Study area

Four pinyon pine forest communities were analyzed in different municipalities of the state of Zacatecas:

San Juan de los Hornillos, municipality of Fresnillo. This community is located between coordinates 23° 36' - 22° 49' N and 102° 29' - 103° 31' W, in the center of the state of Zacatecas. The predominant climates are temperate subhumid with rains in summer (Cw₀) and semi-dry with rains in summer (BS); the average annual precipitation is 457 mm, the average annual temperature is 15 °C, the elevation varies from 2 250 to 2 700 m (^{Medina & Ruiz, 2004}) and the predominant slope is 45°.

Sierra de Órganos, municipality of Sombrerete. The study area is located in Sierra de Órganos National Park in the northwest of the state of Zacatecas. The extreme coordinates are: 23° 44’ 58’’ - 23° 48’ 06.39’’ N and 103° 46’ 37’’ - 103° 48’ 57.93” W. The predominant climate is dry (BS₁ kw) and in the high parts the climate is Cw₀ (^{Medina & Ruiz, 2004}). The average annual temperature is 15 °C, the average annual precipitation is 522 mm (^{Enríquez, Koch, & González, 2003}) and the elevation ranges between 2 120 and 2 650 m.

Concepción del Oro, municipality of Concepción del Oro. The community is located between coordinates 24° 37’ N and 101° 25’ W, in northeastern Zacatecas. The predominant climate is dry or steppe, semi-warm (BS or BW). The average annual precipitation is 420 mm, the average annual precipitation is 17 °C (^{Medina & Ruiz, 2004}) and the elevation ranges between 2 600 and 2 900 m.

Cerro de Piñones, municipality of Juchipila. This site belongs to the Sierras y Valles Zacatecanos physiographic subprovince of the Sierra Madre Occidental province. The extreme coordinates are 21° 20’ - 21° 23’ N and 103° 12’ - 103° 15’ W. The climate is Cw; the average annual temperature is 16 °C and the average annual precipitation is 684 mm (^{Medina & Ruiz, 2004}). The pinyon pine community develops at an elevation of 1 900 to 2 500 m.

Sampling

Multivariate statistical analysis

Cluster analysis was used to classify plant communities into categories or groups, based on the floristic composition of each community. The clustering was hierarchical, polythetic and agglomerative, using the average among groups as a method of union, and the Sørensen index as a measure of similarity (^{Rocha, Chavez, Ramírez, & Cházaro, 2012}). To define the groups formed in the cluster analysis, a standard cut level was chosen in which 50 % of the remaining information was considered (^{McCune & Grace, 2002}). The influence of the edaphic factors (pH, EC, OM, N, P, K, Ca and Mg) in the floristic composition of the communities was determined with a canonical correspondence analysis. Both analyses were carried out with the PC-ORD version 6 calculation program (^{McCune & Mefford, 2011}).

Floristic and structural characterization of vegetation in pinyon pine forests

A total of 243 species of vascular plants were recorded in the four pinyon pine communities in the state of Zacatecas. The species belong to 58 families and 167 genera (Appendix 1). The best represented families in number of species were Asteraceae (49), Poaceae (24), Fabaceae (19), Asparagaceae (15) and Cactaceae (13). The general characteristics of each community are described below:

San Juan de los Hornillos, municipality of Fresnillo. In this community, 60 species of vascular plants were recorded, corresponding to 47 genera and 22 families. The community had a density of 1 187 tree and shrub individuals per hectare. Pinus cembroides Zucc. was the most relevant species since it had a higher density and was more frequent (Table 1). This species is part of the tree stratum, along with Yucca filifera Salm-Dyck and Quercus laeta Liebm. as companion species; the average height of the canopy was 3 m, with few emergent individuals of no more than 8 m (Figure 1). In the shrub stratum, the most relevant species were Dasylirion cedrosanum Trel. and Dodonaea viscosa L. Jaqc. The perimeter region of the pinyon pine forest of San Juan de los Hornillos is used as a tourist recreation and livestock foraging area, so the anthropogenic impact is evident, since there is soil loss, pollution and removal of wild plants, mainly. In areas farther from the edge, trees reach bigger sizes and there are more pinyon pines, due to the fact that human settlements are practically abandoned, owing to insecurity and migration, thus reducing the extraction of resources, including the pine nut.

