Land use, climate, and recent patterns of wildfire severity across a transboundary ecoregion of the United States-Mexico borderlands

Background/Question/Methods

Mountain ranges in the Madrean ecoregion of the southwestern United States and northern Mexico have similar species assemblages and topographic characteristics, but have been managed in strikingly different ways since the mid-1800s. Differences in land use intensity and forest management, particularly active wildfire suppression in the U.S. and lack thereof in Mexico, have led to contrasting fire regimes which may provide information to help guide forest restoration in the U.S. and inform Mexico’s fire management plans. In the U.S., federal agencies developed the Monitoring Trends in Burn Severity (MTBS) national database of fire location and burn severity, however similar data for many remote areas outside of the U.S. are generally sparse. To help fill this data gap, we developed a semi-automated process to identify historical wildfire occurrence in Mexico using Landsat Thematic Mapper data time series from 1985-2011. We circumvented the need for a prioriknowledge of fire occurrence by combining differenced Normalized Burn Ratio (dNBR) images covering sequential and overlapping seasonal blocks, identifying 83 large (> 1,000 acre) wildfires in the northern Mexico portion of the Madrean ecoregion during the period. We compared fire severity of these fires to fires in the U.S. using dNBR and the relative dNBR values, and examined patterns in the number, size, and timing of wildfires across the ecoregion.

Results/Conclusions

Comparison of dNBR and the relative dNBR values across border indicate increasing severity at high elevation forests in the U.S., but no similar trend in forests in Mexico. In contrast, savanna and woodland fires showed increases in fire severity over time across the ecoregion. Fire years with numerous large burns were generally different across the border from 1985-1997, but became more synchronous from 1997-2011. This transboundary analysis of fire severity suggests that apart from climate, land use history may partially explain the recent increase of high severity fires in the U.S., but regional climate and drought may be contributing to fire severity at lower elevations across the ecoregion.