Widespread Fire Years in Conifer Forests are Contingent on Both Winter and Monsoon Precipitation in the US-Mexico Sky Islands

Abstract

The climate of the Southwestern United States and northwestern Mexico is marked by a bimodal precipitation regime with the majority of moisture arriving during (1) the cool season via intermittent frontal storm systems (November to February) and (2) intense, convective storms during the North American Monsoon (NAM) (July to September). Fire season in the region occurs primarily during the arid fore-summer in May and early June prior to the arrival of the NAM. Most long-term tree-ring based studies of fire-climatology in the region have evaluated only the role of winter precipitation. We used tree-ring width based reconstructions of both winter and monsoon precipitation, coupled with fire scar based tree-ring reconstructions of fire history from twelve mountain ranges in the US and Mexico at the northwestern margin of the NAM to quantify the historical role of both seasons of precipitation in modulating widespread fire years. Winter precipitation was the primary driver of widespread fire years in the region, but years with drought in both seasons had the highest fire frequency and most widespread fires. The timing and amount of monsoon precipitation can be important factors in ending the most severe fire weather conditions. Monsoon drought coupled with earlier snow melt in the spring appears to contribute to an extension of fire weather for longer periods, leading to more and larger fires.