Changes in Snag Populations in Northern Arizona Mixed-Conifer and Ponderosa Pine Forests, 1997–2002

Abstract

Abstract Snags (standing dead trees) are important components of forests that contribute to ecological processes and provide habitat for many life forms. We monitored dynamics of snag populations on 1-ha plots in southwestern mixed-conifer (n = 53 plots) and ponderosa pine (Pinus ponderosa, n = 60 plots) forests in north-central Arizona from 1997 to 2002. Of 2,240 snags marked in 1997, at least 76% remained standing in 2002, 17% had fallen and were relocated as logs, 0.5% were cut down, and the remainder (6%) were unaccounted for. Snag density increased significantly in mixed-conifer forest but not in ponderosa pine forest. Snag density increased mainly in the smaller size classes in both forest types. Density of large snags (≥46 cm in diameter and 9 m in height) increased slightly but nonsignificantly from 1997 levels in both forest types, and fell below current targets for retention. Species composition of snags differed between 1997 and 2002 in ponderosa pine forest, but not in mixed-conifer forest, and decay-class distributions differed among years in both forest types. A classification-tree analysis indicated that a ratio of height to diameter, species, and diameter were key variables in predicting whether a snag survived from 1997 to 2002. These results provide empirical data on snag populations and dynamics in this area and establish a basis for longer-term monitoring of snag populations and dynamics. FOR. SCI. 51(5):396–405.