Twenty-five-year response of Larix occidentalis stem form to five stand density regimes in the Blue Mountains of eastern Oregon

Abstract

Upper-stem diameter measurements from a levels-of-growing-stock study on Larix occidentalis in the Blue Mountains of eastern Oregon were collected over a 25 year period with an optical dendrometer. The following hypotheses were tested with these data: 1) stand density regime has a significant effect on stem form; and 2) crown size controls the response of stand density regime to stem form. The five stand density regimes corresponded to five different growing-stock levels (GSLs) defined by bole surface area. A variable exponent taper model was fit to the data from each individual tree and the resulting parameters were treated as indices of stem form for comparisons among density regimes. MANOVA performed on these parameters estimates confirmed statistically significant differences in stem form both between the two lowest levels-of-growing stock and between these two treatments and all three regimes with higher levels of growing stock. The individual tree parameter estimates were also tested in the context of a seemingly-unrelated regression (SUR) analysis. The two stem profile parameter estimates were closely related to diameter! height ratio and crown ratio, respectively. Indicator variables for the different density regimes did not contribute any explanatory power beyond diameter/height ratio for the first parameter or crown ratio for the second parameter. DBH, height and crown ratio are concluded to account for the variation in stem form imposed by differences in stand density management.