Study area and sampling sites

Field work was carried up during the Winter of 2019 in four locations of the mixed pine-oak forest located in the Sierra Madre Oriental mountain system, with a 5 km in-between distance; three are found in Galeana municipality and the fourth one in Aramberri municipality of the state of Nuevo León, between 23°43'51''-25°26'45'' N and 99°28'30''-100°55'48'' W (Figure 1).

Figure 1 Location of the sampling sites in the study area. 

The climate in the region is temperate sub-humid with rains in summer; its pluvial regime oscillates between 600 and 1 200 mm, with annual average temperature between 14 and 18 ˚C.

Litosol soil type predominates and the types of vegetation present are coniferous forests made up of pine-oak and oak-pine associations (^{Inegi, 2017}).

Four sampling plots were established in 2 500 m² (50 × 50 m), in the following spots: Canoas (S1), Agua Blanca (S2), Cañada “El Llorón” (S3) and “El Alamillal” (S4); in each of them, the geographical coordinates and their physiographic characteristics were recorded (Table 1) and all individuals of tree species greater than 7.5 cm in normal diameter (diameter at a height of 1.3 m above ground level) were censored, according to the methodology used in the forest and soil research sites developed by ^{Corral-Rivas et al. (2009)}. The species and tree number of each individual were identified and measurements of total height (h), normal diameter (d _1.30 ) and crown diameter(d _copa ) were taken.

Data Analysis

Floristic Composition

During the collection of information in the field, botanical material was collected from all the tree species present at the sampling sites, where a terminal portion of a branch of each species was taken, made up of leaves and the reproductive structure. For later taxonomic identification, such specimens were taken to the Graduate School of Forest Sciences of the Universidad Autónoma de Nuevo León. The validity of the scientific names for each of the identified species was verified based on the ^{The Plant List (2013)} platform.

Similitude coefficient

To determine the similarity in the composition of the species between the four localities, an analysis was applied with the BioDiversity Professional version 2.0 program (^{McAleece et al., 1997}), using an algorithm that allows analyzing the similarity of the samples, through the Percentage calculation with intervals from 0 to 100 %, the result of which is represented in a Bray-Curtis similarity-dissimilarity dendrogram (^{Bray and Curtis, 1957}).

Diversity and richness indices

The following diversity and richness indices were estimated:

The Shannon-Weaver diversity index (H'), which expresses the heterogeneity of a community based on two factors: the number of species present and their relative abundance (^{Shannon, 1948}; ^{Castellanos-Bolaños et al., 2008}).

Where:

S = Number of present species

ln = Natural logarithm

P _i = Individuals found of the i species ratio; it is calculated through the (n _i /N) ratio

n _i = Number of individuals of the i species

N = Total number of individuals

The Simpson diversity index (D), which estimates whether a given community is made up of very abundant species, since it adds the abundances of each one to the square and, thus, gives importance to the species with high value (^{Lamprecht, 1962}).

Where:

P _i = Ratio of the i species in the community (n _i /N)

n _i = Number of individuals of the i species

N = Total number of individuals

The Margalef richness index (D _Mg ), which determines the biodiversity of a community based on the numerical distribution of the individuals of the different species, based on the total number of individuals in the analyzed sample. It combines the number of species (S) and the number of individuals (N) (^{Magurran, 2004}).

Where:

ln = Natural logarithm (e basis)

S = Total number of present species

N = Total number of individuals

The Menhinick wealth index (D _Mn ), which is based on the relationship between the number of species and the total number of individuals observed, which increases with increasing sample size.

Where:

S = Number of species

N = Number of present species

Diametric categories

The Weibull function was used to create histograms of diameter distributions, which were evaluated using an X-square goodness-of-fit test, with the Minitab version 16 software (^{Minitab, 2014}). This function is used in forest science for its ease of application in similar analyzes of some species of the Pinus genus (^{Bailey and Dell, 1973}).

Horizontal structure

To evaluate the horizontal structure of each species, its abundance was determined, according to the number of trees, its dominance as a function of the canopy area, and its frequency based on the presence at the sampling sites. The results were used to calculate the Importance Value Index (IVI), which acquires percentage values on a scale of zero to 100 (^{Müeller-Dombois and Ellenberg, 1974}; ^{Mostacedo and Fredericksen, 2000}).

