Snag density and composition of snag populations on two National Forests in northern Arizona

摘要

Snags (standing dead trees) provide important habitat for forest wildlife, as well as a source of coarse woody debris important in forest succession. Because of their importance, some land-management agencies have standards for snag retention on lands under their jurisdiction (e.g. US Forest Service, British Columbia Ministry of Forestry). Despite these guidelines, however, little information is typically available on snag numbers or dynamics on these lands. As part of a long-term effort to monitorsnag dynamics, snag populations were sampled on 114 one-hectare plots randomly located across 6 Ranger Districts on 2 National Forests in northern Arizona. Sixty plots were located in ponderosa pine (Pinus ponderosa) forest, with the remainder in mixed-conifer forest dominated by Pseudotsuga menziesii and/or Abies concolor. Small snags and snags in later decay classes numerically dominated snag populations. Because large snags are most useful to forest wildlife, this suggests a need to retain large trees as future snags. Only 6.7 and 16.7% of plots in ponderosa pine and mixed-conifer forest, respectively, met or exceeded current US Forest Service standards for retention of large snags (defined as snags >=46 cm in diameter at breast height and 9 m in height) in this geographic region. However, even plots with no evidence of timber or fuelwood harvest seldom met targets for retention of large snags. Only 30 and 32% of unlogged plots met or exceeded standards in ponderosa pine and mixed-conifer forest, respectively. This suggests that current standards for snag retention may be unrealistic, and that those standards may need to be reconsidered. Snag guidelines should be based on an understanding of both, snag dynamics and the requirements of snag-dependent wildlife species.