Sierra de Órganos, municipality of Sombrerete. The arboreal vegetation consists of Acacia schaffneri (S. Watson) F. J. Herm., Buddleja cordata Kunth, Juniperus deppeana Steud., Quercus grisea Liebm., Y. filifera and P. cembroides, the last being the most important for the community (Table 1). These species, together, form an open canopy 6 m high, while the shrub stratum, consisting of species such as Ziziphus obtusifolia (Hook. ex Torr. & Gray) Gray, Arctostaphylos pungens Kunth, Dasylirion wheeleri S. Wats. ex Rothr. and Condalia fasciculata I. M. Johnst, does not exceed 2 m (Figure 1). This site, being a protected natural area, is in a relatively better state of conservation. It has the greatest floristic richness with 82 species of 70 genera and 30 families, and it has the lowest arboreal and shrub density with 197 individuals·ha^-1; in addition, the stems are larger, indicating the forest’s advanced age (^{van der Maarel & Franklin, 2013}).

Concepción del Oro, municipality of Concepción del Oro. The flora includes 70 species, 54 genera and 22 families, and the density of species with arboreal and shrub habitat is 535 individuals·ha^-1. In the arboreal stratum, the plant community presented an association of P. cembroides and P. cembroides var. bicolor Little, the latter with a higher RIV due to its density and basal area. On average, the arboreal stratum composed of Yucca carnerosana (Trel.) McKelvey, Nolina parviflora Kunth and Juniperus pinchotii Sudw is low (5 m, Figure 1); the emergent species is P. cembroides, reaching heights of 10 m. On the other hand, the shrub stratum is poor, D. cedrosanum being the most frequent species (Table 1). In this area another species of pinyon pine is reported: Pinus pinceana Gordon & Glend. (^{Villarreal, Mares, Cornejo, & Capo, 2009}); however, it was not recorded, so it is necessary to expand the sampling area.

Cerro de Piñones, municipality of Juchipila. In this community, 78 species were identified, divided into 69 genera and 38 families, with a density of 1 170 individuals· ha^-1. On the plateaus of the Juchipila mountain ranges, forests are dominated by Pinus maximartinezii Rzed. (Table 1), a species in danger of extinction and of restricted distribution. This pinyon pine has populations of 621 individuals·ha^-1 and is part of the low arboreal stratum, 4 m on average, in association with Quercus eduardii Trel., Quercus resinosa Liebm., Prosopis laevigata (Humb. & Bonpl. ex Willd.) M. C. Johnst., J. pinchotii and Parkinsonia praecox (Ruiz & Pav.) Hawkins. This area has the highest specific richness in the stratum. On the other hand, the most representative species of the shrub stratum are A. pungens, D. viscosa and Jatropha dioica Sessé (Figure 1). The community is under constant disturbance due to cattle ranching and the gathering of pine nuts, despite which it develops in difficult-to-access areas and under the protection of the inhabitants. In the state of Durango, P. maximartinezii is also distributed (^{González, González, Ruacho, & Molina, 2011}); however, populations that have a discontinuous distribution may have marked genetic differences, so it is important and necessary to establish areas for their conservation and increase the survival of juvenile trees (^{González-Elizondo et al., 2011}; ^{Ledig et al., 2001}).

