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Surface fire has increasingly been regarded as one of the most critical threats to tropical forests (Laurance 2003). However, much of the research documenting degradation and destruction of tropical forests by fire comes from the low-elevation humid tropics (Cochrane 2003). Although fire has been acknowledged as an important part of the ecology of high-elevation tropical forests, it has received little research attention (e.g. Smith & Young 1987). Similar to low-elevation tropical forests, high-elevation tropical forests face threats including climate change, livestock grazing and illegal logging (Toledo-Aceves et al. 2011). High-elevation tropical forests are important conservation targets because they form isolated archipelagoes of rare ecosystems, often separated by low-elevation ecosystems and human development.
The fire regime in the high-elevation tropical forests of central Mexico is relatively unknown; no long-term dendrochronological reconstructions of fire have been carried out in the area. Rodríguez-Trejo & Fulé (2003) suggested that three categories of forest exist in Mexico: (i) forests that have had less fire in recent decades as compared to historical fire occurrence due to human-induced fire exclusion, (ii) forests that have continued to burn at frequencies and severities similar to historical patterns and (iii) forests that have experienced excessive fire with deleterious ecological consequences due to human practices of setting fire for agricultural or other uses (as documented in Román-Cuesta, Retana & Gracia 2004). Fire history studies that have been conducted in Mexico to date have found either regular, frequent fires continuing up to the present (e.g. Fulé et al. 2011), or an abrupt cessation of fires correlated with increased human land use including livestock grazing, road building and timber harvesting, and often associated with the formation of ejidos (e.g. Heyerdahl & Alvarado 2003; Yocom et al. 2010).
In central Mexico, a series of tall volcanic peaks form a chain of geographically isolated islands of high-elevation tropical forest. At the tree line on these volcanoes, the forests consist of monospecific stands of Pinus hartwegii Lindl., a fire-resistant species found in Mexico's highest-elevation forests (Rodríguez-Trejo & Fulé 2003). As P. hartwegii is restricted to the uppermost elevations, its populations are highly isolated and this has led to significant genetic divergence (Schaal & Leverich 1996). Fire history studies are rare in high-elevation tropical forests (but see Martin & Fahey 2006) and in tropical forests in general, but two fire history studies have been completed in P. hartwegii forests, both several hundred kilometres north of Pico de Orizaba in the Sierra Madre Oriental of north-eastern Mexico. Those studies showed that the historical fire regime was characterized by frequent surface fires with mean fire intervals (MFIs) ranging from 8·6 to 16·4 years (Yocom et al. 2010; Yocom 2011). However, the range of P. hartwegii extends from northern Mexico south to Guatemala and Honduras, and it is possible that other populations of this species, including populations south of the Tropic of Cancer, may experience different fire regimes. This study is the first of its kind in high-elevation forests south of the Tropic of Cancer in Mexico (Biondi, Hartsough & Galindo Estrada 2005), designed to analyse the ecological effects of fire on these rare and valuable ecosystems.
Pico de Orizaba is a high volcanic peak in a national park in Mexico (Fig. 1) where forest degradation has been attributed to human-caused fires. Researchers who studied the timberline at Pico de Orizaba in the 1970s observed that people using the lower slopes of the mountain for livestock grazing set fires for agricultural reasons (Lauer & Klaus 1975). Although Lauer & Klaus (1975) did not quantitatively study the fire regime, nor did they specify exactly when human-caused degradation began, they stated that recent human-ignited fires on these volcanoes were different in several ways from natural lightning-caused fires: (i) they took place almost every year, whereas natural fires were estimated to occur every 6–7 years, (ii) they were usually set in February and March while natural fires occurred at the beginning of the rainy season in May and (iii) they were started below the timberline and swept up into the crowns of trees by the upslope wind, while natural fires typically started at timberline and moved downslope as surface fires. They speculated that the upper timberline at Pico de Orizaba had become lower in elevation due to human-caused fire (Lauer & Klaus 1975; Lauer 1978). The current study tests several of their ideas.
Figure 1. (a) Map of Mexico with a black square showing the location of Pico de Orizaba National Park (square is not to scale). Circles indicate 775 weather stations with at least 90% complete data for at least 30 years. Filled circles indicate a negative correlation (<−0·1) between precipitation and Southern Oscillation Index values, and unfilled circles indicate a positive correlation (>0·1). Circle size is proportional to strength of the correlation. (b) Map showing timberline location of six sites between Pico de Orizaba and Sierra Negra in Pico de Orizaba National Park. Image from Google Earth. (c) Photograph of Site 1, with Sierra Negra in the background. (d) Map showing distribution of Pinus hartwegii in isolated sites in Mexico and Central America (Data from Critchfield & Little 1966).
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The geographical location of Pico de Orizaba National Park is also ideal for testing fire relationships with El Niño–Southern Oscillation (ENSO). The dipole between northern Mexico, where La Niña events tend to be correlated with dry conditions, and southern Mexico, where El Niño events tend to be correlated with dry conditions, is located close to the Tropic of Cancer (Fig. 1). In north-western Mexico, several studies have linked fire occurrence to La Niña events (Fulé & Covington 1999; Heyerdahl & Alvarado 2003; Fulé, Villanueva-Díaz & Ramos-Gómez 2005; Skinner et al. 2008). In north-eastern Mexico, the situation is more complicated, with a finding at Peña Nevada in the Sierra Madre Oriental that fires were associated with La Niña historically, but the relationship was unstable over time (Yocom et al. 2010). In southern Mexico, fire is more likely to occur during El Niño events such as those of 1983 and 1998 (Román-Cuesta, Gracia & Retana 2003). The current study allowed us to investigate long-term climate–fire relationships in this region south of the dipole, which were previously undocumented.
Based on the observations of Lauer & Klaus (1975), we tested the following hypotheses related to forest ecology: (1a) fire regime changes in the mid-twentieth century included an increase in fire frequency, a change in fire type from surface fire to crown fire and a change in seasonality of fire to more early-season fires; (1b) forest degradation is evidenced by high tree mortality and little regeneration; and (1c) forest structure or demographics make the forest less resilient to future disturbance such as drought, fire or insect attacks. We also tested two hypotheses related to fire–climate relationships: (2a) El Niño events and low Palmer Drought Severity Index (PDSI) values are associated with fire occurrence; and (2b) the relationship between ENSO and fire has been consistent over time. Investigating the influences of human and climate drivers of fire along with the ecological effects of fire in Pico de Orizaba National Park provides us with a picture of the ecological role of fire in a high-elevation tropical forest, the current conservation status of a rare ecosystem and identifies information gaps for further research.
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We thank CONANP, H. Rojas and the Pico de Orizaba National Park for permission to work in the park. J. Cerrano, J. Villanueva-Díaz and D. Rodríguez provided valuable support to the project. R. de Jesús Amador, O. Hernández de la Torre, K. Kent, I. Zamudio and C. Vásquez García were terrific assistants in the field. We appreciate S. Curran's assistance with the ERICIII data base. R. Sheridan's help in the laboratory was greatly appreciated. Thanks also to D. Normandin, other staff and students at the Ecological Restoration Institute. We thank two anonymous reviewers, whose comments helped greatly improve this manuscript. This research was supported by a National Science Foundation Doctoral Dissertation Enhancement Project grant (OISE-1003845) and grant DEB-0640351, and the Ecological Restoration Institute at Northern Arizona University.