The study area was in a place known as Rincones de la Viga, which belongs to the community of San Miguel Topilejo in the Mexico City borough of Tlalpan, within a previously burned area. In this area there are abundant large rock outcrops, product of the eruption of the nearby Xitle volcano in 300 AD. The forests in this area are dominated by Abies religiosa. but there are also other less abundant species, namely Pinus hartwegii, Arbutus xalapensis Kunth and Buddleia cordata Kunth. Part of the affected area is near flat terrain.

The diversity of trees in these forests is low, although A. religiosa is associated with Pinus, Quercus and others. However, these forests have high species diversity at the understory and fauna level (Rzedowski, 1978). Although they cover slightly less than 1% of forested land in Mexico, sacred fir (A. religiosa) forests are the climax vegetation in cold-temperate areas. Abies forests are located between 2,400 and 3,600 masl, in areas with a mean annual temperature of 7-15 °C and where mean annual precipitation exceeds 1,000 mm. Normally there are only four dry months and snowfalls occur almost every year. Sacred fir are mainly located on deep, dark brown soils with sandy loam or loamy texture, abundant organic matter and slightly acidic pH (Rzedowski, 1978). The species is primarily found in the Trans-Mexican Volcanic Belt and in the mountains of southern Mexico, where they form large stands, although they are also found in the north, albeit to a lesser degree (Challenger & Soberón, 2008). In the Valley of Mexico sacred fir prefers wetter exposures, from the NW to NE (Madrigal-Sánchez, 1967). However, in the Cofre de Perote, near the Sierra Madre Oriental, it commonly occurs on SW, W, NW, N and NE exposures (Sánchez et al., 1991).

In this area on March 3, 2011, a forest fire occurred in a sacred fir forest, affecting 197 ha. The fire was mostly a crown fire with varying intensity but it occassionally spread as surface and ground fires as well. The ground fire was due to the accumulation of combustible materials in the cracks between rocks. The cause of the fire was human activity (CORENA, 2012) (Fig. 2).

The sampling included 335 Abies religiosa trees (≥ 1.5 m tall), selected on a transect in the fire affected area. Field data collection began six months after the fire, in the months of September and October 2011. The variables considered were qualitative and quantitative. The former included the following: substrate where the tree grew (soil, rocky, or natural rock pot filled with earth), damage on the trunk caused by an agent unrelated to fire, and whether the tree was alive or dead. Quantitative variables included: total height, crown base height (height from ground to the base of the original crown), crown length (length from the base of the original crown to the tip of it), lethal scorch height (height from ground to the top of the crown zone with leaf and bud mortality, without resprouting of leaves), crown kill percentage ((length of dead crown/original crown length)(100)), height of the visible scar on the trunk bark and diameter at breast height. The material used included rope, metric and diametric measuring tapes, a Haga height-measurer, a clinometer, a global positioning system (GPS) and a digital camera.

For the statistical analysis, we used logistic regression (Hosmer & Lemeshow, 2000), used individually or in combinations the explanatory variables obtained in the field. The dependent variable was the probability of tree mortality (P):

The sacred fir stand had an average height of 11.3 m, with a maximum of 32 m, and an average DBH equal to 36.7 cm and a maximum of 92 cm. As is typical of the species, crown base height was low (average of 2.1 m, minimum of 0 m), but with high crown length and crown length/height ratio (the latter was 0.79) (Table 1, Fig. 3). Although not sampled, abundant regeneration of A. religiosa was observed in various areas of high mortality. However, three years later, it was observed that most of them did not survive.

Fig. 4 shows a slight trend, with low R², where the tallest trees have the largest diameters. However, more notable is the large variation in heights that each diameter class presents. A closer correlation is presented by crown length and tree height (Fig. 5). That was not the case for crown base height and lethal scorch height (Fig. 6).

