Análisis de la estructura y distribuciones diamétricas en bosques templados bajo la perspectiva del régimen potencial de fuego

Ernesto A. Rubio-Camacho¹; Marco A. González-Tagle^2*; Eduardo Alanís-Rodríguez²; Álvaro A. Chávez-Durán¹; Oscar A. Aguirre-Calderón²

¹ Instituto Nacional de Investigaciones Forestales, Agrícolas y Pecuarias (INIFAP), Campo Experimental Centro Altos de Jalisco. Carretera libre Tepatitlán-Lagos de Moreno km 8. Apartado postal 56, C. P. 47600. Tepatitlán de Morelos, Jalisco, MÉXICO.

² Facultad de Ciencias Forestales, Universidad Autónoma de Nuevo León. Carretera Monterrey-Cd. Victoria km 145. Apartado postal 41, C. P. 67700. Linares, Nuevo León, MÉXICO. Correo-e: marco.gonzaleztg@uanl.edu.mx Tel. +52 (821) 212 4895 ext. 144 (*Autor para correspondencia).

ABSTRACT

The ecological role of fires in ecosystems can be studied through the fire regime. The objective of the study was to evaluate the structure and species composition in an area with a potential regime of frequent and low severity fires under two frequency scenarios in Iturbide, Nuevo León. Two 1-hectare parcels were established with different fuel loads (P1 = smaller load, P2 = larger load); the diameter, size, and coverage of the smaller and larger species were recorded. The forest mensuration variables were analyzed in order to obtain knowledge of the species composition of the forest. In the larger woodland, the Pinus group dominates in both parcels; this species has higher density and coverage values in P1, unlike the Quercus group and Others group (broadleaved). In the smaller woodland, all groups showed higher coverage values in P1, indicating active regeneration in the area with a shorter fire absence period. The diameter distributions were adjusted to the Weibull distribution and reflected the fire regime. It is therefore advised to implement a program for the integral management of fires that encompasses the beneficial effects of the fires.

Keywords: Weibull distribution, succession dynamics, Northeastern Mexico, Pinus pseudostrobus.

RESUMEN

El papel ecológico del fuego en los ecosistemas puede estudiarse a través del régimen de incendios. El objetivo del estudio fue evaluar la estructura y composición de especies en un área con régimen potencial de incendios frecuentes y de baja severidad bajo dos escenarios de frecuencia en Iturbide, Nuevo León. Se establecieron dos parcelas de 1 ha con distinta carga de combustible (P1 = menor carga, P2 = mayor carga) y se registraron las especies, diámetro, altura y cobertura del arbolado mayor y menor. Las variables dasométricas se analizaron para obtener la composición actual del bosque. En el arbolado mayor, el grupo Pinus domina en ambas parcelas; esta especie tiene mayores valores de densidad y cobertura en la P1, caso contrario de los grupos Quercus y Otras (latifoliadas). En el arbolado menor, todos los grupos mostraron valores mayores de cobertura en P1, indicando regeneración activa en el área de menor periodo de ausencia de fuego. Las distribuciones diamétricas se ajustaron a la distribución Weibull y constituyen un reflejo del régimen de fuego en estos ecosistemas. Los resultados aportan conocimiento del régimen de incendios, por lo cual se recomienda la implementación de un programa de manejo integral del fuego que integre los efectos benéficos de los incendios.

Palabras clave: Distribución de Weibull, dinámica sucesional, noreste de México, Pinus pseudostrobus.

INTRODUCTION

Forest fires, although generally perceived as a serious threat to forests, constitute a factor that has played a role in the dynamics of several ecosystems on the planet (Rodríguez-Trejo, 1996; Whelan, 1995). This can be analyzed through the understanding of fire regimes (Jardel-Peláez, 2010), whose main attributes (frequency, severity and pattern) are directly related to the forest structure, the species composition, and the reproductive and protection strategies in case of a fire in different terrestrial ecosystems (Ávila-Flores et al., 2014; Rodríguez-Trejo, 2008; Rodríguez-Trejo & Fulé, 2003; Whelan, 1995).

