Fire regime

Conifer forests experience disturbances; that is, discrete events in time and space that modify environmental conditions, the availability of resources, and the structure of populations, communities, ecosystems, and landscapes. The origin of disturbances may be exogenous or endogenous, natural or anthropogenic (^{Brown, 2013}). Among the most commonly reported disturbances are fires, pests, diseases, winter storms, landslides, floods, droughts and windstorms (^{Timoney, 2003}). Of these, fire is considered the most important natural disturbance agent affecting the regeneration, structure and functioning of conifer forests (^{Frelich, 2002}).

A fire regime is defined as the spatio-temporal integration of individual fire events according to their spatial and temporal attributes, magnitude, type of fire, synergy with other disturbances, and biotic and abiotic controls (^{Brown, 2013}; ^{Conedera et al., 2009}) (Table 1). The fire regime can be characterized as: 1) natural, under which species evolved through geological time scales; 2) historical, under which ecosystems have been subject since humans began to use fire, and 3) current, the one governing forests in the present time (^{Jardel-Peláez, Pérez-Salicrup, Alvarado, & Morfín-Ríos, 2014}). Globally, two main fire regimes have been identified that contrast in conifer forests: 1) surface fires, frequent, of low severity and intensity, and characteristic of forests dominated by Pinus sp. and Juniperus sp., and 2) active or passive crown fires, infrequent, of high severity and intensity, generally present in forests of Abies sp. and Picea sp. (^{Brown & Smith, 2000}; ^{Rodríguez-Trejo & Fulé, 2003}).

Natural fire regime and its effects

Conifers have developed mechanisms to ensure their regeneration depending on the fire regimes under which they have evolved (Figure 1A). At the species level, in forests that experience frequent fires, it is common to find trees with thick exfoliating bark that allows them to withstand fire damage (^{Rodríguez-Trejo & Fulé, 2003}). Some species such as Pinus palustris Miller or Pinus devoniana Lindley present a cespitose stage during the seedling stage, which allows them to develop a deep root while the stem is protected in case of a mild fire (^{Nelson, Weng, Kubisiak, Stine, & Brown, 2003}). By contrast, in places where fires are more severe, species such as Pinus halepensis Miller and Pinus contorta Douglas ex Loudon produce serotinous cones that open when exposed to the heat of the fire, in such a way that, in the clearings generated, their seeds are the first to germinate and establish (^{Keeley, 2012}). Likewise, Abies sp. cones disintegrate releasing a rain of seeds that, potentially, germinate in the clearings formed by stand-replacing fires (^{Cremer et al., 2012}). Other adaptive traits in Abies sp. are microphyllous leaves, volatile compounds and retention of dry leaves and branches, which increase the flammability of fuels and generate highly severe fires (^{Keeley, 2012}).

At the population level, two establishment patterns can be characterized (Figure 1B). First, in sites that experience frequent fires of low severity and intensity, seedling establishment is continuous, and the population structure is composed of individuals of different age categories (^{Brown, 2006}). By contrast, in places that experience infrequent fires of moderate to high severity and intensity, seedling establishment occurs synchronously or by pulses; that is, discrete and massive establishment events during short periods following stand-replacing fires. In this case, stand individuals show little age variation (^{Flaver, Jonnson, Jönsson, & Esseen, 2008}).

The composition, structure and dominance of tree communities also depends on fire magnitude and frequency. First, following the intermediate disturbance hypothesis (^{Svensson et al., 2007}), the number of tree species and, in general, spatial heterogeneity are greater in sites with a mixed fire regime; that is, with fragments subject to small-area, low-severity, low-intensity fires and, in turn, with fragments subject to stand-replacing fires. This allows the coexistence of diverse tree species, contrary to the few species that dominate in sites with contrasting fire regimes (^{Kaufmann, Huckaby, Fornwalt, Stoker, & Romme, 2003}). Second, the magnitude and frequency of fires affect ecological succession patterns and allow the coexistence of dominant tree species (Figure 1C). For example, the mixed forests of Pinus ponderosa Douglas ex Lawson (pioneer species) and Pseudotsuga menziesii (Mirb) Franco (late species) are considered a successional stage, which is maintained by frequent surface fires that allow the co-dominance of both species (^{Sherriff & Veblen, 2006}). Third, conifer regeneration is mediated by both fire and biotic interactions; for example, the nursing (facilitation) exerted by Populus tremuloides Michx (pioneer species) on Abies lasiocarpa (Hooker) Nuttall (late species) after severe fires, as it provides protection and shade to sunshine-intolerant A. lasiocarpa seedlings (^{Calder & St. Clair, 2012}).

