Fire plays a key role in regulating the spatial interactions between adjacent vegetation types from the stand to the landscape scale. Fire behavior modeling can facilitate the understanding of these interactions and help managers restore or maintain fire’s natural role. The Valles Caldera National Preserve (VALL), in the Jemez Mountains of northern New Mexico, USA, contains one of the largest montane grasslands in North America and extensive areas of grassland−forest ecotone. We used the Minimum Travel Time (MTT) module in FlamMap to investigate the primary fire-growth vectors on the VALL landscape for the 50th, 90th, and 99th percentile of fire weather conditions. We evaluated whether modeled fire-growth vectors tended to follow the grassland−forest ecotone or if fire traveled directly across the grasslands and over the upland forest with a chi-square test. Our results indicated that the ecotone is a primary corridor for fire growth on the VALL landscape. Regular fire spread along the grassland−forest ecotone may help stabilize the boundary zone between these two dynamic communities by preventing forest encroachment into the grassland and maintaining an open stand structure. Identifying the dominant fire corridors will help land managers re-establish the spatial and process dynamics of the natural fire regime.