basal area a 30% volume gain, while an unharvested control had a 22% gain in volume relative to residual stand volumes. Although we found the 10-year volume growth varied less than 1 cord ac(-1) between residual growing stock volumes retained in this study, we do not have strong evidence to support the Langsaeter hypothesis for red pine.