There is also a relationship between the fire regime and the incidence of other disturbances (Figure 1D). For example, after strong winds, such as hurricanes or winter storms, a large number of trees fall and this increases the fuel load that induces high- magnitude fires during dry years (^{Buma, 2015}). Similarly, the likelihood of bark insect infestation may increase in remaining trees weakened by fires of moderate severity and intensity; in turn, the death of infested trees increases the fuel load, generating high-magnitude fires (^{Pelz & Smith, 2012}).

Figure 1 Effects of the fire regime in conifer forests at individual scale (A), which are manifested in adaptive traits such as bark thickness, grass-stage and serotinous cones (^{Keeley, 2012}); at population scale (B), where they determine patterns of continuous or pulsed establishment (^{Brown, 2006}); at community scale (C), where they can stop forest succession and generate co-dominance of species (^{Sheriff & Veblen, 2006}); at ecosystem scale (D), where other disturbances induce high-magnitude fires (^{Buma, 2015}). 

Alteration of fire regime and effects on conifer forests

Fire regimes can be modified by the direct or indirect action of biophysical factors (climate variability, elevation, changes in global circulation phenomena, climate change and natural disturbances) or human factors (increase in ignition sources, fire suppression and forest degradation) (^{Morgan et al., 2003}). In conifer forests, alteration of the fire regime has repercussions on regeneration patterns, stand dominance and at ecosystem scale (Figure 2). For example, at population scale, in sites that experienced frequent and less severe fires, their suppression caused the accumulation of fuels, greater tree density in the stands, trees with small diameters, fires of high severity and establishment by pulses in species such as P. ponderosa, whose regeneration occurred continuously (^{Fulé, Korb, & Wu, 2009}). On the contrary, in sites with infrequent and more severe fires, the increase in frequency generated higher mortality rates, especially in seedlings and juveniles, which resulted in lower tree recruitment of dominant species (^{Schwartz et al., 2015}).

Figure 2 Effects of the alteration of the fire regime by biophysical and human factors on conifer forests, depending on the frequency, extent, severity and intensity of the events.  

The structure of the communities is also affected, favoring or restricting the dominance of certain species; for example, the reduction in the fire return interval could displace shade-tolerant species, such as Abies sp. and Quercus sp., and favor the establishment of Pinus sp. (^{Frelich, 2002}). In contrast, fire exclusion can generate the replacement of Pinus sp. populations by Quercus sp. or Abies sp. (^{Brose & Waldrop, 2010}). Interactions between dominant species are also modified; for example, when fire cycles are shorter, the dominance of P. tremuloides is maintained because it limits the development of A. lasiocarpa, due to its intolerance to insolation (^{Calder & St. Clair., 2012}).

Synergies between fire and other disturbances are also affected by the altered fire regime. For example, the occurrence of more severe fires due to the effect of suppression may induce greater incidence of bark beetles, due to the weakening of trees affected by fire (^{Kulakowski & Jarvis, 2011}). On the other hand, the conditions of greater aridity, generated by the effect of climate change, induce the death of trees, increasing the fuel load that can trigger higher-intensity fires (^{Buma, 2015}).

