Manipulation of density of <i>Pseudotsuga menziesii</i> canopies: preliminary effects on understory vegetation

Abstract

Managing second-growth forests to conserve biodiversity has been proposed by both foresters and conserva- tion biologists. Management, however, can have unintended consequences, including reduction in native species diver- sity and increased invasion by exotic species. Our goal was to determine if inducing heterogeneity in managed forest canopies could promote a diversity of native species without contributing markedly to invasion by exotic species. We examined 1- and 3-year responses of understory plants to variable-density thinning of 55- to 65-year-old stands of Pseudotsuga menziesii (Mirb.) Franco. 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 conven - tional thinning. Variable-density thinning initially resulted in decreased understory cover but increased importance of 20 native species, including 2 species of trees. Two native species, however, decreased in importance, and 11 exotic spe - cies increased in importance. Within 3 years, understory cover recovered, species richness increased by >150%, only four exotic species persisted with increased 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 holis- tic silvicultural systems applied across landscapes throughout stand rotations and as a technique to restore vegetative complexity to closed-canopy second-growth forests. Resume : Les forestiers autant que les biologistes qui pronent la conservation proposent d'amenager les forets de se- conde venue avec l'objectif de conserver la biodiversite. L'amenagement peut cependant avoir des consequences non souhaitees, incluant la reduction de la diversite des especes indigenes et une augmentation de l'invasion par les especes