Fire frequency, characterized by its recurrence interval, is one of the main attributes of a fire regime (Agee, 1993). Frequency changes could directly affect the severity and pattern of fires; that is to say, the longer the recurrence interval of a fire, the higher the accumulation of combustion material, therefore increasing the potential of more intense and severe fires (Swanson, Jones, Wallin, & Cissel, 1994). In addition, this tends to decrease the ecosystem's resilience to forest fires, altering the structure and species composition (Jardel-Peláez, Alvarado-Celestino, Morfín-Rios, Castillo-Navarro, & Flores-Garnica, 2009).

In several forest ecosystems, fires act as a regulating agent for species composition (Pyne, Andrews, & Laven, 1996; Rodríguez-Trejo, 1996). Depending on the intensity and severity, fires can wipe out certain species while facilitating the development of others, so that, in theory, the dominance of favored species by the fire shall be evident in certain fire regimes (Rodríguez-Trejo, 2008). Similarly, the suppression of forest fires in ecosystems that have adapted to fires can affect their structure and composition, allowing the shade tolerant species to dominate the ones that are not shade tolerant (Gilliam & Platt, 1999). In this sense, the characterization of the forest structure, species composition and fire ecology present a comprehensive picture of the species behavior in high or low forest fire frequency conditions.

This study was done in the juniper pine-oak forests of the Sierra Madre Oriental in Mexico. This type of forest, due to its bio-climatic and physical characteristics, has a potential fire regime of high frequency and low severity fires (0 - 35 years), and low natural severity, which facilitates the preservation of diversity in the structure and species composition (Jardel-Peláez et al., 2009; Rodríguez-Trejo, 2008). However, adjacent areas with different fuel loads are present, which indicates a difference in the absence period of fires (Sackett & Haase, 1996; Whelan, 1995). The study of the structure and species composition in the same ecosystem but in different fire incidence scenarios such as this one could bring forth new knowledge on the dynamics and relation sustained with forest fires.

Other studies that characterize the diversity of species and their vertical distribution have taken place in the same location as this study (Campus Ecológico Iturbide, Nuevo León) (Jiménez, Aguirre, & Kramer, 2001; Rubio-Camacho, González-Tagle, Jiménez-Pérez, Alanís-Rodríguez, & Ávila-Flores, 2014). Based on these references, the necessity arises to compare the diameter distributions by groups of species and their structural characteristics, including the smaller trees of less than 7.5 cm in diameter and a height of 1.30 m. The general objective of this work is to provide knowledge on the temperate forests in Northeastern Mexico, in the context of the potential fire regime. In order to achieve this objective, two adjacent areas with different fuel loads were selected: P1 (smaller load) and P2 (larger load), conditions that indicate a longer period of fire absence in P2. The specific objectives were: 1) Characterize the forest structure using the parameters of the stand (density, diameter, height and coverage); 2) Compare the structure between the studied adjacent areas; and, 3) Adjust the diameter distributions to a probability distribution function. The starting hypothesis considered that: 1) The actual regime (potential) of forest fires benefits the species that are adapted to fires, such as pine trees; 2) The presence of extended fire absence periods benefits the species tolerant to shade, such as broadleaved trees; and, 3) According to the potential fire regime, it is assumed that the diameter distributions are not adjusted to a normal distribution.

MATERIALS AND METHODS

The research work was carried out in the area of Campus Ecológico Iturbide, managed by the Universidad Autónoma de Nuevo León. The campus is located 15 km southeast of Iturbide in the state of Nuevo León, Mexico between the 24° 42' N and 99° 51' W coordinates and covers an approximate area of 1,035 ha with an elevation range between 1,200 and 1,890 m. The sampling area is located in the juniper-pine-oak forest region.

Sampling design

The permanent sampling parcels are 100 x 100 m. The parcels were oriented in relation to the slope and were established with a Vertex IV Hypsometer (®Haglöf Sweden AB) and a compass. Within each parcel, 20 x 20 squares (sub-parcels) were defined; there were 25 sub-parcels in total on each parcel. The sub-parcels were defined in order to facilitate field sampling.

