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Mexican forest ecosystems are exposed to a great variety of natural disturbances such as fire, which is a key disturbance process influencing tree survival and regeneration (Poulos et al., 2013). In ecosystems such as coniferous forests, the presence and the severity with which a fire develops help to maintain or eliminate the growth dynamics of the species, change its composition, diversify the dimensional structure, and create a patchwork of ages (Collins et al., 2011; Avila Flores et al., 2014).

Nevertheless, forest fires are one of the primary causes for which a large portion of the forests in Mexico is lost (Comision Nacional Forestal [CONAFOR], 2018). According to Mexican statistics, the number of fires is increasing; during the year 2018 about 488,000 ha have been affected, recording more than 6,900 fires distributed across the country (CONAFOR, 2018). In addition, the Sierra Madre Oriental (SMO), in the northeastern part of Mexico, has also experienced a remarkable increase in human development in areas close to forests, thereby creating and expanding a wildland urban interface. With this expansion comes an increased risk to human life and property as severe wildfires become common.

In response, federal agencies have advocated fire control and fire exclusion among land managers and agencies in charge of the natural resources in order to mitigate risk and hazard of severe wildfires. Nevertheless, with a limited budget available for an accurate fire management and continually increasing costs of fire exclusion and control, these policies are neither sustainable nor helpful for fire hazard mitigation planning in Mexican forests. Therefore, Mexican ecosystems are in need of research on fire regimes. In the south part of Mexico, little is known about the historical role of fire; in the north part in comparison, there has been some related research (Fule and Covington, 1999; Gonzalez Tagle et al., 2007; Yocom et al., 2010; Avila Flores et al., 2014). However, the increasing interest in fire regimes in Mexican forests has heightened the need for more studies in the SMO.

Thus, our objectives for this research were to determine: (1) the pattern of occurrence of forest fires, and (2) their relationship with precipitation and atmospheric circulation patterns for coniferous forests in the Cerro El Potosi located in the SMO. The study area, because of its geographic location, provides an opportunity to investigate the influence of atmospheric circulation patterns in fire occurrence. It is located at a latitude where the phases of El Nino Southern Oscillation (ENSO) are likely to show contrast behavior, switching from drought association with El Nino to the south and with La Nina to the north (Caso et al., 2007; Yocom and Fule, 2012).

Materials and Methods--Study Area--El Ceno El Potosi is located in the SMO in the state of Nuevo Leon, Mexico, about 80 km south of Monterrey (Fig. 1). El Potosi belongs to the subprovince of La Gran Sierra Plegada, which is composed of a series of intermountain canyons, oriented north-northwest to south-southeast. El Cerro El Potosi, with a maximum elevation of 3,700 m above sea level, is the highest mountain in northeastern Mexico. The climate is dry and temperate and the range of annual precipitation is between 400 and 600 mm, with low rainfall throughout the year. The annual average temperature range is between 12 and 18[degrees]C (Arevalo Garcia and Elizondo Gonzalez, 1991). The vegetation types at the summit of Cerro El Potosi are alpine and subalpine prairie, Pinus eulminicola Anclresen & Beaman (Pinaceae), shmblancl, mixed forest of Pinus eulminicola--Pinus hartwegii Lincll. (Pinaceae), mixed forest of P hartwegii Lincll.--P eulminicola Anclresen & Beaman, mixed forest of Abies vejarii Martinez (Pinaceae)Pseudotsuga menziesii (Mirb.) Franco (Pinaceae)-P. hartwegii Lincll.--Pinus strobiformis Engelm. (Pinaceae), and forests of P hartwegii Lincll (Faijon et al., 1997). Endemic and rare plant species in the area include P. eulminicola Anclresen & Beaman and Juniperus sabinoides Hiunb. ex Lincll. & Gordon (Cupressafceae), among others (Jimenez Perez et al., 2005).

Sampling--The selected area for sampling is a part of the coniferous forests, between 2,900 and 3,450 m above sea level, with a northeast aspect and a slope range of 22-38% (Fig. 1). We established an area of approximately 25 ha for sampling. The forest in the sampling area is composed of species such as A. vejarii, P strobiformis, Pin us greggii, P. hartwegii, and P menziesii. In order to reconstruct the forest's fire histoty, we took 22 samples from living trees, stumps, logs, and snags with at least one fire seal' using a chain saw. We sampled the species P strobiformis, P hartwegii, and P. menziesii.