The estimation of relative abundance was obtained with the following equation:

Where:

A _i = Absolute abundance of the i species (N ha^-1)

AR _i = Relative abundance of the i species with respect to total abundance

N _i = Number of individuals of the i species

S = Sampling area (ha)

Relative dominance was calculated through the following equation:

Where:

D _i = Absolute dominance of the i species (N ha^-1)

DR _i = Relative dominance of the i species with respect to total dominance

Ab _i = Crown area of the i species

S = Sampling area (ha)

The relative frequency was estimated with the following equation:

Where:

F _i = Absolute frequency (percentage of presence in the sampling sites)

FR _i = Relative frequency of the i species with respect to the sum of frequencies

f _i = Number of sites in which the i species is present

NS = Total number pf sampling sites

The Importance Value Index (IVI) is defined through the following equation (^{Whittaker, 1972}; ^{Moreno, 2001}):

Where:

IVI = Importance Value index

AR _i = Relative abundance of the i species with respect to total abundance

DR _i = Relative dominance of the i species with respect to total dominance

FR _i = Relative frequency of the i species with respect to total frequency

Vertical Structure (A of Pretzsch)

The vertical structure of the species was interpreted by using the Pretzsch vertical distribution index A (^{Pretzsch, 1996}; ^{del Río et al., 2003}), which is a modification of the Shannon index (^{Pretzsch, 2009}), in which A has values between zero and an A _max value; when an A = 0 value means that the stand is made up of a single species that occurs in a single stratum. A _max is reached when all the species are present in the same proportion, both in the stand and in the different strata (^{Corral et al., 2005}). Therefore, three strata were defined with intervals of maximum height of the area, in which the tallest tree represents 100 %, and corresponds to stratum I: 80-100 %, to stratum II: 50-80 % and to stratum III: 0-50 % (^{Aguirre, 2002}; ^{Jiménez et al., 2008}; ^{Pretzsch, 2009}). The A index is calculated using the following equation:

Where:

A = Vertical distribution index

S = Number of present species

Z = Number of strata with respect to height

P _ij = Species percentage in each zone, which is estimated by (n _i;j /N), n _i

j = Number of individuals of the i species in the j stratum

N = Number of individuals

In order to compare the Pretzsch index it was necessary to standardize it and this was done by means of the A _max value, which is calculated with the following equation:

To standardize the A value, the following model was used:

Floristic composition

A total of 10 tree species was identified, which belong to four families and five genera. Fagaceae gathered the greatest richness with five species, followed by Pinaceae with three. These families represent 92.32 % of the total registered in the four study sites (Table 2). The rest were Ericaceae and Cupressaceae with one genus. The species that stood out were of the Quercus (five), Pinus (two) genera; only one taxon was identified of Abies, Arbutus and Juniperus. In relation to the number of individuals for the four study sites, Fagaceae dominated, with 228 and Pinaceae, with 205; while the least abundant were Ericaceae, with 32 and Cupressaceae, with 4.

In the Sierra Madre Oriental, worth mentioning are the floristic studies carried out by ^{García-Arévalo (2008)}, ^{Valenzuela and Granados (2009)}, ^{Aragon-Piña et al. (2010)}, ^{De León et al. (2013)}, ^{Delgado et al. (2016)} and ^{Graciano-Ávila et al. (2017)}, which agree that Pinaceae and Fagaceae were the most abundant families; ^{Valencia (2004)}, ^{Sánchez-González (2008)}, ^{González-Elizondo et al. (2012)} and ^{Gernandt and Pérez-de la Rosa (2014)} argued that the abundance of these two families is explained by their wide diversity of species in Mexico, since 49 are recorded for Pinus and 161 for Quercus. Likewise, ^{Zúñiga et al. (2018)} documented a high value of importance for the Pinus and Quercus genera, to which must be added the great economic interest of both families in the forest ecosystems in a national scope (^{Semarnat, 2014}).

