Snag dynamics and cavity excavation after bark beetle outbreaks in southwestern ponderosa pine forests

In the United States, ponderosa pine (Pinus ponderosa) forests provide habitat for more cavity-nesting wildlife than any other forest type. In 2002-2005, stresses from drought, changes in the fire regime, and increases in forest density contributed to the largest bark beetle (Ips or Dendroctonus spp.) epidemic ever recorded in Arizona. We identified characteristics that predict snag longevity and use by cavity nesters of bark beetle-killed ponderosa pine. We mapped snags and monitored use by wildlife in sites with bark beetle outbreaks in northern Arizona. We measured snag characteristics 3 9 years after outbreaks and used multimodel inference to predict whether a snag was standing or fallen or contained excavated cavities or not. We used spatial statistical tests to determine snag patterns. Although >99% of beetle-killed snags were standing 3 years after outbreaks, <20% remained standing after 7 years. Snags were clumped when initially measured and in subsequent remeasurements. Snags that remained standing had lower surrounding basal area, larger dbh, and more limbs, were more decayed, and were on west-facing lower slopes. Snags with cavities were larger in diameter, had broken tops before falling, and were in open (mean live tree basal area 17 m²/ha) forest. Our data are indicative of snag dynamics likely to occur in ponderosa pine forests of the southwestern United States, given the occurrence of recent droughts and bark beetle outbreaks.