Introduction

Describing the composition, diversity and structure of a forest ecosystem allows to establish strategies for the conservation of biodiversity. Thus, it is necessary to describe the diversity of species and structural aspects through quantitative indexes (^{Suárez and Vischi, 1997}; ^{Wehenkel et al., 2014}; ^{Herrera-Paniagua et al., 2018}). The study of tree richness and diversity and structural ecological parameters of species such as density, dominance, frequency and importance value, are criteria to assess their behavior in the face of possible natural or anthropogenic disturbances (^{Graciano-Ávila et al., 2017}; ^{Hernández-Salas et al., 2018}; ^{Silva-García et al., 2021}).

One of the traditional methods for describing the richness, diversity, structure and value of importance of tree species is the forest inventory. This procedure leads to the collection of vegetation information necessary to achieve this purpose (^{Álvarez et al., 2004}; ^{Delgado et al., 2016}).

In recent decades, Protected Natural Areas PNAs (ANP, for its acronym in spanish) have emerged as an environmental policy instrument to conserve the biological diversity of a given area (^{Cantú et al., 2004}). However, conservation strategies require, in the first instance, a description of tree richness and diversity, and the estimation of elements that characterize forest structure (^{Herrera-Salas et al., 2018}). Mexico currently has 185 PNAs (^{Conanp, 2022}), and new proposals have been made to increase this number, such as the assessment of the ecosystem services they offer to address climate change, mitigate forest deterioration and promote the conservation of resources natural (^{Bertzky et al., 2012}; ^{Oceguera-Salazar et al., 2016}).

In the state of Durango, there are officially six PNAs and their area adds up to a total of 353 229 ha, equivalent to 2.9 % of the national territory; three are of federal jurisdiction and three of state scope (^{Rentería et al., 2011}). Within the latter, the El Tecuán Ecological Park is located under the administration of the Ministry of Natural Resources and the Environment of the Government of the State of Durango (SRNyMA). This Park represents offers the possibility to carry out actions for the conservation of ecological processes and ecosystem services through the promotion and analysis of knowledge of its natural resources (^{POGED, 2018}). Thus, the objective of this study was to describe the diversity of tree species and their ecological importance in the El Tecuán Ecological Park in the state of Durango, Mexico.

Materials and Methods

El Tecuán Ecological Park is located in the Sierra Madre Occidental, southwest of the state of Durango (Figure 1), 57 km from the city of the same name, on Federal Highway No. 40 in its Durango-El Salto section; it comprises 894.57 ha.

Figure 1 Locatión of El Tecuán Ecological Park. 

The type of climate according to the Köppen classification modified by ^{García (2001)}, is temperate with summer rains (Cw). The average annual temperature is 13.9 °C. Is the prevailing soil type in the area (^{INEGI, 2016}).

Vegetation sampling

An inventory was carried out with the support of the Forest Planning System for Temperate Forests (SiPlaFor; http://siplafor.cnf.gob.mx), designed to support the decision-making process in the preparation and execution of forest management programs sustainability in the temperate forests of Mexico. The Park's road surface was generated by means of a multi-resolution segmentation with homogeneous objects in a satellite image of the Sentinel 2A sensor (USGS - https://glovis.usgs.gov - ID: 13QDG) from the year 2017, to then design and define the sample size over the sub-stands or strata.

A stratified systematic sampling was used with 168 circular sites of 1 000 m², reaching a 3.28 % sampling intensity of the area classified as protected forest, which covers 511.16 ha. The rest of the Park area (383.41 ha) was excluded from the sampling as it is classified as grasslands, roads, bodies of water and buildings (cabins) (Figure 2). At each sampling site, for all individuals with a diameter at breast height (DBH, 1.30 m) greater than or equal to 7.5 cm, the scientific name, the diameter at breast height (cm) measured with a Mantax Haglöf Blue caliper were recorded, and the total height (m) measured with a IV Vertex Haglöf hypsometer.

Figure 2 Stand area calculation and sampling sites of El Tecuán Ecological Park. 

Results and Discussion

In total, 354 trees per hectare belonging to 18 species, four families (Ericaceae, Cupressaceae, Pinaceae and Fagaceae) and four genera (Arbutus, Juniperus, Pinus and Quercus) were recorded. The genus Pinus had the highest richness with nine species, and in contrast, Arbutus xalapensis Kunth was the only recorded species of the genus. Table 1 lists the species recorded at the sampling sites.

