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Forests transformed by fire exclusion help us understand climate resilience

Date: November 06, 2017

Climate projections forecast rapid changes resulting in large-scale forest die-offs. Historically, surface fires fostered resilience to climate anomalies and other ecological disturbances


Background

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.

Tree recruitment patterns during a natural fire regime (left) and a fire exclusion period (right).  Once the natural fire interval is exceeded, primary species continue to accumulate and secondary species that are fire-prone also recruit abundantly.
Tree recruitment patterns during a natural fire regime (left) and a fire exclusion period (right). Once the natural fire interval is exceeded, primary species continue to accumulate and secondary species that are fire-prone also recruit abundantly.
     
Results of fire exclusion on four vegetation types in the Pinaleño Mountains of Arizona.  Dry and mesic mixed-conifer forests have undergone the greatest change in species composition and structure, and would benefit the most from restoration treatments.
Results of fire exclusion on four vegetation types in the Pinaleño Mountains of Arizona. Dry and mesic mixed-conifer forests have undergone the greatest change in species composition and structure, and would benefit the most from restoration treatments.

Research

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.

This large Douglas-fir died in 2012 and is surrounded by many smaller Douglas-fir, white fir, and Southwestern white pine that recruited during fire exclusion.  Stand density in mesic mixed conifer forests increased on average 1725% during fire exclusion.
This large Douglas-fir died in 2012 and is surrounded by many smaller Douglas-fir, white fir, and Southwestern white pine that recruited during fire exclusion. Stand density in mesic mixed conifer forests increased during fire exclusion.

Key Findings

  • Fire exclusion can and has produced rapid changes to forest species composition and structure.
  • Restoring disturbance-adapted species composition and structure to forests most changed by fire exclusion may buffer them against future drought, fire, and insect outbreaks.
  • Forests that have not experienced much change as a result of fire exclusion have fewer management options to promote adaptation to future disturbances.
 

Featured Publications

O'Connor, Christopher D. ; Falk, Donald A. ; Lynch, Ann M. ; Swetnam, Thomas W. ; Wilcox, Craig P. , 2017


Principal Investigators: 
Forest Service Partners: 
Craig P. Wilcox, Lincoln National Forest
External Partners: 
Donald A. Falk, University of Arizona School of Natural Resources and the Environment
Thomas W. Swetnam, University of Arizona Laboratory of Tree-Ring Research.
Research Location: 
Pinaleño Mountains of southeast Arizona