The mountain pine beetle. Image from Wildfire Today

Since the turn of the century, forests across the western United States and Canada have been ravaged by bark beetles, particularly the mountain pine beetle (MPB).  While the MPB is native to western forests, the current outbreaks are unprecedented in extent and severity.  Western lodgepole pine and ponderosa pine forests are particularly vulnerable to beetles due to historic management trends, in which widespread timber harvests to support mines and railroads at the beginning of the twentieth century – and aggressive fire suppression since then – has led to homogeneous, overstocked stands of trees that are the ideal host size.  Beetle spread has likely been accelerated by warmer winters increasing beetle survivability, and drier summers weakening potential host trees.

MPB-affect landscape in Canada, showing green-phase (healthy and recently-infected trees), red-phase (needles dead and discolored), and grey phase (needles dropped, but snags remain standing) progression. Image from the University of British Columbia

MPB and other bark beetles infect host trees as part of their reproductive lifecycle.  They swarm trees, boring through the outer bark, and then carving lateral galleries through the water-conducting inner bark in which they lay their eggs.  Healthy trees combat beetles by exuding sap that physically pushes the invaders out, though this defense is overwhelmed if the tree is weak or the beetle too numerous.  While some trees seem better adapted to resist beetles than others, most trees will die rapidly from a combination of loss of water conductance (girdling) and infection from symbiotic blue-stain fungus introduced by the beetles, and mortality often reaches >80% in pure pine stands. These dead trees change from green- to red -to grey-colored as their needles gradually die and drop, though the dead trunks typically stay standing for 5-10 years until rot and wind knock them over.  Beetle-killed stands switch from carbon source to sink, likely subject to more sever wildfire (though not more frequent).