Frequent surface fires in western forests fostered resistance to drought, insect outbreaks, and extreme fire-weather conditions. Fire exclusion in the 20th century resulted in abrupt shifts in tree abundance and species composition. For example, relatively sparse ponderosa pine-Douglas fir forests were transformed into high-density forests that include fewer ponderosa pine but orders of magnitude more Douglas-fir and a massive influx of white fir. This altered state maxes out the site capacity and is susceptible to high-intensity fire and insect outbreaks. Many forests are already at their biological limits for productivity and climate projections suggest a warmer, drier future; making them vulnerable.
Dendrochronology (tree-ring) studies of pre-fire exclusion forest species composition and structure and of changes over the past century help identify those forests most likely to benefit from restoring fire as an ecosystem process. These studies can also indicate where restoration is unlikely to influence vulnerability to future disturbances. Prioritizing management actions based on the ability to promote resilience in these altered forests can help avert forest die off from drought, insect outbreaks, and unprecedented extreme fire behavior.
Future studies could look at management actions that can be taken to mitigate climate impacts in forests that have always been constrained by site productivity.