The estimated species diversity with the Shannon index was 1.89, which suggests that species diversity in the study area is medium to high. This index is higher than that of ^{Méndez et al. (2018)} who observed a richness of five species and an index of 0.78 in a similar forest in the Sierra Madre del Sur. Likewise, ^{Graciano-Ávila et al. (2017)} recorded a Shannon index of 1.81 and a richness of 13 species in a mixed pine-oak forest under management for the El Salto region in the state of Durango. The greater species richness and the similarity between the diversity value of this study and the value reported by ^{Graciano-Ávila et al. (2017)} could be attributed, among other reasons, to the fact that the park's forest, despite being located in an area with less forest potential, is protected from timber harvesting, extensive livestock farming and different anthropogenic activities that could affect the diversity of species.

Figure 3 shows the distribution of the number of trees per hectare and by category of diameter and height. It is observed that in both variables the distributions fit an inverted-J or negative exponential type. This type is frequently observed in forests under an irregular management scheme, as is the case of the mixed and irregular forests of the Sierra Madre Occidental (^{Návar-Cháidez y González-Elizondo et al., 2009}). Likewise, they have been described in many forests in protected status throughout the world. For this reason, it is considered as the structure that represents the ideal number of trees per diameter class for a given basal area, and, therefore, it is used as a reference when developing forest management methods with selective felling or continuous cutting cover (^{Buongiorno and Michie, 1980}).

Figure 3 Distributions of diameter (top) and height (bottom) per hectare of trees studied in El Tecuán Ecological Park. 

The greatest number of trees accumulates in the diameter classes of 10 and 15 cm (80 and 76 trees ha^-1, respectively) and the number of individuals decreases as the diameter size increases. The foregoing results mainly from natural mortality events in this type of individuals, since commercial exploitation is not allowed in the Park.

These results agree with those of ^{Hernández-Salas et al. (2013)}, who evaluated the composition and diversity of species in temperate forests under exploitation in the state of Chihuahua, in a similar way, they recorded the highest densities for the smaller diameter classes, a finding attributable to the effect of timber use of trees in stages of maturity.

According to the results in Table 2, Pinus cooperi C. E. Blanco was the species with the highest density (32.15 % and 112 trees ha^-1) and relative dominance (37.77 % and 6.15 m² ha^-1). This species has been recorded as the most abundant in studies of temperate climate forests in the state of Durango (^{Návar-Cháidez and González-Elizondo, 2009}; ^{Valenzuela and Granados, 2009}; ^{Graciano-Ávila et al., 2017}). The genus with the highest density and dominance in the Park was Pinus with a density of 204 trees per hectare (57.60 % relative density) and a basal area of 11.33 m² ha^-1 (69.56 % relative dominance). This result coincides with previous studies in which the Pinus genus has been recorded as the most abundant and dominant in temperate forests of the country (^{Hernández-Salas et al., 2013}; ^{Delgado et al., 2016}; ^{Humano, 2020}).

In spite of the high density of the Pinus genus, Quercus sideroxyla Bonpl. was the most frequent species (18 % of the relative frequency), since it was recorded in 123 of the 168 sampling units, while Pinus maximinoi H. E. Moore and Quercus eduardii Trel. were the least frequent with relative frequency values of 0.15 % (Table 2). This result agrees with that obtained by ^{García et al. (2019)} who analyzed the diversity and vertical structure of unmanaged coniferous forests in Chihuahua, Mexico, also reporting the highest densities and frequencies for Pinus and Q. sideroxyla.

Pinus cooperi recorded the highest ecological importance value (28.23 %), followed by Q. sideroxyla (21.68 %). On the contrary, the species with the lowest importance values were P. maximinoi and Q. eduardii with 0.07 % and 0.06 %, respectively (Table 2 and Figure 4). The Pinus genus stood out for the highest VIE with 57.73 %, followed by the Quercus genus with 32.29 %, Arbutus (6.22 %) and Juniperus (3.76 %). These results coincide with those obtained by ^{Hernández-Salas et al. (2013)}, who referred to two Pinus species with the highest values of ecological importance, followed by Quercus species, in a study carried out in the El Largo y Anexos ejido, Madera municipality, in Chihuahua State, Mexico.

Figure 4 Estimation of the abundance, dominance, frequency and value of ecological importance for the tree species recorded in El Tecuán Ecological Park. 

Conclusions

In El Tecuán Ecological Park, the Pinus genus predominates (with nine species), followed by Quercus (with seven species). The diversity of tree species described through the Shannon index is considered to be between medium and high. Pinus cooperi is the species with the highest density, relative dominance (AB) and ecological importance values. The results obtained from this study can be useful as a baseline to assess possible changes in the diversity and tree structure of the Park over time, and by having its forest in protected status, they also represent scientifically based information to develop key indicators of sustainability for the conservation of timber species in other forests subject to logging.