At ecosystem level, the alteration of the fire regime has the ability to drastically modify local environmental conditions and the physiognomy of landscapes. For example, in forests subject to frequent fires of low severity and intensity, such as those of P. ponderosa, fire suppression led to high-intensity fires that transformed the ecosystem from pine forests to scrub (^{Rother & Veblen, 2016}). On the other hand, in forests subject to infrequent fires, but of greater intensity and severity, the effect of suppression is less evident, but in the long term it could lead to an atypical accumulation of fuels and generate catastrophic fires (^{Fulé & Laughlin, 2007}). On a global scale, some effects of the alteration of the fire regime could be manifested in the modification of terrestrial carbon stocks, the concentration of carbon dioxide and an increase in global temperature (^{Pan et al., 2011}). Hence, it is essential to document natural and altered fire regimes at both local and larger scales.

Reconstruction of fire regimes

There are direct and indirect methods for reconstructing fire regimes, which often focus on knowing seven attributes: frequency, seasonality, severity, intensity, type of fire (surface or crown), area and spatial complexity (^{Yocom, 2014}). Among the most useful direct methods is the dating of fire scars, characterized by the presence of reaction wood that gradually envelops the affected trunk area. Based on annual growth rings and by means of partial or total cuts of the trunk, it is possible to locate the exact year of the fire and to infer the historical frequency of fires at stand scale (^{Grissino-Mayer, 2001}). The second direct method is the analysis of the sediment layer in the soil, where carbon residues from generally intense and severe fires accumulate. These carbonized remains are deposited and covered by other sediments. The return interval of these fires can be accurately dated by paleoecological techniques, depending on the sedimentation rate (^{Conedera et al., 2009}). The third direct method consists of describing fires through historical records such as photographs, logbooks and permanent monitoring. In this case, the area, severity and intensity of the fires are generally documented (^{Yocom, 2014}).

Among the indirect methods, the analysis of the age structure stands out, under the assumption that the history of tree establishment is correlated with the cyclic intervals of fires. An even-aged structure, with great age variation, suggests that trees experience frequent, low-severity, low-intensity fires (^{Brown, 2006}), while a pulsed age structure suggests infrequent, moderate-to-high severity and intensity stand-replacing fires (^{Flaver et al., 2008}). The second indirect method is remote sensing analysis of fire scars in the landscape, which consists of generating georeferenced polygons of fragments or clearings produced by fires, from satellite images. This method evaluates the extent of the fire and estimates the times of scar appearance and disappearance in the landscape; that is, the frequency and return intervals, as well as the severity, seasonality and spatial extent (^{Hudak & Brockett, 2004}). The third indirect method is reconstructing frequency, intensity, type of fire and severity through interviews with owners, firefighters and forest managers. This technique is quite useful for recovering the empirical knowledge of rural communities about fire management (^{Raish, González-Cabán, & Condie, 2005}).

Each method has strengths and weaknesses, and its implementation depends on the objectives of the study, the properties of the fire regime they address (Table 2), and the ecosystems in which they are applied. For example, the fire scar dating method has been used the most, although it is only applicable to sites with low-to-medium severity and intensity fires, since trees with scars are not always found in sites with stand-replacing fires (^{Yocom, 2014}). In these cases, studies of age structures are generally used, assuming that the establishment is associated with this type of fire (^{Flaver et al., 2008}).

An interesting application is to know the effect of climate on a fire regime by means of superposed epoch analysis, in which the values of precipitation, temperature or global circulation phenomena, such as the El Niño Southern Oscillation (ENSO), are correlated with the fire occurrence years. This allows knowing, for example, if prior or during the fire year the precipitation values were statistically low (^{Grissino-Mayer, 2001}). Another important aspect is knowing whether tree regeneration responds to historical fire occurrence intervals. This involves carrying out, in conjunction with fire chronologies, studies of age structures and evaluation of population establishment patterns in order to know whether they are correlated with fire frequency (^{Brown, 2006}).

A subject that has received attention is the potential existence of an altitudinal or latitudinal gradient in the fire return interval. Although there is no systematic review, there are some indications that the fire return interval is relatively lower in montane conifer forests developing within the intertropical zone (^{Yocom & Fulé, 2012}). Therefore, it would be important to analyze whether these patterns can be observed in tropical conifer forests of central and southern Mexico, where in the higher mountains it is possible to find genera characteristic of higher-latitude zones such as Abies, Pinus, Picea and Pseudotsuga (^{Gernandt & Pérez de la Rosa, 2014}).

