Introduction

Anthropogenic activities such as recreation, ecotourism, agriculture, livestock, among others, have led to changes in ecosystems; an example of this is the disturbance generated by the fire regime (^{Paysen et al., 2000}). The fire regime is a mixture of temporal and spatial scenarios that depend on the frequency, intensity, extent, and seasonality of fire, which have a decisive influence on the patterns of ecological succession (^{Medina, 2007}).

Certain types of vegetation throughout history have shown resilience to the presence of forest fires, which has given them adaptive value to the new environmental conditions imposed by the high frequency regime. However, there is also an alteration in species composition and structure that results in a transformation of their original state (^{González et al., 2007}; ^{Alanís-Rodríguez et al., 2012}; ^{Carbone et al., 2017}).

Certain species of the Pinus genus, due to the constant presence of fire, have developed adaptive strategies to cope with it, e.g.: thick bark (to protect the cambium), serotinous cones, protected apical buds, and the cespitose or branching stage (^{Rodríguez-Trejo, 2001}).

Also, another unusual phenomenon has been observed in response to fire: the appearance of shoots from the base of the stem of affected individuals, which can be perceived as part of an adaptation that favors them in the face of alterations in the fire regime (^{Pausas et al., 2016}). Although this phenomenon is not exclusive of the Pinus genus (^{Juárez et al., 2012}; ^{Lilly et al., 2012}; ^{García-Jiménez et al., 2017}), there are few records for it (^{Sánchez et al., 2014}; ^{Gómez-Mendoza and Rodríguez-Trejo, 2021}). Therefore, the objective was to characterize the trees of the Pinus genus and their shoots by type of fire severity under the hypothesis that there are significant differences in shoot density between fire severities.

Materials and Methods

Study area

The study was carried out in a fraction of the Eastern Sierra Madre, in the Cumbres de Monterrey National Park (CMNP), specifically in the community of El Tejocote (25°20'38.2" N, 100°14'53.0" W), located west of Santiago municipality, Nuevo León State. The area is located at an altitude between 2 000 and 2 300 m, with a vegetation consisting mainly of pine-oak forest, where the presence of the following species stands out Pinus teocote Schltdl. & Cham., P. pseudostrobus Lindl., P. greggii Engelm. ex Parl., Quercus canbyi Trel., Q. laceyi Small and Q. polymorpha Schltdl. & Cham. The mean annual temperature is 14.1 °C, the maximum values range from 16.8 to 29.8 °C, and the minimum values from 0.8 to 9.4 °C, and the annual precipitation is 766.5 mm, with a maximum of 89.6 mm and a minimum of 13.5 mm (^{Cuervo-Robayo et al., 2015a}; ^{Cuervo-Robayo et al., 2015b}). A fire occurred in the CMNP in March 2021 that affected 14 453.60 ha of land (Figure 1).

Source: Prepared by the authors.

Figure 1 Location of the study area and surface area affected by the fire by severity type. 

Results

In the analysis of the difference index of the normalized burning coefficient (dNBR), three different fire severities were determined: (I) Low, with a surface area of 74.73 ha, (II) Moderate-low, with 91.02 ha, and (III) Moderate-high, with 40.52 ha (Figure 2).

Figure 2 Fire severity and estimated surface areas. 

The highest number of individuals occurred in the low-fire severity area; 31 % produced shoots. Two types of tree conditions were identified: (1) Living burnt and (2) Charred; the latter resulted in the highest number of individuals with shoots in all three severities. Moderate-high severity recorded more trees with shoots. In the three severity levels, the dominant proportion of trees did not bud (Table 1). Of the total number of individuals sampled, 83 % corresponded to the species Pinus teocote, and 17 % to Pinus pseudostrobus.

Over 50 % of the individuals in all fire-severity categories had no shoots. In the trees that exhibited shoots, the number of shoots per tree was higher in the low level, from one to two shoots in the three severities. The presence of five or more shoots per tree was the least frequent (Table 2).

The mean values of the mensuration variables of the trees in the moderate-low severity were higher than the others. Shoot height was greater in the moderate-high severity in a range of 8 to 83.1 cm, with an average of 32.3 cm, and the diameter of the shoots ranged between 0.3 and 1.9 cm, with a mean of 1.1 cm. The values were lower in the low-severity areas (Table 3).

In the area with low fire severity, there were 12 diameter categories, the other fire-severity levels had nine. The 5 and 10 cm categories in the low and moderate-low fire severity levels had the largest number of trees, trees with shoots, and shoots; in the moderate-high severity areas, they were observed in the 10 and 5 cm categories. There was a larger number of trees with shoots and shoots in the moderate-high severity areas (Table 4).

The analysis of variance between the number of shoots in relation to fire severity showed that there is no significant difference (p>0.05); therefore, the hypothesis is rejected.

The results of the correlation tests for diameter and height category with respect to the occurrence of shoot break showed a strong significant association (p<0.05). The smaller the diameter of the shaft, the higher the probability of sprouting; when the diameter category variable increases, the probability of sprouting decreases.