In each sub-parcel, the largest woodland area was inventoried through the recording of the species of each specimen, the diameter at a height of 1.30 m (d₁₃ ≥ 7.5 cm) from the ground, total height (h) and diameter of the treetop (Benavides-Solorio, Rubio-Camacho, & Rueda-Sánchez, 2010; Rohman de la Vega, Ramírez-Maldonado, & Treviño-García, 1994; Olvera-Vargas, Moreno-Gómez, & Figueroa-Rangel, 1996). The smaller woodland area (2.5 ≤ x ≤ 7.5 cm) was measured in the center of each sub-parcel in circular nesting sites with a radius of 2 m, wherein the species, the diameter at 1.30 m (d₁₃) from the ground, the height, and the treetop diameter of each specimen were recorded (Brown, Oberheu, & Johnston, 1981).

Parameters of the adjacent area

The parameters of the adjacent area were determined based on the most important species reported in the study of Rubio-Camacho et al. (2014). According to the authors, the species with the highest ecological importance values were Pinus pseudostrobus Lindl., Juniperus flaccida Schltdl. and Quercus canbyi Trel.; for this reason, the adjacent areas were characterized using groups. Four types in total were studied: Pinus, Juniperus, Quercus and the Others group that comprises the rest of the broadleaved species (Rubio-Camacho et al., 2014). Based on this classification, the following parameters of the adjacent area were determined: mean diameter , density (N), basal area (G), mean height and canopy coverage (Cd) (Gadow, Sánchez, & Álvarez, 2007; Prodan, Peters, Cox, & Real, 1997).

Analysis of the information

The statistical and graphical analyses presented in this work were done through the free license software R v3.0.1 (R Development Core Team, 2011). The generated routines were created with the R-Studio v0.9 (RStudio, 2014) software.

Parameters of the adjacent area. The parameters of the adjacent area were compared among parcels, differentiating between the larger and the smaller woodland areas. In the case of the larger woodland area, the parameters of adjacent areas P1 and P2 were directly compared, not as pseudo-replicas, to prove the hypothesis that the fires have an influence on the structure of the vegetation. Regarding the diameters and heights , and because the distribution of the information was not adjusted to the normal distribution, the non-parametric Mann-Whitney-Wilcoxon (Zar, 2010) test was used. The N and Cd were analyzed by comparing proportions with the "prop.test" function in R (Crawley, 2007). In the case of N, the total number of specimens was compared between parcels (N-P1 and N-P2) and groups (v. g. N-Pinus-P1 and N-Pinus-P2) (Gemma-Rutten, 2014); the coverage was also compared in a similar manner (Swamy & Terborgh, 2010).

In the case of the smaller woodland area, two nested sites with a radius of 2 m (25 per parcel) were considered. The information of each sub-parcel was extrapolated to hectares and in this manner, the means of the parameters of the adjacent areas of the smaller woodland were compared using the Mann-Whitney-Wilcoxon test (Zar, 2010).

Weibull probability distribution function (pdf). The Weibull probability distribution function (pdf) of two parameters (Weibull 2P) and its cumulative density function are given by models 1 and 2, respectively. Meanwhile the probability distribution function of three parameters (Weibull 3P) is given by model 3 and its corresponding probability distribution function by model 4 (Bailey & Dell, 1973).

where:

f(x) = Density probability function of the random variable

α, β, γ = Form, scale and position parameters, respectively.

The α, β, and γ parameters were estimated using the maximum likelihood method because the real distribution of the diameter classes of the parcels studies is known (Diéguez et al., 2009). The 2P model was calculated using the fitdistrplus library (Delignette-Muller, Dutang, Pouillot, & Denis, 2014), and for the 3P model, the method described by Robinson and Hamann (2010) was used. The maximum likelihood method was chosen because authors such as Návar-Cháidez and Contrera-Aviña (2000) recommend it as one of the most efficient methods for the estimation of parameters of Weibull's pdf.

