Influence of climate and local factors on fire in high-elevation forests of Mexico

Factors that affect fire occurrence operate on a continuum of scale, from fine-scale microsite variation in topography, fuels, ignitions, and weather to broad-scale global climate oscillations. Fine-scale factors interrupt fire synchrony across landscapes, while broad-scale factors synchronize fire across landscapes and even continents. This dissertation assesses the influence of fine-scale and broad-scale factors on fire occurrence in eastern and northern Mexico.;At Pena Nevada in northeastern Mexico, a temporal change occurred in the association between the broad-scale climatic factor El Nino Southern Oscillation (ENSO) and fires; before the 1830s La Nina events were significantly associated with fire years, while after the 1830s this association was not significant. This result suggests that ENSO effects have changed over time in this location and that phases of ENSO are not consistent indicators of precipitation, fire occurrence, or fire behavior in this part of Mexico.;At the northern end of the Sierra Madre Oriental in northeastern Mexico, three parallel mountain ranges were chosen to distinguish the influence of broad-scale vs. fine-scale factors. The mountain ranges received nearly identical broad-scale climatic influence, but spread of fire between the ranges was unlikely. Broad-scale control would be indicated by high fire synchrony among mountains while fine-scale control would be indicated by asynchrony in fire occurrence. Fires were asynchronous among mountains, indicating a strong influence of fine-scale factors on fire occurrence.;In southeastern Mexico, Pico de Orizaba is North America's third-tallest peak. In 1975, researchers reported that increased human-caused burning was degrading the forests in this location. We investigated the fire regime and forest structure in this area and found that in the twentieth century a fire was recorded in at least one of six sites in 90 of 100 years; this very frequent surface fire regime consisted of mostly small and asynchronous fires. Inter-annual climatic variability was not an influential driver of fire, and no evidence of forest degradation was found in the research plots. A trend in the 21 st century toward decreased fire could be cause for concern, as this could lead to an increase in tree density and a loss of resilience in the face of climate change and other future disturbance.;Finally, the influence of climate on the occurrence of fire across northern Mexico was assessed using a network of 52 sites in 5 regions in the Sierra San Pedro Martir, the Sierra Madre Occidental, and the Sierra Madre Oriental. Across-region synchronous fires in northern Mexico were significantly associated with negative (La Nina) phases of ENSO and cool phases of the Pacific Decadal Oscillation (PDO). Although climate was a strong driver of fires historically and through the twentieth century at some sites, dates of fire regime interruption across northern Mexico were highly variable within and among regions. This result suggests that human land use change is the strong driver of fire iii regime interruption, and climate played little or no role in the widespread cessation of fire across much of western North America in the nineteenth and twentieth centuries.;In summary, this dissertation showed that both broad-scale and fine-scale factors influenced the occurrence of fire in eastern and northern Mexico, and many factors that affect fire occurrence cannot be disentangled. Understanding the historical influence of climate variability on fire occurrence may help us understand how future climate change will affect fire activity in western North America.