Stand structure and small mammals in intensively managed forests: Scale, time, and testing extremes

Abstract

Stand thinning and fertilization have the potential to create a diversity of forest habitat conditions to meet the goals of biodiversity conservation while sustaining wood and biomass production. Three factors are crucial to a better scientific understanding of how wildlife habitats develop in managed forests: real-world scale of treatments using adaptive management, several decades of time to monitor responses, and the testing of extreme treatments. Communities of arboreal and forest-floor small mammals occupy young coniferous stands and may serve as indicators of change in forest structure and function, and hence biodiversity. We tested the hypotheses (H) that, when compared with unmanaged (unthinned and old-growth) stands, large-scale pre-commercial thinning (PCT) and PCT with repeated fertilization, up to 16–21years after the onset of treatments, would enhance (H1) stand structure, (H2) abundance of southern red-backed voles (Myodes gapperi) and species diversity of forest-floor small mammals, and (H3) abundance of northern flying squirrels (Glaucomys sabrinus), in young lodgepole pine (Pinus contorta) stands in the south-central interior of British Columbia, Canada.Mean diameters and heights of crop trees in thinned stands were less than those in old-growth stands, but the low-density stands did have greater mean diameters than those in unthinned stands. Mean crown volume of pine crop trees was significantly greater in the low- and medium-density stands than in the high-density, unthinned, or old-growth stands. Mean density of overstory (range of 1010–1590 stems/ha) and understory conifers were similar in the thinned and old-growth stands. Mean species and structural diversity of total conifers were significantly higher in the heavily thinned (⩽1000stems/ha) than other stands. For vascular plants, mean abundance of understory herbs and total structural diversity were higher in heavily thinned and fertilized stands. Thus, some aspects of stand structure supported H1 while others did not.Mean abundance of M. gapperi was higher in old-growth forest than the younger managed and unthinned stands, except in years of low numbers, and hence did not support H2 that these treatments over a longer time-frame would enhance abundance of this species. Fluctuations in numbers, drier forest ecosystems, and slow development of forest-floor habitat with decaying woody debris and fungi may contribute to flexible habitat occupancy for M. gapperi. The second part of H2, that mean species diversity of forest-floor small mammals would be enhanced in heavily thinned and fertilized stands tended to be supported, particularly in the period prior to canopy closure. H3, that abundance of G. sabrinus would be maintained at levels recorded in old-growth forests by PCT of young stands, was supported for medium- and high-density stands.A sustained flow of wood products and structural features with their habitats for wildlife and biodiversity should be achievable with a wide range of stand densities from PCT and variable regimes of nutrient additions in intensively managed forests. Essential key elements include large-scale (operational-size) treatment units and long-term monitoring of installations over decades, preferably at 5- to 10-year intervals.