With the information estimators, the frequencies of the diameter categories were calculated through the cumulative density function. The goodness of fit was determined with the Kolmogorov-Smirnov test (K-S). The model with the best fit was chosen based on the error index known as relative discrepancy (RD), that evaluates the similarity between the frequencies observed and estimated, generated based on the cumulative density function. The results of the statistical analysis are shown to be between 1 and 0, where the values closer to 1 indicate that the distribution observed and the one calculated do not have anything in common, and the 0 value indicates that the distributions are identical (Gadow et al., 2007; Quezada & Trincado, 2002).

RESULTS AND DISCUSSION

In Table 1, the dasometric characteristics characteristics of the studied parcels can be observed. The groups were ordered according to their ecological importance (Rubio-Camacho et al., 2014); the first is the Pinus group, followed by Quercus, Juniperus and lastly the Others group.

The general comparison between parcels showed that P2 has a significantly (P > 0.001) higher number of trees (669) than P1 (561); the median diameter is statistically smaller (P = 0.03) in P1 than it is in P2, same as the height average (P = 0.02). The results reveal that P2 is a denser area; however, the trees have a larger dimension in diameter and height in P1 (Table 1). This is consistent with the study of Holden, Morgan, Rollins, and Kavanagh (2007) who found that the areas with frequent fires tend to be less dense, but have higher values when it comes to mensuration characteristics.

The groups also present significant differences, more notably in density and coverage. Regarding density, the number of specimens from the Pinus group is higher in P1 than in P2 (P < 0.001); Quercus presents a higher density in P2 than in P1 (P < 0.001), same as the Others group (P = 0.003). Regarding coverage, the Pinus group shows a larger area in P1 than in P2 (P < 0.001), meanwhile the Quercus group and the Others group also registered significant differences (P < 0.001) being larger in P2 than in P1 (Table 1). In addition, in the case of Juniperus, the density and coverage were statistically equal in the two parcels. The broadleaved trees have a stronger presence and coverage in P2. This could be the result of the absence of fire in longer periods of time in which the species that are adapted to fire and are intolerant to shade give way to the broadleaved trees (Oliver & Larson, 1996). Pinus pseudostrobus is intolerant to shade, resistant to superficial fires and regenerates its treetop in younger trees when it has been subjected to low severity fires; therefore, when there are long periods without fires, the species could present problems in their development and settlement (Jardel-Peláez, 1991; Rodríguez-Trejo & Fulé, 2003).

Parameters of the stand (smaller trees)

In the smaller trees are those, those trees and shrubs with a diameter between 2.5 and 7.49 cm, measured at 1.30 m from the ground (d_1.3), are considered. Based on the general comparison between parcels, the density and coverage values are significantly higher (P < 0.001) in P1 (Density: P1 = 3,200 N·ha¹ and P2 = 2,400 N·ha^-1; Coverage: P1 = 4,068 m² and P2 = 2,614 m²). The diameter and general height did not show any significant differences.

According to Table 2, the Pinus and Quercus groups registered a higher density in P1 (P < 0.001). The density of the Others group was significantly higher in P2, whereas the density of the Juniperus group was similar in both parcels. The Pinus and Quercus groups have higher values in P1 than in P2, which could be the result of their fire history, due to the fact that fires provide an opening of the canopy which helps in the regeneration of the broadleaved trees (Gilliam & Platt, 1999; Oliver & Larson, 1996). In addition, this analysis highlights the case of the Pinus group, which indicates that the regeneration of P. pseudostrobus decreases in scenarios where there is an absence of fire, such as is the case of P. palustris Mill., which was demonstrated in a similar study conducted by Gilliam and Platt (1999).

Diameter distributions and Weibull PDF models

Parameter estimation. The estimated parameters per parcel and per group are shown in Table 1. It is necessary to point out that predictions and validation of the parameters were not done in this work because the entirety tree density is known.

