Differing effects of mortality agent on snags persistence in southwestern ponderosa pine forest

Authors: Joy Mast*, Carthage College, Carol L Chambers, Northern Arizona University
Topics: Biogeography, Mountain Environments, Physical Geography
Keywords: Arizona, Dendroctonus, Drought, Ips, Pinus ponderosa, Standing dead trees, Wildfire
Session Type: Poster
Day: 4/5/2019
Start / End Time: 3:05 PM / 4:45 PM
Room: Lincoln 2, Marriott, Exhibition Level
Presentation File: Download



We compared two key mortality agents, high-intensity crown fires and an outbreak of bark beetle, in southwestern ponderosa pine (Pinus ponderosa Laws). Our objectives were to conduct a longevity analysis to predict the lifespans of standing dead trees (snags) from large mortality events and determine characteristics of snags that remaining standing. We documented ponderosa pine snag dynamics in northern Arizona with repeat sampling of 14 variables in 18 1-ha permanent plots across two national forests. Specifically, we followed two high-intensity wildfires that occurred in 1996 and 2000 and 4 bark beetle outbreaks from 2002-2005. After field sampling >2500 snags for 1 to 13 years post-mortality, we estimated a 7.6 (± 0.1) year time period for a snag to remain standing. Mortality agent was the strongest predictor of how long a snag stood, with the hazard of falling for beetle-killed snags 2.5 greater than for fire-killed snags. Beetle-killed snags with intact tops, higher degree lean, and smaller diameter were most likely to fall. Inversely, the snags more likely to stand longer were fire-killed, with broken tops, straighter (low degree lean), and larger diameter. Our long-term study provided a useful model to forest managers seeking to select longer-standing snags for wildlife immediately following a high-intensity fire or beetle outbreak. When combined with increased drought-related tree stress due to climate change, and the similarity of southwestern snag dynamics to the west US, snag demography reported in our research indicates future conditions in a broader geographic region.

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