The 3/4 power law in forest management: how to grow dead trees

Abstract

Structural complexity of managed forests is proving to be vitally important for the conservation of wildlife and ecological processes. We test for the factors most important in determining the availability of snags (dead standing trees) in fire-origin boreal forests of northwestern Ontario. The strongest correlate of snag basal area (m 2/ha) was the amount of live tree basal area, which accounted for 68% of variation in the production of snags. The relationship between live and dead tree basal area followed a 3/4 power law with a slope of 0.74 for the forest community and differing slopes for forest stands and forest age. A relationship between tree mortality and metabolic rate is consistent with rate of self-thinning theory. This invariance indicates that total community biomass is likely to be insensitive to species diversity. Mixed hardwood forest types produced the greatest basal area of snags, followed by poplar, mixed conifer, jack pine and spruce stands. The percentage of stems that were dead was greatest in young forests (18% of stems in 0–60-year-old forests), decreased to a low percentage of 12% in 61–80-year-old forests, and thereafter increased with age of forests from 15 to 16% in 81–100, >100-year-old forests, respectively. In contrast, the number of small diameter (10–22 cm dbh) snags (stems/ha) was greatest in 61–100-year-old forests, whereas medium diameter (22–30 cm dbh) snags and large diameter (>30 cm dbh) snags were greatest in old-growth (>100 years) forests. The management implications are that increased live tree density due to environmental conditions, or possibly intensive forest management practices following clear cutting, may secondarily result in greater snag production and the structural complexity that favors wildlife biodiversity. Forest management guidelines that incorporate allometric relationships are needed to ensure maintenance of biodiversity and habitat heterogeneity following timber harvesting.