Goodness of fit. Tables 3 and 4 show the goodness of fit of the two and three parameter models (2P and 3P). For the 2P models, the null hypothesis (H₀) that indicates that the data follows a specific distribution- in this case the Weibull distribution- was accepted in five of the eight occasions, and for the 3P models, it was accepted in seven of the eight tests. According to the RD test, the model with the best fit was Juniperus-P2 of 3P. Similarly, it was observed that the best models for the Quercus, Juniperus and PQJ (Pinus-Quuercus-Juniperus) groups were the 3P ones, whereas it was the 2P for the Pinus group.

Classes for the Pinus group. The distribution of diameter classes of Pinus-P1 graphically presents a slight tendency to bimodality, even though it adjusts well to that of Weibull of 2P (Table 3). For Pinus P2, the best fit was with Weibull 2P (Table 4). In both groups it can be observed that the probability of finding more trees with 40 cm diameter categories is lower than 6 % (Figure 2). Comparing both parcels, P1 has diameter distributions that show a slight tendency to lower classes, which indicates a higher recruitment in this parcel. These results are consistent with other studies, where the recruitment of intolerant species to the shade can be observed in areas with frequent fires (Gilliam & Platt, 1999).

Classes for the Quercus group. The groups formed by Quercus present a very similar distribution in the P1 and P2 parcels. The best fit for this group in both parcels was found in model 3P, through which a low probability of finding trees (< 10 %) from the diameter categories of ≥ 25 cm was observed, whereas there was a higher probability (< 30 %) of finding trees in the diameter categ ories of 10 and 15 cm (Figure 2).

Classes for the Juniperus group. The Juniperus groups in P1 and P2 present the majority of their diameter classes in the smaller categories. The probabilities of finding trees in the categories of ≥ 20 cm are fewer than 15 % in P1 and fewer than 10 % in P2 (Figure 2). The probabilities were calculated with Weibull 3P for both parcels.

These results particularly coincide with those shown by Návar-Cháidez (2010), who documented that the majority in the forests of the state of Nuevo León are in early-intermediate succession stages. The Pinus groups, followed by Quercus and Juniperus, had the most presence in Weibull distribution, showing that they have a mix of species in different development stages. This shows that the aforementioned forests have been exposed to natural or anthropogenic disturbances, and that those disturbances have affected Juniperus and Quercus more than Pinus.

The parameters of the diameter distributions reflect a small change toward the larger diameter categories in P1. These changes are consistent with other studies, where the areas with frequent fires present less trees but with larger dimensions (Holden et al., 2007). However, in both adjacent areas the diameter categories leaned towards the smaller and intermediate diameter categories, which reflects the success in the establishment of regeneration in favorable environmental conditions and in the variable effects of thinning due to frequent fires (Holden et al., 2007; Savage, Brown & Feddema, 1996).

CONCLUSIONS

According to the results obtained, it can be concluded that the forest structure is a reflection of the incidence of fires. In both parcels, the Pinus group (resistant to fires and intolerant to shade) presented higher values in the parameters of the adjacent areas. However, in the area with the most prolonged period without a fire (P2), the broadleaved trees presented significantly larger structural dimensions. In stead of decrease in trees, should be: the smaller diametric categories. The diameter distributions in the shape of an inverted J adjust to Weibull's pdf and tolerate the presence of low severity fires that eliminate the percentage of trees leaving only the most resistant, which explains the dominance of the Pinus group in the larger diameter categories. In later studies, it is recommended to characterize the spatial distribution of the trees with the fire ecology of the species in order to detect competition/association. It is also recommended to implement a fire management program, where the knowledge of the natural regime of forest fires is included and objectives pertaining to the development of the ecosystem are established.

We thank CONACYT for the scholarship that was granted to us (329047) and to the UANL (PAICYT CA1080-11) for the support and facilities provided during the execution of the research work. In the same manner, we thank INIFAP (2011-007) for the support during the development of the study. Finally, we thank the personnel that collaborated in the collection of information on the field, with special mention to Magnolia Mendívil.

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