El Niño Southern Oscillation (ENSO) is a climate-forcing mechanism that has been shown to affect precipitation and the occurrence of wildfires in many parts of the world. In the southwestern United States and northwestern Mexico, warm events (El Niño) are associated with moist conditions and fewer fires, while cool events (La Niña) tend to favor dry conditions and more fires. The opposite is the case in southern Mexico: El Niño years tend to be dry and La Niña years tend to be wet. In a region of northeastern Mexico, midway in latitude between locations where El Niño has wetting effects (northwestern Mexico) and drying effects (southeastern Mexico), and where ENSO has displayed variable patterns, we characterized the fire regime by collecting 100 fire-scarred samples of trees from three high-elevation sites. We then analyzed the relationship between fire occurrence and ENSO, drought, and other climatic influences.
The study area was characterized by frequent surface fires before the 1920s. After 1929, fires were nearly eliminated, most likely due to human causes. The three sites were not particularly synchronous; the three sites burned simultaneously only seven times between 1774 and 1929. In 1998, however, after a very strong El Niño event, all three sites experienced a severe, stand-replacing fire. We found a temporal change in the association between ENSO events and fires: before 1830, La Niña events were significantly associated with fire years; after 1830, the relationship was no longer significant, and in 1998 the most severe fire in the last 500 years occurred during the most extreme El Niño event of the century. Historically, fires tended to occur during dry La Niña years, but El Niño events have been associated with both very wet years and very dry years (and severe fires) in this region in the last 3 decades. Our results suggest that the influence of ENSO on precipitation and fire has changed over time in this site.