Similitude coefficient

The dendrogram derived from the Bray-Curtis similarity-dissimilarity matrix (Figure 2), groups the study sites into two well-defined sets. The first is made up of S2 and S3, which were the most similar in the composition of species, with 48.03 % similarity and correspond to a mixed oak-pine forest plant community. The second group is made up of S1 and S4, with 43.72 % similarity, and a pine-oak plant community. The clusters are influenced by the number of individuals of the taxa with the highest floristic composition, because their abundance was very similar, despite the fact that their richness was very different between the sampling sites.

Figure 2 Similitude-dissimilitude Bray-Curtis dendrogram of the study sites. 

Diversity and richness indexes

The highest values of the of the Shannon-Weaver and Simpson diversity indexes as well as the Margalef richness index were recorded in S4, because it has the highest number of species; however, the highest value of the Menhinick richness index was determined in S3. The lowest diversity values of Shannon-Weaver and Simpson were recorded in S1, but the richness indices of Margalef and Menhinick in S2, represented by four species (Figure 3).

Figure 3 Analysis of the diversity and richness indexes of the study sites. 

When comparing the results of this study with the values of species diversity and richness in other regions of the country, it is observed that the value of the Shannon-Weaver index falls within the ranges recorded by ^{Solís et al. (2006)} of H´ = 0.72 and 1.21 and ^{Návar-Cháidez and González-Elizondo (2009)} of H´ = 0.53 to 1.33 in temperate forests of Durango; by ^{Hernández-Salas et al. (2013)} of H´ = 1.23 in a temperate pine-oak forest in Chihuahua; by ^{López-Hernández et al. (2017)} of H´ = 1.37 in temperate forests of Puebla; by ^{Domínguez et al. (2018)} of H´ = 0.93 to 1.51 in four sites of the SMO; and by ^{Buendía-Rodríguez et al. (2019)} from H´ = 0.83 to 1.78 in 10 sites in northeastern Mexico.

According to the Margalef index, some authors such as ^{Hernández-Salas et al. (2013)} refer values of D _Mg = 0.90 to 1.04 in temperate forests of northeast Mexico; ^{López-Hernández et al. (2017)} of D _Mg = 1.35 in temperate forests of Puebla. In a similar way, for the Menhinick index, ^{Návar-Cháidez and González-Elizondo (2009)} and ^{Ríos-Saucedo et al. (2019)} recorded higher values of D _Mn = 0.82 and 0.95 in temperate forests of Durango and Simpson richness index values of D = 0.73. Based on the results of the diversity and species richness indexes at the study sites, they are classified as low to medium diversity.

Diametric categories

Diametric distributions indicate irregular masses, as they reflect both inverted J trends and normal distribution curves. In this case, forests with dominant young trees with diameters from 10 to 30 cm, values similar to those reported by ^{Návar-Cháidez (2010)} and ^{Delgado et al. (2016)} for temperate forests of Nuevo León and Durango. Individuals with diameters greater than 40 cm were observed, which as the diameter increases, the structure of the sites decreases in density, reflecting an adequate flow of regeneration with a reserve of young individuals, which in the future could replace the larger diameters, so that the survival of the ecosystem is guaranteed. Therefore, thinning could be carried out, since there are trees that have reached their maximum development (Figure 4).

Figure 4 Distribution of trees by diametric categories at the study sites. 

Horizontal structure

A total of 469 trees were recorded. S4 corresponds to the largest number of families with four, followed by S3 with three, S1 and S2 with two. Six species were identified in S3 and seven in S4, which are the highest figures, perhaps due to its northeast exposure, since it favors favorable temperature and humidity conditions, since forests with this exposure show less water deficit due to the low temperature and evapotranspiration during the day (^{López-Gómez et al., 2012}). S1 and S2 follow a lower trend, with four families, with a southwest exposure. At all sites, there was at least one taxon of the genus Pinus and Quercus. The rarest species, one for each site, were Abies vejarii Martínez, Juniperus flaccida Schltdl., Quercus canbyi Trel. and Quercus laeta Liebm.