Laboratory Procedures--We air-clriecl sampled sections and prepared them for analysis. In order to improve visibility of tree rings and fire scars, we used a sequence of increasingly fine grits of sandpaper. We cross-dated samples using standard clenclrochronological techniques (Stokes and Smiley, 1996). For samples coming from dead trees, we made skeleton plots that were cross-dated using a Pinus hartiuegu Lindl. ring width chronology made for the Cerro El Potosi (Villanueva-Diaz, 2016a). Subsequently, we dated each fire scar in the samples to the year of its formation.

We identified the seasonality of fire occurrence by determining the exact location of the fire scar within the annual growth ring (Baisan and Swetnam, 1990) using the categories from Grissino-Mayer (2001): D = dormant, E = early earlywood, M = middle earlywood, L = late earlywood, and A = latewood. Afterward, we grouped the categories into two periods: (1) spring (D+E), and (2) summer (M+L+A).

Data Analysis--In order to create the master fire chronology and perform statistical analysis, we used the FHX2 software, version 3.2 (Grissino-Mayer, 2001). We calculated summary statistics for the period of adequate sample depth, also known as the "period of reliability," which consists of the period between the first and last occurrence of a minimum proportion of scarred samples in any given fire year (Touchan and Swetnam, 1995). For every year, we determined the appropriate period, when at least 25% of the total samples had scars. That is, beginning and ending years were years when fire was first and last recorded on four or more samples. The period that showed an adequate record spanned 123 years from 1881 to 2004. Using all fire dates as well as those that scarred at least 25% or more of the recording trees, we calculated the composite mean fire interval for this period. The recording trees are trees that are more susceptible to form new fire scars because they had an open wound after being scarred previously (Rornnie, 1980). We used the Weibull distribution to statistically describe the fire interval data because of its flexibility and ability to model skewed distributions (Grissino-Mayer, 1999). The Weibull distribution results also provide a standard way to compare fire regimes across ecological gradients (Grissino-Mayer, 1999). We calculated descriptive statistics from the fire intervals as mean fire interval, median fire interval, Weibull median interval, Weibull modal interval, and minimum and maximum fire interval. We determined the dominant seasonality of fire occurrence from the distributions of the positions of fire scars in the growth rings.

To analyze climate conditions related to fire occurrence in the study area, we used Superposed Epoch Analysis (SEA) according to Grissino-Mayer (2001). We used this analysis to determine climate conditions during fire years, for 5 years prior to fire years, and for 2 years after fire years. We used an annual precipitation reconstruction (1796-2010) from P. culminicola of the Cerro El Potosi (Villanueva-Diaz, 20166; Villanueva-Diaz et al., 2018). Additionally, we related fire occurrence with the ENSO index NIN03 for the period 1856-2011. Data were obtained from IRI/LDEO Climate Data Library (http://iridl. We used a bootstrapping procedure to assess the statistical significance of the influence of climate on fire occurrence.

Results--Fire Regime Characteristics--From the 22 collected samples taken front P strobiformis, P. hartwegii, and P. menziensii, we were able to cross-date 16 (83%) samples, while in the case of six samples (17%), we were unable to date them because of rings that were too tight, and some samples presented an insufficient number of rings to allow reliable cross-dating. Tree rings analyzed could span a period of 296 years (1715-2011) by cross-dating the 16 samples. All in all, 15 forest fires, registered by 34 fire scars, could be identified covering a timeframe of 191 years from the first fire event in 1807 to the last in 1998 (Fig. 2). Concerning the fire events, we recorded mainly each of the early events in 1807, 1825, 1830, and 1888 by one sample or fire scar, whereas three or four fire scars could be associated with fire events in 1945, 1955, and 1976. We recorded eight scars or 50% of the samples (Fig. 2) for the latest forest fire (1998) at the study site identified until 2011.

Most of the fire events (53%) occurred between 1907 and 1955. According to the results of the present study, 73% of the forest fire events occurred before the establishment of the ejido "18 de Marzo" (communal land) near the study site in 1945, whereas fire frequency seemed to decrease after the establishment of the ejido (Fig. 2). The master chronology depicts three periods where no fire records were found. The largest period with the absence of fire was from 1834 to 1888 (54 years), then the second period was from 1955 to 1976 (21 years), and the third period with an absence from 1976 to 1999 (22 years).