As is to be expected from an intense, predominantly crown fire, the severity in the sacred fir forest was high, as mortality reached 73.1% and is likely to increase by some degree in the first 2-3 years after the fire. Such delayed mortality is a response to stem and root injuries, as well to insect attacks (McHugh et al., 2003). Another measure of the high severity is 83.9% of crown kill (Fig. 3). There is not a strong direct relationship between crown length and lethal scorch height. The opposite was expected, for longer and lower crowns can catch surface fire easily.However, if values with 0 lethal scorch height are removed, of trees with non fire-affected crowns due to variation in the intensity and severity of the fire, the relationship would undoubtedly be more direct.

After running the regressions for all variables, both one-by-one and using combinations of them, there was only significance for estimating the probability of tree mortality (P) when individual variables such as crown base height (CH), diameter at breast height (DBH), lethal scorch height (LH), total height (TH), crown length (CL) and crown kill (CS) were used. Below is a list of the models, and Table 2 provides data on the significance.

Height. Shorter individuals generally have even thinner bark and less protection against fire than tall trees, so they have higher mortality. Such trees also incur more crown injury due to a greater portion of their foliage and buds being killed because they are closer to the surface flaming. Fig. 4 showing the relationship between height and diameter of the sample, demonstrates that almost all diameter categories have a significant proportion of individuals up to 15 m high; according to Model 2 and Fig. 7a, trees 15 m high have a 61% probability of mortality. The great variability of heights, with values ranging from 2-32 m, helps explain the vertical continuity of fuels.

Diameter. Sacred firs with a DBH of 20 cm have a close to 80% probability of mortality. By contrast, that probability for P. hartwegii with the same diameter in high-intensity prescribed fire treatments is only 10% (Rodríguez-Trejo et al., 2007). The larger diameter is associated with thicker bark that better protects the vascular cambium from lethal temperatures. In seven conifers in the U.S. and Europe, greater bark thicknesses delayed the time to kill the cambium. For example, in Pseudotsuga menziesii, with a DBH of 30 cm this time was 10 min, but with a DBH of 60 it was delayed by 43 min (Ryan et al., 2010). However, in the U.S., Abies lasiocarpa has very thin bark, while Abies grandis (Dougl. ex D. Don) Lindl., A. procera Rehder, A. amabilis Douglas ex J. Forbes, A. magnifica A. Murray and A. concolor all have medium-thick bark (Miller, 2000; van Mantgem & Schwartz, 2003). A. religiosa bark is considered to be medium-thick.

Crown base height. Lower crown base heights are related to higher levels of mortality in the sacred fir, evidence that the lower they are the greater the possibility that the fire will transit from surface flaming to torching or crowning and damage the photosynthetic, conductive and supporting tissues. Alexander (1988, after Van Wagner, 1977) proposed a model to determine the critical intensity (I_O) that a ground fire must have to start a crown fire as a function of live crown base height (LCBH) and foliar moisture content (FMC):

When a surface fire severely affects tree crowns, the bottom of the crown is burned (without foliage). Above this section is another zone called the scorch area (with dehydrated foliage), above which there is another zone, not significantly affected, with green foliage. The scorch area is divided, in turn, into two, because the lower part includes dead buds, while the upper has living buds and has the potential to regrow (Hood, 2010; Ryan et al., 2010; Fowler & Sieg, 2004).

Crown kill. The highest levels of crown kill and thinnest bark thicknesses are related to higher mortality in Abies lasiocarpa and six other North American conifers, Pseudotsuga menziesii, Larix occidentalis Nutt., Picea engelmannii Parry ex Engelm., Pinus contorta Douglas, Thuja plicata Donn ex D. Don and Tsuga heterophylla (Raf.) Sarg. (Ryan & Reinhardt, 1988). If the crown kill is total or near total, non-fire-tolerant trees will lack living buds and carbohydrates stored by twigs and stems that can allow recovery. The greater the crown area left unaffected or affected at low severity, the more buds will have survived and there will be some carbohydrate reserves to supply their activation and growth. In addition, severe damage to the tree increases the possibility of pest attack.

