Stand structure and the abundance and diversity of plants and small mammals in natural and intensively managed forests

Abstract

Little attention has been given to investigating biodiversity in managed forests. Pre-commercial thinning (PCT) and fertilization have been used successfully to increase growth of coniferous trees, vegetative succession, and overall stand structure in second-growth stands. This study was designed to test the hypotheses (H) that PCT and repeated fertilization of young (20–25 years) even-aged lodgepole pine ( Pinus contorta) stands would enhance (H1) coniferous stand structure; (H2) abundance and diversity of understory vegetation; and (H3) abundance and diversity of forest-floor small mammals, to levels found in mature and old-growth forests. Replicate study areas were located near Summerland and Kelowna in south-central British Columbia, Canada. Each study area had six stands: young plantation, thinned stand (1000 stems/ha), thinned–fertilized stand (1000 stems/ha), unthinned stand, mature forest, and old-growth forest. Coniferous stand structure, understory vegetation, and forest-floor small mammals were sampled during a 5-year period from 1999 through 2003. The smaller tree sizes (diameter, height, basal area) in the young lodgepole pine stands did not support the tree size part of H1. Similar abundance of overstory and total conifers did support the abundance part of H1. The diversity component was supported by the similarity in species and structural diversity of total conifers among the intensively managed and older unmanaged stands. Response of understory vegetation was dominated by the abundance of herbs in the thinned–fertilized stands. There was no difference in shrub abundance among stands, but mosses and terrestrial lichens were most numerous in the mature and old-growth stands. Our results supported the abundance part of H2, at least for herbs and shrubs. Species richness and diversity of vascular plants were similar in managed and old-growth forests, and richness was lowest in the mature stands. H3 was supported for total abundance, species richness, and diversity of small mammals, and for the generalist species Peromyscus maniculatus and Tamias amoenus, but not for the old forest specialist Myodes gapperi. Two insectivores, Sorex monticolus and Sorex cinereus, were at comparable or higher abundance in the managed stands than in the older unmanaged forests. Three other microtines: Microtus pennsylvanicus, Microtus longicaudus, and Phenacomys intermedius were early successional vegetation specialists, and hence did not fit the prediction of H3. Thus, despite overall quantitative differences in stand structure and species-specific variability between our intensively managed and older natural stands, old-growth attributes seem to be developing in a time-span of decades rather than the centuries depicted by long rotations.