CANCELLED - Top-down and bottom-up control of fire regimes in montane grasslands of the Valles Caldera, New Mexico, USA

Background/Question/Methods

Montane grasslands are widely distributed across the western United States, including the southern and central Rocky Mountains, but little is understood about their historic fire regimes. These ecosystems provide habitat for numerous species of flora and fauna, while benefiting local economies through livestock grazing and recreation. Land managers require specific information about past fire regimes in these systems, including the frequency, extent, and seasonal timing of fires, their spatial complexity, and the role of climate variability. To provide this historical perspective, we reconstructed the historic fire regimes at multiple scales in the montane grassland/forest ecotone in the Valles Caldera National Preserve (VCNP) of the Jemez Mountains in northern New Mexico.  We employed dendrochronological methods to develop a spatially-explicit reconstruction of historic fire occurrence within and among valles (montane grasslands) as recorded in the surrounding fire-scarred ponderosa pine and mixed conifer species. This fire reconstruction will cover an area of ~ 40,000 ha in one of the most diverse landscapes in the American Southwest. Using a spatially-explicit sampling design, we collected 610 fire-scarred samples from predominately relict wood in 27 sites surrounding the perimeter of the eight valles.

Results/Conclusions

We identified 181 fire years from AD 1479 to 1900, with adequate sample size after AD 1700. Preliminary results confirm pre-1900 historic occurrence of high frequency, low-severity surface fires over multiple centuries (mean fire interval = 2.33 yr). In some fire years, synchronous fires burned across the grasslands and into the surrounding forests over much of the ~40,000 ha Caldera (greater than 35% of total sites burning), indicating dominant top-down climate control of fire occurrence. These widespread fires typically occurred at ~10.5 year intervals. In other years, fires burned relatively small portions throughout the Caldera (between 10 and 35% of total sites at ~6 year intervals), creating asynchronous burn patterns. Asynchronous fires suggest stronger bottom-up controls driven by fuels, fuel moisture, weather conditions, topography, and human ignitions. Results of this study will be used in planning forest and grassland restoration actions, and reinstituting pre-fire-suppression fire regimes for prescribed and natural fire management.