Western United States

The Greater Yellowstone Ecosystem and the Colorado Rockies enhance the international component of WildFIRE PIRE, because they extend the gradients of climate and land-use, span a range of fire regimes, and utilize extensive ongoing research on climate-fire-human linkages.

Land-use gradients range from wildland reserves at high elevations to ex-urban development, logging, and grazing at middle elevations, to irrigated agriculture and growing communities at lower treeline. Hunter-gatherers occupied the Rocky Mountain region for at least 12,000 years, but unlike NZ and Tasmania, the extent and impacts of deliberate burning are geographically unclear.

High-elevation mesic conifer forests seem to be little altered by prehistoric or recent human activities, because they naturally experience long fire return intervals and high fuel build-up. Fires at the lower forest/grassland transition are naturally more frequent and currently represent areas of high fire risk. In the last 20 years, both regions have experienced large severe fires, as a result of early snowmelt, wet springs, and dry summers.

Catastrophic fires are projected to increase in both areas with future climate and land-use change. Forests are currently under attack by native bark beetles and budworm and non-native blister rust, and large tracks of dead and dying conifers are altering fuel conditions and raising major management concerns. Likewise, invasive grasses and forbs have increased in density and spatial extent at all elevations. The life history of many non-native grasses (e.g., Bromus tectorum and Taeniatherum caputmedusae) has changed the timing of the fire season and the success of ignitions, and increased fire frequency has created feedbacks that perpetuate annual invasive plants and decrease native perennial plant reproduction and survival.

In the western U.S. regions, we will examine:

  • Historical range of variability in fire regimes over the last 5000 years, filling in key information gaps. We will develop charcoal and pollen-based fire and vegetation reconstructions in 3-4 new watersheds in the subalpine zone of northern Colorado to complement ongoing investigations based on tree-ring research. In the GYE, new tree-ring studies at the lower forest-steppe ecotone will build on existing pollen and charcoal studies in the region.
  • Site-specific human histories to better assess the consequences and timing of different land-use practices with respect to past fire regimes and climate conditions, building on current studies that extend back to 1950 AD with archaeological, documentary and remotely sensed data to evaluate current conditions and recent fire events.
  • Fire feedbacks on plant-invasion processes and thresholds by explicitly linking with plant invasion and fire behavior models. For example, variables, including fire, which influence cheatgrass invasion in the western U.S. will be examined.
  • Interactions between fire, climate change, and insect outbreaks over the last 5000 years, maximizing overlap between proposed and existing tree-ring and lake-sediment records in the GYE and CR, and integrating multiple disturbances into long-term disturbances histories.