Manipulation of density of Pseudotsuga menziesii canopies: preliminary effects on understory vegetation

Managing second-growth forests to conserve biodiversity has been proposed b y both foresters and conservation biologists. Management, however, can have unintended consequences, including reduction in native species diversity a nd increased invasion by exotic species. Our goal was to determine if induc ing heterogeneity in managed forest canopies could promote a diversity of n ative species without contributing markedly to invasion by exotic species. We examined 1- and 3-year responses of understory plants to variable-densit y thinning of 55- to 65-year-old stands of Pseudotsuga menziesii (Mirb.) Fr anco. Our study stands had been managed either with retention of old-growth legacies (large live trees, dead trees, and fallen trees) and no thinning or with removal of legacies and twice-repeated conventional thinning. Varia ble-density thinning initially resulted in decreased understory cover but i ncreased importance of 20 native species, including 2 species of trees. Two native species, however, decreased in importance, and 11 exotic species in creased in importance. Within 3 years, understory cover recovered, species richness increased by > 150%, only four exotic species persisted with incre ased importance, eight native species (including four graminoids) increased in importance, and seven native species decreased in importance. Variable- density thinning shows promise as part of holistic silvicultural systems ap plied across landscapes throughout stand rotations and as a technique to re store vegetative complexity to closed-canopy second-growth forests.