Snag persistence rates and patterns following mountain pine beetle epidemic in the Elkhorn Mountains, Montana, USA

Abstract

Bark beetle (Dendroctonus spp.) epidemics generate snags, providing nesting and foraging opportunities for woodpeckers and other disturbance-associated wildlife. Forest managers need to know how long snags persist to include wildlife habitat conservation in their objectives, but published studies of snag persistence following disturbance under-represent beetle-killed forest. We analyzed snag persistence rates in relation to environmental covariates following a mountain pine beetle (Dendroctonus ponderosae) epidemic in the Elkhorn Mountains (Montana, U.S.A.) over a 6-year study period (2010–2015). We evaluated hypothesized relationships with topography, local tree density, and snag characteristics. Consistent with our hypotheses, we found snags with greater diameter at breast height (DBH), younger snags, and broken-top snags persisted relatively long. Also consistent with our hypotheses, snags located in areas of relatively rugged topography and greater tree densities persisted longer. Moreover, we estimated a diameter-height ratio ≥ 4.8 cm × m−1 as a useful threshold for distinguishing broken-top snags likely to persist longer than other snags. Greater snag persistence on north-facing slopes and on topographic high points were inconsistent with our hypotheses, suggesting these conditions may be less useful for understanding snag dynamics. Our study contributes to general knowledge capable of helping managers identify snags most likely to persist and thereby provide extended habitat value for nesting woodpeckers.