In both Petran montane forests of the southwestern US and Madrean montane forests of northern Mexico, similar ecological processes of frequent, low-intensity fires maintained relatively open forests of large trees in the past. A comparison of examples from an unharvested ponderosa pine forest of northern Arizona, USA, with over 100 years of fire exclusion, and an unharvested mixed-species pine forest of northwestern Durango, Mexico, with approximately 50 years of fire exclusion, show that tree populations have irrupted, small trees are densely aggregated in clumped stands, and fuel loading and vertical continuity have increased, raising the susceptibility of these sites to high-intensity crown fires. In contrast, a reconstruction of the pre-fire-exclusion structure of the Arizona site as well as data from another Durango site currently under a frequent fire regime both share open forest structures with low fuel loads and little vertical fuel continuity. These ecological similarities suggest that Mexican forest managers may benefit from studying the deleterious effects of long-term fire exclusion in the US, while both US and Mexican managers could make use of unharvested, frequent-fire sites in Mexico as benchmarks of ecosystem process and structure.