Low-severity fires increase susceptibility of lodgepole pine to mountain pine beetle outbreaks in Colorado

Both fire and insect outbreaks are critical components of many forest ecosystems and understanding the two-way interactions between these disturbance types is an important goal for researchers, resource managers, and policy makers. Much recent research has focused on understanding the effects of outbreaks on subsequent fires, but the effects of fires on subsequent outbreaks are also important in shaping ecosystem dynamics.In the current study we examined how low-severity fires influence susceptibility of lodgepole pine to mountain pine beetle (MPB). We examined 607 lodgepole pine trees in stands that were affected by low-severity fire in 2002 and subsequent MPB outbreak in Routt National Forest, Colorado. For each tree we recorded effect by fire (no visible effect; visible effect – i.e. charring), recent effect by MPB (no visible effect; infestation – i.e. presence of MPB entry or exit holes; or mortality), dbh, and age (based on increment core samples).Tree diameter (dbh) was the most important factor in determining susceptibility to MPB such that larger trees were more susceptible to MPB. But once dbh was taken into account, trees that were charred were more likely to have been attacked and killed by MPB. Previous work has found that stand-replacing fires reduce susceptibility of lodgepole pine stands to MPB in these ecosystems. The current results highlight the fact that fires that are below a high threshold of severity and instead injure or otherwise weaken trees, may increase susceptibility to MPB. Fire-affected lodgepole may act as vectors for the spread of outbreaks during moderate outbreak conditions or as refuges during endemic population phases. It is important to consider how low-severity fires, including prescribed burns, may increase forest susceptibility to outbreaks at local to landscape scales.

► Both fire and insect outbreaks are critical components of many forest ecosystems.
► We examined how fires and tree attributes affect susceptibility to mountain pine beetle (MPB).
► Tree size was most important in determining susceptibility in early stages of the outbreak.
► Among larger trees, low-severity fires increased susceptibility of lodgepole pine to MPB.
► Fire-affected lodgepole may act as vectors for the spread of outbreaks.