Testing a pyroclimatic hypothesis on the Mexico–United States border

Abstract

The "pyroclimatic hypothesis" proposed by F. Biondi and colleagues provides a basis for testable expectations about climatic and other controls of fire regimes. This hypothesis asserts an a priori relationship between the occurrence of widespread fire and values of a relevant climatic index. Such a hypothesis provides the basis for predicting spatial and temporal patterns of fire occurrence based on climatic control. Forests near the Mexico-United States border offer a place to test the relative influence of climatic and other controls in mountain ranges that are ecologically similar and subject to broadly similar top-down climatic influence, but with differing cultural influences. We tested the pyroclimatic hypothesis by comparing fire history information from the Mesa de las Guacamayas, a mountain range in northwestern Chihuahua, with previously published fire data from the Chiricahua Mountains, in southeastern Arizona, approximately 150 km away. We developed a priori hypothetical models of fire occurrence and compared their performance to empirical climate-based models. Fires were frequent at all Mesa de las Guacamayas study sites through the mid-20th century and continued uninterrupted to the present at one site, in contrast to nearly complete fire exclusion after 1892 at sites in the Chiricahua Mountains. The empirical regression models explained a higher proportion of the variability in fire regime associated with climate than did the a priori models. Actual climate-fire relationships diverged in each country after 1892. The a priori models predicted continuing fires at the same rate per century as prior to 1892; fires did in fact continue in Mexico, albeit with some alteration of fire regimes, but ceased in the United States, most likely due to changes in land use. The cross-border comparison confirms that a frequent-fire regime could cease without a climatic cause, supporting previous arguments that bottom-up factors such as livestock grazing can rapidly and drastically alter surface fire regimes. Understanding the historical patterns of climate controls on fire could inform the use of historical data as ecological reference conditions and for future sustainability.

Linking old‐growth forest composition, structure, fire history, climate and land‐use in the mountains of northern México

“…We measured all dead and live 1000-hour timelag fuels (.7.61 cm at the narrowest point), and the depth in cm of the organic (duff and litter) layers every 5 m. We recorded the presence or absence of canopy cover by vertical projection every 1 m along the transect. We collected and crossdated 157 partial cross-sections from fire-scarred trees on the four sites and used them for fire history analyses (Fulé et al 2012). …”

“…Old-growth forests provide key habitat for species that are not found in secondary or harvested forests (Lammertink et al 1998, Miller and and act as sinks in terrestrial carbon dynamics (Carey et al 2001, Luyssaert et al 2008, Keeton et al 2010. They also can provide information regarding processes that may affect ecosystems over time, such as climate, fire, and other disturbances (Swetnam 1993, Swetnam and Baisan 2003, Kaufmann et al 2007, Fulé et al 2012. However, the area covered by OGFs has decreased dramatically worldwide in the past century (Boucher et al 2009, Wirth et al 2009).…”

“…Despite the limited information about its biological and ecological systems, it has been estimated that temperate forests of the SMOc have a plant endemism rate of 70%, the highest identified for all vegetation types in México (Felger et al 1997). Its temperate forests also offer unique opportunities to study systems with maintained fire frequencies that in some cases have remained unaltered into the 21st century (Fulé et al 2011, Fulé et al 2012.…”

“…While fire suppression policies have been in place in México since the early 1900s (Rodríguez Trejo 1999), loose enforcement and relatively sparse resources-compared to the United States-have allowed historical fire regimes to continue up to the present in some remote sites of the SMOc (Heyerdahl and Alvarado 2003, Stephens et al 2008, Fulé et al 2011, Fulé et al 2012. Previous studies in the SMOc suggest a strong relationship between fire occurrence and large-scale climatic patterns, such as the El Niño Southern Oscillation.…”

“…We studied four old-growth sites at Mesa de las Guacamayas in northwestern México, measuring forest structure, composition, age distribution, tree regeneration, surface and canopy fuels, and the characteristics of large snags. We used these data, together with fire history information from a companion study (Fulé et al 2012), to answer the following questions: (1) What are the attributes of the structure and floristic composition of the overstory, regeneration, and fuel loads? (2) What are the interactions of fire, regional climatic variability, and human-induced land-use changes across a ;250 year timeline in the landscape represented by the four study sites?…”