Stand structure and growth of Abies magnifica responded to five thinning levels in northeastern California, USA

A 60-year-old red fir stand with 23,950 stems/ha was thinned to five stand densities. Thinning occurred in 1972, 1976, and 1980. Height, dbh, and crown characteristics were measured seven times at 4–7-year intervals from 1972 to 2002. Tree rings were measured retrospectively to determine growth of individual years. Periodic annual increment (PAI) was analyzed for quadratic mean diameter (QMD), height, crown volume, and basal area per ha. We found significant difference for all variables among thinning levels and among measurement periods (P < 0.002). Trees in heavily thinned plots grew more in height, dbh, and crown volume than trees in lightly thinned plots. The trends were true for the eight tallest trees per plot (=100 trees/ha) for height and dbh. Yet, both total and PAI stand basal area were greater in the lightly thinned plots than in the heavily thinned plots. Thinning intensity accelerated the transformation in diameter distribution from either reverse ‘J’ or skewed shape to a bell-shape. In addition, the heavier thinned plots increased stand vigor and health as indicated by less snow damage and stem deformities than the lighter thinned plots. Annual diameter growth was significantly related to average minimum temperature during the growing season. We conclude that thinning young red fir stands to a wide spacing will enhance stand development and minimize damage to the residual trees.