Stand structure and small mammals in young lodgepole pine forest: 10‐year results after thinning

Management of forested landscapes for biological diversity is a major objective across North America. Perhaps the greatest potential to diversify future forests lies in the vast areas of young second‐growth stands which may be managed silviculturally to accelerate ecosystem development. This study was designed to test the hypotheses that large‐scale precommercial thinning, at ages 17–27 yr, to various stand densities would, over the 10‐yr period since treatment, enhance: (1) productivity of lodgepole pine (Pinus contorta) crop trees, (2) stand structure attributes, and (3) species richness and diversity of forest floor small‐mammal communities. Study areas were located near Penticton, Kamloops, and Prince George in south‐central British Columbia, Canada, in three forest ecological zones. Each study area had three stands thinned to densities of ∼500 (low), ∼1000 (medium), and ∼2000 (high) stems/ha, with an unthinned juvenile pine and old‐growth pine stand for comparison. Understory vegetation was measured in all stands in 1990, 1993, and 1998, and coniferous tree layers were measured in 1998. Small‐mammal populations were sampled intensively in 1990, 1991, and 1998. Mean diameter increments of trees in the low‐density stands were significantly higher than those in the medium‐ and high‐density stands at all study areas. Mean height increments of trees were similar in the medium‐ and high‐density stands and significantly higher than that in the low‐density stands at Penticton and Prince George. Crown volume index (biomass) of herbs was highest in the thinned stands by 1998, but there was no difference among stands for shrubs and trees; volume of mosses was highest in the old‐growth stands. Mean species richness and diversity of herbs, shrubs, and trees were similar among stands at 2, 5, and 10 yr after thinning. However, mean species diversity and structural diversity of coniferous trees were significantly higher in the low‐ and medium‐density stands than in the high‐density and unthinned stands 10 yr after thinning. Total structural diversity of all vegetation in the low‐density stands was significantly greater than that of the medium‐density, unthinned, and old‐growth stands in 1998. Mean total abundance of all small mammals was similar among stands in 1990–1991, but the low‐density and old‐growth stands had the most mammals in 1998. Mean abundance of southern red‐backed voles (Clethrionomys gapperi) was consistently higher (2.1–3.3 times) in the old‐growth stands than in unthinned stands. In seven of nine cases, mean abundance of red‐backed voles was similar among old‐growth and thinned stands. Mean species richness and species diversity of small mammals were highest in the low‐density and medium‐density stands. Heavily thinned lodgepole pine stands developed structural attributes such as large diameter trees, large crowns, and structurally diverse vegetative understories. Forest floor small‐mammal communities reflected the compositional and structural diversity of these managed stands.