However, there is not a direct relationship for all species between crown scorch level and mortality or adverse effects. In fire-adapted species, a low level of crown scorch (up to 1/3 of it) improves secondary growth the next year, as has been found in P. elliottii Engelm. (Johansen, 1975), P. palustris Mill. (Wade, 1983) in North America, P. pinaster Ait. (Botelho et al., 1998) in Europe, and P. hartwegii in Mexico (Rodríguez-Trejo et al., 2007). According to the logistic model obtained for this study, the sacred fir tolerates up to 60% crown scorch. From that point on mortality increases. According to Ryan & Reinhardt (1988), different conifers with a high level of crown kill have a high probability of mortality. Both, fire-resistant species such as most hard pines (Keeley, 2012), and fire-susceptible species characteristic of late successional stages, like sacred fir, increase their mortality with high levels of crown kill.

Foliage sprouting. Characteristics that increase the likelihood of a crown fire, such as a low crown base height and high crown bulk density (VanWagner, 1977), or high stand density are generally considered to be deleterious to tree survival. However, regeneration of new crown from epicormic buds is an important fire adaptation to the damage inflicted by fires occurring in such stands (Bond & Midgley, 2001, 2003; Hanson & North, 2006). Likewise, fire tolerance is evidenced by basal sprouting or if there is consistent sexual regeneration to recolonize burned sites. Sacred fir has the stand characteristics that make it prone to crown fires when the environment is dry enough, yet it has foliage sprouting when the fire is not very intense.

Sexual regeneration. There is evidence that under intermediate levels of damage by crown fire in El Chico Park, Hidalgo, Mexico, sacred fir regeneration is more abundant than with high levels of damage in the crowns or ground fire areas or unburned sites, so there fire is part of the sacred fir regeneration niche (Ángeles & López, 2009). Lara et al. (2009), have documented that A. religiosa regeneration is best in small gaps (most with between 44 and 88 m² and an average age of 7.6 years) than under canopy, with densities of 4,239 ha^–1, in the Cofre de Perote, Veracruz. In Japan, Narakawa & Yamamoto (2001) have reported that the regeneration of other species of Abies is favored in gaps. Other researchers have typically denoted A. religiosa as a shade-tolerant species. González et al. (1991) note that in Zoquiapan, State of Mexico, the seeds of this species germinate better under a closed canopy (87.3%) than under open canopy (76.9%) or intermediate canopy (60.2%). They also included prescribed ground burn treatments in intermediate and open canopies, obtaining germination of 61.1 and 48.5% respectively.

Bark thickness. The authors know of no comparative analysis of the morphological characteristics of the genus Abies. However, a synthesis for 25 Abies species grown in a common setting at the Arnold Arboretum in Massachusetts indicates several species develop moderately thick, furrowed bark at maturity (Warren & Johnson, 1988). This is particularly evident for species habiting cool mountain areas proximal to more fire-adapted pines (Pinus sp.) and oaks (Quercus sp.), in contrast to those from cold, boreal regions. In North America Abies concolor, Abies grandis, A. magnifica, and A. procera are moderately resistant (Miller, 2000; Stephens & Finney, 2002; van Mantgem & Schwartz, 2003; Schwilk et al., 2006; van Mantgem et al., 2013), mainly due to their bark thickness. These species grow in cool temperate forests in the Sierra Nevada and Rocky Mountains, with A. concolor extending into northern Mexico. They experience a pronounced dry season where lightning is common and fires frequent pine and oak forests lower on mountain slopes. In contrast A. balsamea, A. amabilis and A. lasiocarpa are fire-sensitive. These boreal and subalpine species burn predominantly in long fire return interval stand replacing fires (Miller, 2000).