Figure 1 Semi-realistic profile of four pinyon pine sites in the state of Zacatecas: a) San Juan de Hornillos, Fresnillo; b) Sierra de Órganos, Sombrerete; c) Concepción del Oro (dominated by Pinus cembroides var. bicolor); d) Concepción del Oro (dominated by Pinus cembroides); e) Cerro de Piñones, Juchipila. Species: 1. Pinus cembroides, 2. Yucca filifera, 3. Arctostaphylos pungens, 4. Dodonaea viscosa, 5. Agave spp., 6. Nolina parviflora, 7. Jatropha dioica, 8. Opuntia sp., 9. Quercus laeta, 10. Dasylirion cedrosanum, 11. Juniperus deppena, 12. Ziziphus obtusifolia, 13. Vachellia schaffneri, 14. Quercus eduardii, 15. Dasylirion wheeleri, 16. Condalia fasciculata, 17. Quercus grisea, 18. Juniperus pinchotii, 19. P. cembroides var. bicolor, 20. Celtis pallida, 21. Yucca carnerosana, 22. Purshia plicata, 23. Quercus resinosa, 24. Parkinsonia praecox, 25. Prosopis laevigata. 

Classification and ordination of pinyon pine forests

Structurally, in the pinyon pine forests of the state of Zacatecas, three physiognomic variants can be distinguished: Pinus cembroides forests, the P. cembroides - P. cembroides var. bicolor association and the forest dominated by P. maximartinezii; however, floristically, the cluster analysis showed two well-defined groups at a cut-off level with 50 % remaining information (Figure 2).

Figure 2 Hierarchical dendrogram showing the floristic similarities among four pinyon pine communities in the state of Zacatecas. 

According to Figure 2, the forests of P. cembroides and P. cembroides-P. cembroides var. bicolor have greater floristic affinity with each other than with the forest dominated by P. maximartinezii. This floristic difference may be due to the geographic distance between the sites, since the closest communities could present similar environmental conditions in comparison with the more distant ones; that is, the floristic similarity decreases with the increase in distance (^{Tuomisto, Ruokolainen, & Yli-Halla, 2003}), as has been reported in forests dominated by P. pinceana (^{Villarreal et al., 2009}).

Soil composition is a relevant factor for determining structural differences in plant communities (^{Chavoya et al., 2016}; ^{Granados et al., 2015}; ^{Sánchez-González & López-Mata, 2003}; ^{Sardinero, 2000}). The canonical correspondence analysis determined that the predominant edaphic factors in the floristic composition of the pinyon pine forests are pH, EC, K concentration and soil texture, especially the percentage of clay (Table 2; Figure 3).

Figure 3 Ordination diagram based on the canonical correspondence analysis of the sampling sites with respect to the soil factors of the pinyon pine communities analyzed in the state of Zacatecas. 

Table 3 shows the physicochemical properties of the analyzed soils. The forests dominated by P. cembroides (Fresnillo and Sombrerete) develop on soils with slightly acid pH (5), low EC, low concentration of Ca, and rich in K and N. Acid pH values are common in pine forests due to the decomposition process of the organic waste (^{Granados, López, & Hernández, 2007}). On the other hand, communities with codominance of P. cembroides var. bicolor and P. cembroides inhabit sites with neutral pH (7) and higher EC due to the high Ca concentration (^{Osman, 2013}). The plant species found in the Concepción del Oro community have relatively greater resistance to salinity and nutrient-poor soils (^{Gandullo, 2004}). In the case of P. maximartinezii, the forest is located in the zone of greatest precipitation and lowest elevation with respect to the other communities in the state of Zacatecas. The soils are slightly acid (5.9) with loam texture, considered the most propitious for plant growth, since it favors infiltration and good drainage; it also has high fertility and adequate water retention due to its proportion of sand, silt and clay (^{Osman, 2013}; ^{Rodríguez-Fuentes & Rodríguez-Absi, 2002}).

Environmental factors such as temperature and slope of the land were rejected because the four communities had a certain climatic and orographic affinity, and in other regions of the country, with the same type of vegetation, these variables were not considered relevant to explain the assembly of species (^{Chavoya et al., 2016}; ^{Romero et al., 2014}; ^{Romero-Manzanares, Flores-Flores, Luna-Cavazos, & García-Moya, 2016}).