The descriptive statistics were calculated for the period of reliability between 1881 and 2004. They show that the mean fire interval for all fires was 11 years and the mean fire interval for more widespread fires (those that scarred >25% of all samples) was longer (15.7 years; Table 1). The Weibull distribution results indicated an appropriate fit for the data. The Kolmogorov-Smirnov test uses d-statistic, which measures the maximum distance between the cumulative frequency distribution of the data and the fit of the Weibull distribution. Therefore, low values for d and high values for the probabilities are desirable (Grissino-Mayer, 1999). The values of the KolmogorovSmirnov test for the goodness of fit of the distribution show that the data come from a Weibull distribution (d = 0.19; P > 0.82) for all fires, and for those that scarred 25% or more of the samples the Kolmogorov-Smirnov was d = 0.20 and P > 0.93. The Weibull median probability interval for all fires was 10.1 years. The Weibull median probability interval increased (14.5 years) when data were filtered to include only fire years in which 25% or more of recording samples were scarred (Table 1). The Weibull model resulted in shorter return intervals for all fires (7.0 years) as well for those that only scarred 25% or more of the samples (11.3 years).

We identified the position of fire scars in the growth rings for all the samples dated. The results of the analysis of seasonality indicate that the highest percentage of fires (77.4%) occurred in summer (M+L+A), with fewer fires (22.6%) registered in spring (D+E). The distribution of fire scars according to their position within growth rings for all samples shows that most of the scars were located in the middle of the earlywood (M) and the fewest were located in the latewood (L; Fig. 3).

Climate-Fire Interaction--The results from the SEA analysis indicate that, for the period from 1881 to 2004, there was no significant relationship between fire occurrence and ENSO events (P > 0.05). Fires tended to occur in dry years, but the relationship was not statistically significant (P> 0.05). Similarly, we found that the occurrence of fire was preceded by a year of wet conditions, but not significantly (P > 0.05; Fig. 4).

Discussion--Historical Fire Regime Characteristics--Although the reconstruction is limited by the small number of samples (n = 16), we were able to derive some important conclusions for the area. The historical mean fire intervals of 11 years (all fires) and 15.7 years (>25% of all samples) indicate that fires occurred frequently and possibly with low intensity in the conifer woods of the Cerro El Potosi. The nearest reference from this study site is the fire chronology at Pena Nevada, about 130 km straight line from the Cerro El Potosi in the state of Nuevo Leon (Yocom et al., 2010). In general, fires events were not synchronous when comparing the results of the present study with those of Yocom et al. (2010). However, four fire events (1807, 1920, 1955, and 1998) occurred at both sites concurrently. This synchrony in fire events in the two areas can be explained with the relationship between reconstructed precipitation at the Pena Nevada and extreme El Nino and La Nina years (Stahle and Cleaveland, 1993). The four forest fires registered in both areas matches some ENSO extreme events. For instance, between 1800 and 1810, two extreme La Nina events coincided with minimum precipitation (fire recorded in 1807). Additionally, two extreme La Nina events were recorded in the 1920s and one more in the 1950s (fire recorded in 1920 and 1955). Conversely, for the 1998 fire, an extreme El Nino event was registered which coincided with low rainfall (Stahle and Cleaveland, 1993).

Concerning the fire interval statistics, the results of the present study are similar to those found in the P. hartwegii forests at Pena Nevada (Yocom et al., 2010) as well as to those found in the more-northerly dry forests of northwestern Mexico (Cerano-Paredes et al., 2010; Poulos et al., 2013; Meunier et al., 2014) where they also presented a historical low-severity surface fire regime.

A feature of the Cerro El Potosi fire history is that most of the fires occurred during summer (77.4%) and only a small portion (22.6%) of the fires occurred in the early earlywood (late winter-early spring). In contrast, the results found in Pena Nevada, where most of the fires (92%) have occurred during the dormant season, and the rest (8%) registered correspond to summer fires (Yocom et al., 2010). Of the fires registered during the summer in this study, 20% occurred in the latewood and 57.4% of the fires occurred in the middle earlywood.

Climate-Fire Interaction--Climate-fire relationships elsewhere in northern Mexico and the southwestern United States have typically shown strong relationships between fire occurrence, drought, and ENSO events (Yocom et al., 2010; Yocom and Fule, 2012; Poulos et al., 2013; Meunier et al., 2014). Nevertheless, for this study the SEA analysis indicated as not statistically significant the role of dry years as well as for the subsequent wet conditions from 1881 to 2004. We believe that one of the factors that drive this result was that the length of the present reconstruction was fairly brief because of the relatively small sample size. Despite this problem, some tendencies can be drawn from the SEA analysis. We identified the same pattern prior to fire for the precipitation between the Cerro El Potosi and Pena Nevada, according to the SEA analysis for all fires after 1832 (Yocom et al., 2010). In contrast, the SEA analysis using the NIN03 sea surface temperatures value, the subsequent years for the Cerro El Potosi likely occurred in La Nina conditions, but not for Pena Nevada, where the subsequent years occurred likely on El Nino conditions (Yocom et al., 2010).