Comparison with a fire-adapted species.Pinus hartwegii, a species with which the sacred fir forms some associations and ecotones, and which succeeds it on the next altitudinal level, is one of the better fire-adapted pines. This pine has resistance mechanisms (thick bark) and even a fire tolerance level (it recovers much of its crown when it is affected by fire and some populations are able to emit abundant basal resprouts), in addition to recolonizing burned areas (winged seed, the bed generated by fire is suitable for the germination of its seeds, and it is a shade-intolerant species); thus, many populations present a grass stage (Rodríguez-Trejo, 1996, 2001, 2006; Rodríguez-Trejo & Fulé, 2003). Even crown scorch not exceeding one third of the crown generate greater secondary growth than that of unburned controls during the year following a prescribed fire (González-Rosales & Rodríguez-Trejo, 2004). With a low-intensity fire applied mid-season, young pine survival is very high, in both the first and second year (Rodríguez-Trejo et al., 2007; Vera-Vilchis & Rodríguez-Trejo, 2007). Also the comparison among a pine and a true fir, both from North America, may be useful. Van Mantgem et al. (2013) found similar mortality rates for large (DBH > 50 cm) ponderosa pine (Pinus ponderosa) and white fir (Abies concolor) five years after prescribed burning in the Southwestern United States. For smaller trees (DBH < 50 cm) white fir was more likely to die. Prescribed fires are commonly less intense than wildfires, but not necessarily. These authors accounted for actual fire injuries (crown volume scorched and stem char height), so results should be broadly applicable to these species. In general surface fuels can be expected to burn out before lethal heat can penetrate to cambia beneath bark thicker than about 1 cm (Ryan, 1990).

It is the sacred fir fire-adapted?Abies religiosa has no marked fire adaptations, except for the following evidence: the foliage of A. religiosa resprouts after a fire of moderate severity (Rodríguez-Trejo, 2007); it regenerates well in partially shaded burned areas (Ángeles & López, 2009); and has a thick bark when old. Based on the evidence of this study and in the literature review, A. religiosa is considered to be a moderately fire-resistant species adapted to mixed-severity fire regimes.

Sacred fir forests are critical habitat for wintering Monarch butterflies. Logging and other land use changes threaten these forests (Ramírez et al., 2003; Toone & Hanscom, 2003). Forest fires originating from land clearing, charcoal production, and other activities increasingly damage these forests (Brower & Malcolm, 1991;Brower et al., 2012). The most critical years in terms of forest fires for Mexico have been 1998 and 2011. In both years crown fires in sacred fir forests were relatively common. Various global climate change predictions indicate drying trends for Mexico (Magaña et al., 1997; Villers-Ruiz & Trejo-Vazquez, 1997, 1998; Karmalkar et al., 2011; Sáenz-Romero et al., 2012) with anticipated increases in fire, which could have a major impact on Monarch butterflies (Villers-Ruiz & Trejo-Vazquez, 1998; Peterson et al., 2002). Longer and stronger dry seasons (Karmalkar et al., 2011) coupled with the structural characteristics of sacred fir, leads to the conclusion that crown fires will become more common in sacred fir forests, as well as in adjacent forest types. In light of this situation, the silvicultural practices of pruning low branches to raise crown base height, thinning and managing forest fuels where they are excessive, including materials resulting from silvicultural treatments, are factors that can help to reduce the incidence of fires in the vicinity of sacred fir forests. There may be a roll for judicious use of prescribed burning under mild weather conditions to effectively treat fuels, raise crown base height, and create fire-resilient sacred fir forests. However, fire use should be timed such that the butterflies are not directly affected, i.e., scheduled for times when butterflies are not present. Air pollution in general, and smoke in particular, is known to adversely affect Monarch butterflies (Mayerle, 1992; Toone & Hanscom, 2003). Given the critical importance of sacred fir forests for Monarch butterflies, these forests should be managed in a landscape context, particularly in the face of climate change and increased land use pressure (Peterson et al., 2002; Vos et al., 2008; Sáenz-Romero et al., 2012). Management should attempt to develop a fire resilient landscape where the risks of major habitat loss due to fire are reduced. Research on the role and use of fire in managing these landscapes is critical to the development of informed policy and management. Such research should integrate Monarch butterfly ecology and monitoring to reduce the risks.