Thinning Red Alder: Effects of Method and Spacing

Abstract

Abstract A 14-year-old fully stocked red alder (Alnus rubra Bong.) stand on a site index 30 m (50 yr) site in northwest Oregon was precommercially thinned and growth measured for 5 years. Treatments were (1) 4.3 x 4.3 m spacing (541 trees/ha), chemically thinned, (2) 6.1 x 6.1 m spacing (269 trees/ha), chemically thinned, (3) 6.1 x 6.1 m spacing, chain-saw thinned, and (4) control (1754 trees/ha). Chemical thinning was done by trunk injection of 2,4-D and caused some damage (flashback) to neighboring crop trees. Thinning increased individual-tree radial growth 49% to 100% and decreased tree height growth by as much as 56%. In combination, this resulted in no significant change in tree volume growth with thinning. The lack of tree volume growth increase combined with the decrease in stem density with thinning produced a net decrease in stand volume growth with thinning. A review of the literature showed height growth reductions to be common among other alder studies. Tree basal area growth on the chemically thinned plots did not increase as much as on the chain-saw thinned plots, suggesting a prolonged growth reduction effect from the herbicide treatment. Diameter growth decreased with increasing degree of herbicide-induced crown damage. Thinning increased the size but not the numbers of epicormic branches. The longevity or effects on wood quality of these branches is not known. For. Sci. 35(1):16-29.