Study Area

The study described below was conducted in an area of Pinus-Quercus forest in the Atoyaquillo ejido, Coyuca de Benítez municipality, which is part of the Sierra Madre del Sur in the state of Guerrero, Mexico. The area is located at 17°09'46.34 "north and 100°03'55.76" west. The altitudinal interval is from 1 000 to 1 100 m. The average annual rainfall is 1 100 mm (Figure 1) (^{Inegi, 2002}).

Figure 1 Location of the study area. 

Analysis of vegetation

To evaluate the condition of the vegetation of a Pinus-Quercus forest, five 1 000 m² circular plots (radius =17.84 m) were established. This design was chosen because only one control point is required at the center, unlike rectangular or square plots that consider four (^{McRoberts et al., 2015}). They were randomly distributed in areas that presented the same physiographic conditions of slope, exposure and altitude. In the sampling plots, a census of all woody species with a DBH> 10 cm (d0.10 m) was carried out. The variables quantified for each individual were: total height (h), with a Suunto Pm-5 hypsometer; the diameter (DBH), with a 1270mm Haglöf Mantax Blue caliper; and a crown diameter with a Truper ^® fiberglasss measuring tape of 100 m (north - south and east - west).

The density by species was determined with respect to the total number of trees; coverage, depending on the area of the cup; and its frequency based on the presence of the species in the sampling plots. The calculated variables were used to calculate the Importance Value Index (IVI), which is a weighted value at the taxon level, and which acquires percentage values on a scale of 0 to 100 (^{Mostacedo and Fredericksen, 2000}).

Data analysis

To evaluate the contribution of the species to the structure of the plant community of interest, the estimation of the following variables was used: abundance, coverage, frequency and value index of importance. For the species richness the Margalef index (D _Mg ) was used and for the alpha diversity the Shannon-Wiener index (H') (^{Magurran, 2004}) (Table 1).

The vertical structure of the vegetation was characterized with the index of A (^{Pretzsch, 2009}) (Table1), which considers minimum values of 0 and maximum of A _max , and in which the vegetation is represented in three strata, according to the maximum recorded height: stratum I: 80 %-100 % of the maximum height of the vegetation; stratum II: 50 %-80 %, and stratum III: from 0 to 50 %.

If A is equal to 0, the stand is constituted by a single species in a single stratum, whereas A _max occurs when all the species are in equal proportions in the stand and in the different strata (^{Pretzsch, 2009}).

Diversity

The ^{Margalef index (1951}) yielded a value of 0.72, which implies a low diversity according to the same author's measurement criteria, in which values lower than 2.0 indicate a low species richness. The ^{Shannon index (1948)} of 0.78 stands for a low diversity according to the parameters of the same author. In an investigation conducted under the same methodology in a temperate forest of the Sierra Madre del Sur of Mexico, ^{Méndez et al. (2014)} obtained a similar value in the Margalef index, but higher in the Shannon index.

In another investigation, ^{Alanís et al. (2011)} recorded higher values in both indices in a study on tree regeneration after fire in the Chipinque Ecological Park (northeast of Mexico); ^{López et al. (2017)} calculated higher figures when evaluating the composition and diversity of forest species in temperate forests of the state of Puebla, based on the same dasometric variables estimated in this study. ^{Graciano et al. (2017)} also obtained outstanding values when studying pine - oak forests in different regions of northeastern Mexico. The low diversity of tree species in this type of ecosystem is attributed to the high frequency of Pinus and Quercus genera in these plant communities (^{Mora and Alanís, 2016}).

Species richness

A richness of five species was recorded, which belong to two families and two genera (Table 2). Of the two families, Fagaceae is represented by three species, while the Pinaceae family only two. ^{Méndez et al. (2014)} found five families and the same number of species in a Pinus-Quercus forest of the Sierra Madre del Sur after a fire. ^{Juárez et al. (2014)} accounted for a lower number of species in a forest of the Sierra de San Carlos in Tamaulipas, Mexico; whereas ^{Torres et al. (2006)}, ^{Alanís et al. (2011)}, ^{Zacarías et al. (2011)}, ^{Hernández et al. (2013)}, ^{Graciano et al. (2017)}, ^{López et al. (2017)} and ^{Rubio et al. (2017)} registered a greater number of species in different regions of Mexico in this plant association, compared to this study. This low species richness could be due to the influence of anthropogenic factors such as agriculture, illegal logging, local use of species and the constant fires from illegal crops in the region.

Ecological indicators

Density

A total number of 254 individuals (N) ha^-1 were counted, 88.19 % (22 4 N ha^-1) from which are pines and 11.81 % (30 N ha^-1), oaks. Pinus oocarpa had the highest density (77.95 %; 198 N ha^-1), and in contrast, Quercus rugosa had the lowest value (0.79 %; 2 N ha^-1). ^{Torres et al. (2006)}, ^{Alanís et al. (2011)}, ^{Méndez et al. (2014)}, ^{Graciano et al. (2017)} and ^{López et al. (2017)} recorded a higher number of trees per hectare at different regions of pine-oak forests of Mexico.

