Static life-table analysis of fall rates of subalpine fir snags

A conflict exists in managed forests between the maintenance of snags (standing dead trees) for biological diversity and the requirement to remove snags for worker safety. This conflict could be mitigated by the identification of safe classes of snags, those with low rates of fall, that can be easily identified and retained in forest operations. I outline an approach to estimating rates of fall of identifiable decay classes of snags based on static life-table analysis of the current age distribution and relative abundance of classes of snags in a stand, applying the analysis to subalpine fir (Abies lasiocarpa) snags. The two most recent classes of snags, identified by the presence of fine branches and intact bark, showed fall rates of 0.17-0.20%/yr, similar to or lower than live subalpine fir trees. Fall rates increased to 0.70%/yr for the third class of snags, identified by loss of fine branches and cracking of bark, and to 3.79% and 6.16%/yr for the fourth and fifth classes, respectively, identified by increasing loss of bark and incipient softening of outer wood. Snags in the two earliest classes could be retained during forest operations without substantial increase in risk to workers. The static life-table approach to estimating fall rates of snags can complement long-term monitoring of snags in older forests where the assumption of a stationary age distribution is met.