Q. mexicana Humb. et Bonpl was the best represented in S2 with 89 trees (59.73 % of the total), followed by S1 with 44 trees of P. teocote Schiede ex Schltdl. et Cham (61.97 % of the total), P. pseudostrobus Lindl. with 37 trees (35.24 % of the total) in S3, Q. canbyi with 35 trees (24.3 % of the total) in S4. To a lesser extent, Q. mexicana and Q. rugosa Née with 3 trees (4.23 and 2.08 % of the total) in S1 and S4. S2 was the site with the highest number of individuals (149), followed by S4 (144), S3 (105), and S1 (71). The highest coverage corresponded to S2, S4 and S3 with values of 2 853 m² ha^-1, 2 806 m² ha^-1 and 2 779.4 m² ha^-1 respectively; while the site with the least coverage corresponds to S1 with 2 215.02 m² ha^-1. The species with the highest coverage were Q mexicana, with 3 189.44 m² ha^-1 and P. teocote with 3 038.56 m² ha^-1 and with the lowest coverage J. flaccida 77.14 m² ha^-1.

The most abundant species were Q. mexicana, P. teocote and P. pseudostrobus, and to a lesser extent J. flaccida. The dominant species were Q. mexicana, P. pseudostrobus and P. teocote, and with less dominance J. flaccida. The most frequent species was P. teocote (Table 3).

A higher importance value (IVI) was calculated for Quercus in S2, S3 and S4, and for Pinus in S1. The most significant taxa of the sampled sites were P. teocote, P. pseudostrobus, Q. rugosa and Q. mexicana, an expected behavior of the vegetation because they are mixed forests. ^{Guzmán (2009)} pointed out that when a species reaches the value of outstanding importance, it plays a clear ecological dominance, and it is common for a few species, with higher values of importance, to represent the most significant populations of ecosystems.

Conversely, when competition on the site is divided into several species, none of them exceeds 50 % of the importance value. In general, the sampled sites are located in areas under forest management, which have not been intervened so far, and which include species of economic interest, therefore, if the corresponding forestry treatments are applied, this would favor their density, frequency, coverage and basimetric area. However, the generic dominance makes it possible to observe pine-oak or oak-pine forests in a site, which is consistent with what is documented in the Ecological Ordering of the state of Durango for the temperate forests of the area (^{Semarnat, 2007}).

The basimetric area reached the highest value in S1 with 53.59 m² ha^-1, which can be explained by the higher diameters of P. teocote, and to a lesser extent, in S4 with 19.55 m² ha^-1, made up of the same way mostly by P. teocote (Table 3).

The comparative study between sites allowed to detect structural and compositional variations that occur within the same ecosystem. Although in general terms, the same coniferous or broadleaved species can develop in different degrees of slope or orientation, others have preferences for particular topoforms (hills, canyon, valley, slopes, etc.) (^{Martínez-Antúnez et al., 2013}).

Vertical structure (A of Pretzsch)

The vertical structure of the sampled sites is distributed in three strata, of which the upper one (I:> 80 % of the maximum height) was dominated by P. teocote, which is present in S1, S2 and S4 with the heights The most outstanding were from 26 to 30 m, and for S3 it was P. pseudostrobus with 22.93 m. The lowest heights in S1 correspond to A. vejarii; in S2 and S3, to Q. mexicana and Q. rugosa in S3. Diameters at this level range from 41.13 to 94.67 m. In the stratum (II:> 50 % - 80 %) P. teocote dominates in the four sampling sites with heights of 17.90 to 24 m. However, other species begin to appear as P. pseudostrobus in S1, S3 and S4, A. vejarii in S2 and some Quercus species: Q. cambyi and Q. laeta in S4, Q. mexicana in S1, S2 and S3, Q. polymorpha Schltdl. et Cham in S2 and S3, and Q. rugosa in S1 and S3 which is a shade tolerant genus. Diameters at this level range from 31.07 to 66 m. The stratum (III: up to 50 %) of S4 showed the greatest diversity of species, with the most noticing P. teocote height values, but Q. rugosa and Q. mexicana recorded the largest diameters in S1 and S3 (Table 4).

In regard to the values of the vertical index of A = 1.02 to 1.69 and an A _max = 2.48 to 3.04 in the sampling sites, the highest was located in S4 with an A _rel = 55.39 %, indicating that the evaluated site has uniformity mean in height diversity (Table 4). These results are comparable with the results of ^{Buendía-Rodríguez et al. (2019)} from A = 1.5 to 2.53, with A _max = 2.48 to 3.5 at 10 temperate climate sites in northeast Mexico.