Fire regime in montane tropical conifer forests

Montane tropical conifer forests are those that are distributed in the mountain massifs within the intertropical zone; that is, between the Tropic of Cancer and the Tropic of Capricorn. In Mexico, the distribution area corresponds to the Transversal Volcanic System and the Sierra Madre del Sur. ^{Rodríguez-Trejo and Fulé (2003)} indicate that, according to the altitudinal gradient, the fire regime in montane tropical conifer forests differs between forests dominated by Abies sp. and those dominated by Pinus sp.; the former are located at higher elevations and are subject to infrequent, moderate-to-high severity and intensity fires, while the latter are distributed on lower slopes and are subject to frequent, low-severity, low-intensity fires. However, both in forests dominated by Pinus hartwegii Lindley, at elevations above 3 400 m (^{Yocom & Fulé, 2012}), and in forests dominated by Pinus douglasiana Martínez, at elevations below 2 700 m (^{Cerano et al., 2015}), fires are frequent and of low severity and intensity. There are still no chronologies documenting the fire return interval at intermediate elevations (2 700 to 3 400 m), where other species of Pinus and Abies are distributed. On the other hand, there are indications that the regeneration of Abies religiosa (Kunth) Schltdl. et Cham. responds better to moderate-to-high-severity fires (^{Angeles-Cervantes & López-Mata, 2009}). Fuel load seems to be higher in forests dominated by this species with respect to pine forests, which would trigger greater-magnitude forests (^{Villers-Ruiz & López-Blanco, 2004}). However, similar patterns of severity, intensity and tree establishment (continuous) have been detected in stands dominated by Pinus pseudostrobus Lindley and A. religiosa, which suggest that both experience frequent, low-severity, low-intensity fires, and that tree regeneration is not associated with cyclical fire events (^{Pérez-Salicrup et al., 2016}).

It is not entirely certain whether the presence of frequent fires, of low extent, severity and intensity, in montane tropical conifer forests is due to biogeographic causes and is specific to this type of ecosystem in tropical latitudes, or whether it is a consequence of the alteration of the fire regime by human activities. On the one hand, it has been proposed that montane tropical conifer forests have different structural and functional attributes than higher-latitude conifer forests (^{Yocom & Fulé, 2012}). For example, in montane forests, rainfall distribution is tropical with a dry and a wet season (^{García, 2003}), and the fuel load dries and decomposes faster than in higher-latitude forests, which prevents the accumulation and generation of greater-magnitude fires (^{Quintero-Gradilla, García-Oliva, Cuevas-Guzmán, Jardel-Peláez, & Martínez-Yrízar, 2015}). On the other hand, due to the long history of forest management of montane tropical conifer forests, it has been postulated that fire regimes are anthropogenic (^{Myers & Rodríguez-Trejo, 2009}) and that fire frequency is weakly associated with periods of drought (^{Pompa-García, Camarero, Rodríguez-Trejo, & Vega-Nieva, 2018}). In this sense, fires are frequent because they are caused by human activities and are of low severity and intensity because they are fought and suppressed, which does not allow them to spread (^{Martínez-Torres, Castillo, Ramírez, & Pérez-Salicrup, 2016}). For this reason, it is important to continue investigating fire regimes in montane tropical conifer forests in order to know if these characteristics are of natural origin, if they have been altered by humans or are the result of a mixture of both processes.

Timber activity and fire regimes

In addition to the biogeographical aspect, fire regimes can be drastically altered by timber harvesting. In fact, this activity has replaced fire as the main disturbance agent that modifies the structure and function of conifer forests (^{Frelich, 2002}). In forest management, two methods stand out: regular management, which consists of the extraction of large volumes of timber in large areas, commonly known as clear-cutting (^{Gustaffson, Kouki, & Sverdrup-Thygeson, 2010}), while irregular management consists of selective cutting, in order to maintain balanced diameter structures and ensure a continuous supply of timber in the same stand, which leads to the maintenance of some canopy cover (^{Schwartz, Nagel, & Webster, 2005}). So far, the effect of these and other silvicultural methods, such as salvage or illegal logging, has not been systematically evaluated on fire regimes in Mexico's conifer forests. However, some consequences could be inferred from information generated at other latitudes.

