CLIMAS: Overview of Fire Research

Fires that do not kill a tree often leave a scar, which is recorded in the tree's annual growth ring. By carefully examining the tree rings, researchers can determine the year and often even the season in which the fire occurred. Tree-ring cores are obtained from 20 to 30 different trees per acre in each study site; the collection of multiple samples is necessary because no individual tree gives a complete history. Spatial scales of tree-ring analysis range from that of individual stands to the watershed level, entire mountain ranges, up to the broader regional scale. At this larger scale, broad patterns can be discerned through crossdating of tree rings. This allows identification of the year a fire occurred and the synchrony of fire across space. This type of crossdating can provide exact years when fires occurred.

Fire is a key ecological process in the Southwest, and patterns of change in the fire-scar record are interpretable in the context of climatic variation and changes in land use and forest stand structures (including fuel conditions). The Southwestern United States is the only region of the world where extensive networks of century-scale climate and fire reconstructions based on tree-rings have been compiled with annual resolution (and seasonal resolution in the case of the fire reconstructions). The crossdated fire records are called fire histories.

Spatial domain of tree-ring fire histories. (View larger image)

Credit: Baisan and Swetnam

Seasonal fire scar analysis. (View larger image)

Credit: Allen,Swetnam and Betancourt

To examine long-term relations between climate and fire in the southwestern United States, regional values for tree-ring growth in precipitation-sensitive trees growing at sites unaffected by fire in Arizona and New Mexico are compiled. These data explain at least 50% of the variance in water-year precipitation (October prior to the year of tree growth to September of the year of tree growth) with the largest response in autumn and spring. These two seasons exhibit the strongest teleconnections (atmospheric connections between ocean or land surface conditions in one location and related weather effects in a distant location) between precipitation in the Southwest and climatological conditions in the tropical Pacific, where El Niño-Southern Oscillation (ENSO) develops.

The fire history records are then compared with proxy climate records developed from tree rings (and other precipitation proxies, including historical records and coral and ice-core records of ENSO). These comparisons highlight the effects of climate on fire occurrence.

Connection with Climate >