Fire History of Montane Grasslands and Ecotones of the Valles Caldera, New Mexico, USA

Montane grasslands are distributed across the Southwest, but there has been little quantitative study despite their biological and economic value. The study area is the montane forest-grassland ecotone within the Valles Caldera National Preserve (VCNP) northwest of Albuquerque, New Mexico, situated in the heart of the Jemez Mountains. We hypothesize that the fire history record of the ecotone should closely reflect the fire regimes in the montane grasslands throughout the study area and that this zone served as a pathway to encourage the spread of fire to other valles. To test our hypothesis, we reconstructed historic fire regimes at multiple spatial scales in ponderosa pine and mixed-conifer forests surrounding the “valle” grasslands to determine temporal and spatial patterns and then compared variations of historic fire occurrence within and among valles as recorded in fire-scarred trees. Thus, our sampling design is spatially-explicit in order to capture patterns of both spatial and temporal synchrony or asynchrony in the grassland-forest fire regime. We cross-dated 2338 fire scars representing 191 separate fire years from an adequate sample size from AD 1601-1902. Our reconstructed fire regime results confirm pre-1900 historic occurrence of high frequency, low-severity surface fires over multiple centuries in the forest-grassland ecotone. When all fires were considered, mean fire intervals (MFI) were 5.5 to 22.5 years at individual sites; the MFI for all fire dates at sites combined into their respective valles was 2.7 to 10 years; the MFI for all fire dates at the landscape level was 1.6 years. Results of this study will be useful in planning forest and grassland restoration actions, and reinstituting pre-fire-suppression fire regimes through prescribed and natural fire management.