Regular management generally increases the fire risk, since a common practice is to extract commercial timber and leave residual material (poorly-shaped branches and trunks) on the ground, which increases fuel loading and susceptibility to fires of greater severity and intensity (^{Chen et al., 2015}). In irregular management methods, the opening of small spaces promotes the establishment of shade-intolerant species, which can double or triple their density (^{Merschel, Spies, & Heyerdahl, 2014}). If thinning is not applied, the accumulation of fuels and the possible increase in fire severity and intensity are generated (^{Morgan et al., 2003}). On the other hand, salvage logging removes damaged trees and reduces fuel loads, but its application is controversial, as it modifies the natural fuel dynamics (^{Lindenmayer & Noss, 2006}). Finally, illegal logging is characterized by extracting segments that are quickly loaded onto a truck, so that most of the tree is abandoned, a situation that increases the fuel load, fire severity and the speed of spreading to tree crowns (^{Kukavskaya et al., 2013}). In some cases, fires are even deliberately set to eliminate evidence of illegal logging (^{Martínez-Torres et al., 2016}; ^{Mehring & Stoll-Kleemann, 2008}). These situations suggest that timber harvesting should be consistent with fire regimes to ensure forest integrity.

Fire regime management

Integrated fire management emerged as a proposal to prevent and counteract the negative effects of altering fire regimes. Such management is based on the premise of maintaining the structure and function of forest ecosystems through management practices that emulate the natural regime (^{Scott, Bowman, Bond, Pyne, & Alexander, 2014}). For example, the application of silvicultural methods, such as the regular one, may mimic the effect of stand-replacing fires, while the irregular method may have effects similar to surface fires (^{Chen et al., 2015}). Since conifers have evolved under different fire regimes, it is important that the management of this element be incorporated into forest management practices, especially in sites where the regime has been altered (^{Fulé et al., 2009}). Fire management practices include thinning, implementation of firebreaks, removal of excess fuel material, closure of forest supply roads, grazing control, and prescribed burns (^{Brown & Smith, 2000}).

Fire management should consider differences in the fire regimes of each type of conifer forest (^{Fulé & Laughlin, 2007}). For example, for conifer forests in central Mexico, such as those dominated by Pinus sp., it is suggested to maintain a multi-stage age structure and induce frequent, low-severity surface fires. Unfortunately, as previously mentioned, for other forests such as those dominated by Abies sp. or those co-dominated by Pinus sp. and Abies sp., there is no information on fire regimes, which makes it difficult to formulate forest management recommendations in accordance with natural regimes. Therefore, following the uncertainty criterion that should govern ecosystem management, before implementing forest management practices it is necessary to evaluate their effect on ecosystems with the tree dominances previously described. Likewise, fuel loads must be monitored to maintain them at levels compatible with the dynamics of each dominance (^{Battaglia & Shepperd, 2007}).

Fire management faces some barriers. First, the expansion of urban areas has increased the risk of property damage and fatalities from forest fires, which has led to the development of laws and public policies that restrict or prevent the use of fire as a forest management tool (^{Birot, 2009}). The second barrier is the lack of information on fire regimes in particular sites or ecosystems, which prevents having a solid frame of reference to make appropriate decisions (^{Conedera et al., 2009}). In this sense, a paradigm shift from the suppression to the use of fire as a forest management tool is required. Therefore, it is essential to have awareness-raising strategies and negotiation mechanisms to plan fire management among forest resource managers, researchers, authorities and society in general (^{Morgan et al., 2003}). The most important aspect is to understand how fire limits or promotes the regeneration and development of conifer forests, including the effect of global change, in order to maintain resilient and sustainable ecosystems in the long term.