Pine snags persist for different durations depending on conditions after a beetle epidemic

Bark beetle epidemics create standing dead trees, or “snags," which provide important nesting and foraging habitat for woodpeckers. Snags also create habitat for many other species, such as those that use old cavities created by woodpeckers. However, while snag removal via salvage logging can negatively impact wildlife habitat, it can also provide important economic and safety benefits to people. To develop habitat conservation objectives that account for this trade-off, it is helpful for forest managers to know how long these snags remain on the landscape before they break down and fall over.

During 2006-2011, a mountain pine beetle epidemic killed 90 percent of ponderosa and lodgepole pines over 1,700 square miles in the Helena National Forest of Montana. Douglas-fir trees, also part of this forest, were little affected by the beetles. We monitored pine snags at 75 plots distributed across four large study units, 329-665 acres each, during and after the epidemic. We developed a tree-diameter-to-height threshold to distinguish between broken-off snags and intact snags, and then investigated factors that affected how long snags remain on the landscape, called “snag persistence,” with a focus on topography, tree density, and snag characteristics.

We found that wider diameter, younger (more recently dead), and broken-off snags remained standing the longest, as well as snags in rough topography and in plots with greater tree density. This is likely related to the structural integrity and decay states of snags and their exposure to wind. Wider diameter and younger snags have more structural integrity, while broken-off snags have lower centers of gravity, helping them to remain standing. Similarly, rugged topography and dense stands likely shelter snags from wind exposure. Lodgepole pine snags tended to remain standing longer than ponderosa pine snags on average. However, further work is needed to confirm this observation, as well as to investigate relationships between snag persistence and topographic aspect and position that were inconclusive in this study. Taken together, our results may help managers identify areas in beetle-killed forests that could provide longer-term habitat value for nesting woodpeckers and other wildlife.