We reconstructed historical fire regimes of montane forest-grassland ecotones in the ~40,000 ha Valles Caldera National Preserve, New Mexico. We used a targeted approach to sample ancient fire-scarred trees along the ecotone, and compared variations in historical fire occurrence within and among valles in the grassland-forest. The resulting tree-ring record extends from 1240-2008 C.E., comprised of 2,443 fire scars from 330 trees representing 238 fire years during the period of analysis, 1601-1902 C.E. Our results confirm pre-1900 historical occurrence of high-frequency, low-severity surface fires over multiple centuries in the ecotone. Mean fire intervals for all fires were 5.5-22.5 years (~6-123 ha) at individual sites, 2.7-10 years (~67-4955 ha) in individual valles, and 1.6 years (~10 386 ha) across the landscape. Synchronous fires burned extensively and occurred at ~10 year intervals during years with significantly low PDSI. Results will be useful in planning forest/grassland restoration actions and reinstituting fire regimes.