Effects of Thinning and Overstory Removal on Western Larch and Western Larch Dwarf Mistletoe

摘要

Western larch dwarf mistletoe (Arceuthobium lauds) causes an important disease of western larch (Larix occidentalis), resulting in growth loss and mortality. Little is known about the spread, intensification, and impact of this parasitic plant on its principal host, western larch. We examined the impact of precommercial thinning and (or) overstory removal on western larch crop tree height and diameter growth, as well as spread and intensification of western larch dwarf mistletoe over 21 years in a western Montana larch stand. In addition, upward advancement of dwarf mistletoe in crop tree crowns and dwarf mistletoe effects on mortality in both overstory and understory trees were explored. Overstory tree mortality was measured 15 years after treatment, and understory trees were measured 5 times over 21 years. Mulitivariate linear models showed that initial crop tree size and treatment were predictors of stem diameter 21 years after treatment. Models also showed that the initial dwarf mistletoe rating (DMR) of crop trees and treatment were reliable predictors of the changes in DMR after each treatment, but treatment did not predict the dwarf mistletoe index. Dwarf mistletoe upward advancement in larch crowns was greater than the rate of tree height growth in all treatments. Of the larch crop trees that died in the 21 years after treatment, western larch dwarf mistletoe was implicated in about 35% of the mortality. Eight percent of overstory larch rated as DMR 5 or 6 at study establishment were dead after 15 years. Although mean DMRs for crop trees were low at study establishment (average DMR 1), treatments were probably implemented about 15 years later than needed to best demonstrate differences in treatment impacts. Management implications are discussed.