There were 13 height classes in the low and moderate-low severities, and 14 in the moderate-high. At low severity, the 2 m class had the highest number of trees, trees with shoots, and shoots; at moderate-high severity, the 3 and 4 m classes had the highest number of trees, trees with shoots, and shoots. The moderate-low severity area exhibited more trees and trees with shoots in the 2 m class; however, the 3 m class was superior in number of shoots (Table 5).

Discussion

Charred trees and the moderate-high fire severity area had the highest presence of shoots; likewise, ^{Gómez-Mendoza and Rodríguez-Trejo (2021)} refer that the probability of reshooting shows a direct relationship with the percentage of the crown affected by fire in Pinus montezumae Lamb.; ^{González-Rosales and Rodríguez-Trejo (2004)} state the opposite for Pinus hartwegii Lindl., although the classification of tree condition differed between the three studies.

Of the total population sampled in the three severities of fire, 39 % produced shoots. This percentage is lower than the one cited by ^{Sánchez et al. (2014)}, who assessed the regeneration of Pinus oocarpa Schiede ex Schltdl. in the La Primavera forest, in Jalisco State, estimating 83 %. But it is higher than the value indicated by ^{Baumgartner and Fulé (2007)} for Pinus leiophylla Schiede ex Schltdl. & Cham. var. chihuahuana (Engelm.) Shaw, of 34 % in the Mogollon Rim, Arizona.

One strategy for tree recovery after a fire is sprouting. ^{Juárez et al. (2012)} researched this process Pinus teocote in Chignahuapan, Puebla State, and calculated 344 shoots ha^-1 and five shoots per tree; in this work the densities are higher and the level of sprouting is the least common in the areas.

The average height of the shoots in the three fire severities is less than that determined in Chiapas State by ^{Rodríguez et al. (2019)}, which is 40 cm in Pinus oocarpa six months after a fire, and then that cited by ^{Clabo and Clatterbuck (2019)} for Pinus echinata Mill. (73 cm) in Morgan, Tennessee.

^{Gómez-Mendoza and Rodríguez-Trejo (2021)} assessed and modeled the probability of mortality and regrowth in Pinus montezumae, Pinus teocote, and Pinus patula Schltdl. & Cham. nine months after a fire in the Michac ejido, Chignahuapan, Puebla, and they found that younger individuals had greater resprouting power than older ones, similarly to what was observed in the CMNP. The ability to sprout after a fire can vary according to the age of the trees, with older trees of some species producing few shoots (^{Miller, 2000}).

This study supports the observation of ^{Baumgartner and Fulé (2007)} for Pinus leiophylla var. chihuahuana on the Mogollon Rim in Arizona, in the sense that most of the trees that sprouted from the base of the tree were in the 5 cm diameter class, as well as their results for the low and moderate-low severities.

The ability of conifers to sprout in response to disturbances, such as forest fires, can vary in space and time, as was studied as part of the fire ecology of Pinus hartwegii in Mexico City (^{Rodríguez, 1996}); the results show that 28 % of trees between 1.3 and 4 m in height have an average of three shoots per tree, and 97 % of trees less than 1.3 m in height have seven shoots per tree. These values coincide with those of the present study, in which the lowest height classes have the highest shoot presence and an even larger number of shoots per tree.

^{Alanís-Rodríguez et al. (2012)} assessed the composition and diversity of the natural regeneration in post-fire Pinus-Quercus natural communities in the Chipinque Ecological Park; they classified Pinus pseudostrobus as a non-sprouting species, which contrasts with what was described for the CMNP, where Pinus communities exhibited the presence of sprouting as a fire response adaptation.

Growth substances in the roots, particularly cytokinins, assist the sprouting of dormant buds at the base of the tree, though they may already be present in the shoots; however, a decrease in the ratio of auxins to cytokinins provides the stimulus for shoot growth, as the former are inhibitory to sprouting (^{Miller, 2000}).

As a broad generalization, species growing on sites with higher impact or frequent disturbances are likely to sprout more vigorously and retain the ability to sprout longer than species growing on sites with less damage or less frequent disturbances (^{Del Tredici, 2001}).

Conclusions

Pinus teocote and Pinus pseudostrobus species exhibit shoots in the three fire severities; the most represented level of shoots is the lower one, of one to two shoots per tree. The area with the highest intensity of disturbance ―moderate-high severity― has the highest density of trees with shoots, however, there are no differences between severities. Charred trees are predominant as to the presence of shoots. These develop mainly on individuals of the lowest diameter and height categories.

The ability to sprout as a form of regeneration after a fire is an important adaptive advantage; this ability may potentially determine the difference between the dominance of pines versus other species in an area after a disturbance.

Studies are suggested to explain the physiological reasons for sprouting after a fire, as well as what substances and in what proportion are necessary to activate this process because only certain trees of the same species have this capacity.