There is a great variety of species that stand out as the most abundant in pine-oak ecosystems in Mexico. ^{Méndez et al. (2014)} in an analysis of post-fire regeneration in a forest in the Sierra de Guerrero, Pinus radiata D. Don., in a characterization study of post-fire regeneration too, ^{Alanís et al. (2011)} considered Quercus polymorpha Schltdl. & Cham. as an outstanding species in temperate forests of northeastern Mexico; ^{Graciano et al. (2017)} as well as ^{Hernández et al. (2013)} in the same type of ecosystem, selected Pinus cooperi C. E Blanco and Pinus arizonica Engelm. ^{López et al. (2017)} referred to Pinus montezumae Lamb. in temperate forests of Puebla (Table 3).

Dominance (coverage)

The total coverage in the area studied was 15 348 m² ha^-1, which suggests a coverage> 100 %, and, therefore, a crown overlap is confirmed, a percentage with which ^{Alanís et al. agree (2011)}; ^{Hernández et al. (2013)} recorded a lower coverage (88.76 %) in their research in a temperate forest of Chihuahua.

Of the two genera of interest, Pinus predominates with 87.64 % coverage (13 452 m² ha^-1), while Quercus occupies the remaining percentage (1 897 m² ha^-1). The results of ^{Alanís et al. (2011)} and ^{Sánchez et al. (2014)} also pondered Pinus with the highest dominance, as well as various authors. At the species level, Pinus oocarpa had the highest relative coverage with 70.96 % (10 891 m² ha^-1), and Quercus rugosa only 0.57 % (88 m² ha^-1) (Table 3).

Frequency

Pinus was present in all the sampling plots, in particular Pinus oocarpa (41.67 %) and Quercus in three; Quercus elliptica and Quercus rugosa had the lowest relative frequency (8.33 %) and were only identified in one plot (Table 3). This behavior of the pines was ratified by ^{Jiménez et al. (2001)}, ^{Torres et al. (2006)}, ^{Alanís et al. (2008)}, ^{Hernández et al. (2013)}, ^{Méndez et al. (2014)}, ^{Delgado et al. (2016)}, ^{Graciano et al. (2017)} and ^{Hernández et al. (2013)} in their respective investigations; but ^{Zacarías et al. (2011)} and Alanis et al. (2011) highlighted Quercus as the most frequent genus.

Importance Value Index (IVI)

Of the two genera studied Pinus reached 78.05 % of IVI value, and Quercus 21.95 %. At the species level Pinus oocarpa obtained the highest value with 63.53 % and the lowest was Quercus rugosa with 3.23 % (Table 3). The authors who confirmed the highest frequency of pines, also do so for IVI, but do not coincide with the species with the present investigation. ^{Alanís et al. (2010)}, ^{Alanís et al. (2011)} and ^{Zacarías et al. (2011)} registered Quercus with the highest IVI in their research experiences.

Diameter and heights classification

In the histogram it is observed that an exponential trend line decreases as the diametric classes increase. The categories of 10-20 cm and 20-30 cm gather more than 50 % of the individuals included in the study, which acts as an indicator of active regeneration due to the high recruitment level of individuals in the area.

The third category also houses a significant number of individuals, but categories greater than 40 cm of DAP group a smaller amount. The results indicate a forest in regeneration and growth in which the most abundant population is grouped in the first diametric categories (Figure 1). Regarding heights, in the category of 11-20 m, the largest number of trees is concentrated, followed by the interval of 0 - 10 m; and finally the smallest group falls within the category of 21 - 30 m (Figure 2).

Figure 1 Diametric category distribution in a Pinus-Quercus forest in the la Sierra Madre del Sur, Guerrero, Mexico. 

Figure 2 Height class distribution in a Pinus-Quercus forest in the la Sierra Madre del Sur, Guerrero, Mexico. 

For the vertical structural diversity of the species, three strata were determined: high (III), from 23 to 28 m; medium (II), from 15 to 22 m and low (I), from 5 to 14 m. Of the three strata, the highest species density was found in the low, with 50.4 % of the individuals (127 N ha^-1), of which Pinus oocarpa contributed 102 N ha^-1. In the middle stratum, 41.7 % of the total individuals were concentrated, represented by Quercus crassifolia, Quercus rugosa, Pinus maximinoi and Pinus oocarpa with 86 N ha^-1 (81.1 %) (Figure 3).

Figure 3 Species vertical distribution in the study area. 

The result obtained for the Pretzsch index was A = 2.07 with an Amax = 2.70, which indicates that the distribution of species is highly represented by the low stratum (I) with a higher proportion of trees between 5 and 14 m, which decreases slightly as the height increases in the next stratum (II). The high stratum (III) has a low relative density (7.8 %), and is made up of two species (Pinus oocarpa and Pinus maximinoi). These results coincide with those of ^{Rubio et al. (2014)}, who also recorded a high presence of trees in the lower stratum and low representativeness of the upper stratum (Table 4).