Are 21st Century Wildfires Bigger and Hotter Than Ever?

Wildfires have been raging over the past two months throughout the Rocky Mountains, with the most recent of the devastating fires occurring in Idaho, Wyoming and Montana.  Several fires have resulted in significant damage, with at least 200 homes burned in each of the Waldo Canyon (Colorado Springs, CO), High Park (Ft. Collins, CO) and Little Bear (Ruidoso, NM) fires [www.inciweb.org]. When I saw the images of the Waldo Canyon fire, I thought of many friends and family who have struggled with evacuations and loss of homes over the past decade from fires in the Southwest.  While people and forests recover from these tragedies, scientists and managers are continuously working to better understand the role of fire in our forest ecosystems, and to learn how to limit damage from future fires. 

The 2012 fire season began in Arizona in early May with a series of small fires, including the Gladiator (Prescott National Forest) and Bull Flat (Fort Apache Agency) fires in central Arizona.  Meanwhile in New Mexico, the Whitewater-Baldy complex burned in the Gila wilderness of southern New Mexico.  Given the steep, rocky terrain and high winds, the fire reached 100,000 acres within the first week, with huge swaths of forest consumed in a single day (70,000 acres on May 23).  The fire continued to burn throughout June, reaching 297,845 acres by the time of full containment at the end of June.  Ultimately, the Whitewater-Baldy complex fire surpassed the size of the Las Conchas fire (156,293 acres) of 2011, and became the largest fire in New Mexico history.  By comparison, the current and former record-holders in Arizona are the Wallow (538,000 acres in 2011) and Rodeo-Chediski fires (460,000 acres in 2002).  The relatively larger size of the Arizona fires may be due to more gentle terrain and continuous forest cover of the Mogollon Rim, which would have helped these fires to cover more ground.

One thing to keep in mind is that the fire intensity (energy emitted during fire) and effects (impact on the ecosystem) vary considerably within the boundaries of a wildfire.  Once a fire has passed, the resulting damage to the forest and soils is mapped as low, moderate or high severity.  The fire severity classification measures the depth of charring beneath the soil surface and degree of vegetation mortality above ground.  While the photos in the media focus on the high-severity areas of complete tree mortality, there are often large patches of low and unburned forest within the fire perimeter.  These unburned areas serve as refuges for wildlife and sources of seed for future tree regeneration. 

The fires of this current season are part of an increasing trend in burned area since about 1970 across the western US.  The increase over this 40-year period is related to warming temperatures of climate change and land management practices of the past century.  Grazing and fire suppression over the 20^th century prevented natural fires and increased fuels available for burning (see previous blog).  Yet fire occurrence in any given year is related to annual to decadal drought conditions and extreme fire weather (i.e., high winds, low humidity) on a daily to weekly basis (see previous blog).  Given the extreme behavior of recent fires, researchers are very interested in looking further into the past to understand ‘natural’ fire behavior prior to the 20^th century.  Key questions that researchers try to answer are: Were fires as large and severe in the past?  Was there a time period when fires were more frequent than we experience today?  

My research as a PhD student at the University of Arizona documents forest fires over the past 3,000 years using both sediment records and tree-ring samples to reconstruct fire history information.  The setting for my research is the San Juan Mountains of southwestern Colorado, where forest types and climate conditions are similar to northern New Mexico and Arizona.  The forest cover is composed of ponderosa pines with piñon and juniper growing on drier, south-facing slopes and mixed-conifer and spruce-fir in the upper elevation and north-aspects. 

I hike into small drainages along the broad, glacially-carved valleys of the San Juans and look for cuts in the banks of small streams.  Within these natural exposures of stream sediment, I search for charcoal layers within the sediment.  I scrape and dig into the sediment exposures, while taking notes on the character (coarse vs. fine-grained) and thickness of individual sediment layers.  When I find charcoal, I take samples of charcoal separately, which I bring back to the lab for radiocarbon dating.  Aside from the sediment records, I survey the hillslopes above the stream channels for fire-scarred trees.  Fire-scarred trees provide a record of surface fires from the past ~ 500 years by preserving damage from individual fire events in the growth pattern of the tree.  Cross-sections are removed from the trees (usually dead) with a chainsaw and brought back to the lab for dating of fire scars.

Fires have burned continuously over the past 3,000 years (and much longer beyond what I observe in my data), though with variations in the frequency, size and severity of fires.  There was more fire activity during the Medieval Warm Period (see previous blog), a period of increased droughts from 900 – 1300 AD.  During this period, my data shows that fires may have included areas burned as severely as we see today, although the extensiveness of severe burned patches is unclear. This is based on evidence of coarse, bouldery debris-flow deposits (poorly-sorted mixture of clay to boulders flowing down a stream channel or hillslope) covered by thick layers of ash and charcoal.

For the past 500 years, the tree-ring record shows a more detailed picture of the fire history, with evidence of frequent surface fires burning throughout the lower-elevation conifer forests.  During drought years, surface fires were extensive, while small patches (< 25 acres) of severe fire burned on the north-facing slopes.  Fire historians working throughout the Southwest have long observed that frequent surface fires were common in lower elevation forests until the end of the 19^th century.  Natural fires were virtually eliminated from the forests during the 20^th century, contributing to forest overgrowth, and resulting in the catastrophic fires we observe today.  Combined with evidence from the sediment record, my gut feeling at this point is that fires today are more severe than we’ve seen in the past 3,000 years.  They are not anomalous in overall size, but the extensiveness of high-severity burned areas seems to be enhanced by fuel build-up and warming climates today.

Clearly, given climate change predictions, there is a strong indication that future fires will continue to be large and severe.  However, thinning the forests can greatly reduce the risk to southwestern communities, and at a lower cost than fighting catastrophic fires.  If we continue to focus on forest restoration and the translation of science between researchers and managers, then the damage from future fires can be lessened.