Implications for Management--The state policies have implemented fire suppression and if possible, fire exclusion in forest ecosystems. Nevertheless, control efforts in the Cerro El Potosi area have been limited because fire control and suppression are accomplished by local crews and landowners rather than by the use of engines and helicopters. However, since the year 2013, huge budgets have been designated in the state of Nuevo Leon for leasing helicopters for fire control and suppression (CONAFOR, 2013). Fire suppression at the Cerro El Potosi with a fire regime historically characterized by frequent surface fires is not the best policy, as it may contribute to changes in forest structure, fuel buildup, and increased risk of stand replacing fires (Yocom et al., 2010; Avila Flores et al., 2014). The results of this study are useful to understand the historical fire regime of the Cerro El Potosi in order to design a better management approach for this area. Perhaps the application of prescribed fires in the area in order to reduce fuel loads can be a strategic part of the fire management in this forest. In addition, a better and complete understanding is important in order to determinate how climate factors such as ENSO influence precipitation and fire patterns in the area.

The study was funded by the Universidad Autonoma de Nuevo Leon (UANL-PAICYT, No. CT 733-19). We would like to thank the Dendrochronology Laboratory of the Centro Nacional de Investigacion Disciplinaria en Relacion Agua, Suelo, Planta, Atmosfera del Instituto Nacional de Investigaciones Forestales, Agricolas y Pecuarias (INIFAP-CENID-RASPA) for assistance with laboratory procedures, and Ph.D. P. Fule and Ph.D. L. Yocom for their valuable assistance and analytical support during the stay at the School of Forestry at Northern Arizona University. We further would like to thank the anonymous reviewers for their key comments on the manuscript.


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Submitted 22 October 2018. Accepted 10 April 2020.

Associate Editor was Alicia Melgoza-Castillo.

Marco A. Gonzalez Tagle, * Diana Y. Avila Flores, Wibke Himmelsbach, and Julian Cerano Paredes Universidad Autonoma de Nuevo Leon, Carretara Nacional, km 145, 67700, Linares, Nuevo Leon, Mexico (MAGT, WH) Instituto Nacional de Investigaciones Forestales Agricola y pecuarias, Campo Experimental Saltillo, Carretera Saltillo- Zacatecas km 342+19 No. 9515 (DYAF)

Instituto Nacional de Investigaciones Forestales y Agropecuarias, Centro Nacional de Investigacion Disciplinaria en Relacion Agua, Suelo, Planta, C.P 35140, Gomez Palacio, Durango, Mexico (JCP)

* Correspondent:

Caption: Fig. 1--Map of the study site, Cerro El Potosi, southern Nuevo Leon, Mexico. The circles indicate the location where the samples were obtained.

Caption: Fig. 2--Composite fire history chart of Cerro El Potosi, southern Nuevo Leon, Mexico. Horizontal lines represent samples and vertical lines represent fire scars. The bottom axis is a composite showing the dates of all the fire years. The period of sufficient sample depth for calculating fire interval distribution statistics was from 1881 to 2004.

Caption: Fig. 3--Intra-annual position of fire scars for all samples in the coniferous forest of el Ceno El Potosi southern Nuevo Leon, Mexico.

Caption: Fig. 4--Superposed epoch analysis for Ceno El Potosi. Top) annual precipitation (Cerano-Parecles et al., 2009), and bottom) El NIN03 index (IRI/LDEO Climate Data Library, http://iridl.kleo.columbia.edU/SOURCES/.Inclices/.nino/). The lines placed above and below the mean represent confidence intervals of 95% (clashed line), 99% (clot clashed line), and 99.9% (solid line), respectively.
Table 1--Characteristics of fire intervals in the coniferous
forest of Cerro El Potosi, in the SMO, southern Nuevo Leon,
Mexico. We calculated the descriptive, statistics for the period
between 1881 and 2004.

Characteristics                    All fires   25% scarred

Total no. of intervals                10            7
Mean fire interval (years)            11          15.7
Weibull median interval (years)       9.9         14.4
Weibull modal interval (years)        7.0         11.3
Minimum fire interval (years)          2            4
Maximum fire interval (years)         22           31
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Article Details
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Author:Tagle, Marco A. Gonzalez; Flores, Diana Y. Avila; Himmelsbach, Wibke; Paredes, Julian Cerano
Publication:Southwestern Naturalist
Article Type:Report
Geographic Code:1MEX
Date:Sep 1, 2019

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