Study area

The study area is located in the arid region of Querétaro State and its surrounding mountain system that, as a whole, is known as Sierra Gorda. This zone is part of the physiographic province of the Sierra Madre Oriental, at an altitude of 2 000 to 2 900 m. The semi-warm semi- humid weather prevails, which includes Arroyo Seco, Jalpan de Serra, Pinal de Amoles and Landa de Matamoros Municipalities; and to the southwest, the weather is dry and semi-warm with summer rains (^{Conagua, 2002}).

Field work was performed in the following 14 locations of pinyon pine forests: Cadereyta 1 and 2, Los Juárez, Maguey Verde 1 and 2, Pinal de Amoles, Puerto Hondo, Río Blanco 1 and 2, San Javier, San Joaquín, Sombrerete, Villitas and Vizarrón (Figure 1).

Figure 1 Study area. 

Floristic composition

Seasonal samplings were performed from June 2013 to June 2014, as well as 2 000 m long walk rounds by location were made and, thus, annual and perennial herbs, shrubs and trees were collected and identified. The specimens that were not identified in situ were collected and processed for that purpose in the herbarium of the División de Ciencias Forestales and of the Preparatoria Agrícola of the Universidad Autónoma Chapingo.

Description of vegetation

The structural analysis was based on the physiognomical characterization of each location, in which 12 points selected at random were taken at a distance of 20 m at least between them. In each point was applied the central quadrant point technique (^{Cottam and Curtis, 1956}), in which species with a tree life form with a BHD over 10 cm were taken as a reference; in each site two parallel perpendicular lines that form quadrants were drawn, and in each of them the distance to the closest tree to the central point were measured, and the name of the species, height and BHD were recorded.

This variables allowed to make an estimation of the frequency, dominance and density values of the more representative species in each location and were calculated as follows:

The mean basal area is the average of the basal area of all the members of each species; it was calculated with the following formula:

Density was defined as the total number of individuals of all species in 100 m2 and was determined as follows:

Mean distance is the average of all de distances among the individuals of the species recorded in regard to the central point.

The relative values of frequency, density and dominance were combines in the Relative Importance Value (VIR, for its Spanish acronym) based upon de formula by ^{Müeller-Dombois and Ellenberg (1974)}:

With the Relative Importance Values of the species for each locality, the graphical representation of the vertical and horizontal structure of communities through physiognomic type semi-realistic dimensional profiles was performed according to the method of ^{Richards (1952)}. Schemes called danserograms were also developed, which are aimed to facilitate understanding of the organization of plant communities, their classification and selection of methods for their systematic study (^{Granados and Tapia, 1990}). The symbols used in these graphics to describe the most representative characteristics of each species are illustrated in Figure 2.

Figure 2 Symbols for the structural description of the community proposed by Dansereau, 1957 (Adapted by ^{Valenzuela and Granados, 2009}). 

Classification and Ordination

About the environmental variables, in each site the exposition, slope and altitude of each community were noted. In the case of the edaphic variables, it was necessary to take a 1 kg of soil samples from each site with pinyon pine forest for their subsequent analysis at the Laboratorio Central Universitario of the Departamento de Suelos of the Universidad Autónoma Chapingo, where the following was determined:

Vegetation was classified by a cluster or grouping analysis, from which a general area dendrogram came, with binary properties (presence-absence) of the species present at each one of the localities. The Bray-Curtis similitude index was used a distance measure and as a link algorithm of the average among groups at a cut level of 0.5. The analysis were made with the calculation PAST 3.01 program (^{Hamer et al., 2001})

Later, a Canonical Correspondence Analysis (ACC, for its acronym in Spanish), with a direct ordination, in which the species composition is directly related with the environmental variable gradient (^{Sánchez and López, 2003}). Species, sites and the environmental variables taken at each locality were the components correlated with ACC. The calculation program that was used was CANOCO (^{Ter Braak and Smilauer, 1998}).

Floristic composition

A total of 78 species of vascular plants in the pinyon pine forests of Querétaro State were identified, which belong to 63 genera and 39 families (Table 1).

Table 1 Floristic composition of the pinyon pine forests of Querétaro State. 

Cactaceae followed by Asteraceae were the families with a greater number of species (Table 2).

Table 2 Families with greater number of species associated to pinyon forests. 

Classification

From the composition of species in each site, a dendrogram with the cluster analysis was obtained; with a cutoff level of 4.8, four different groups corresponding to four different floristic associations (Figure 3) were recorded; the Pinus cembroides forest -Juniperus flaccida Schltdl., dominated at the canopy by the Pinus cembroides forest, sometimes accompanied by Pinus pinceana, Pinus cembroides-Pinus cembroides var. bicolor forest and forest dominated by Juniperus flaccida accompanied by Pinus cembroides.

Figure 3 Floristic similitude of the 14 sites determined by the Bray-Curtis coefficient. 

The structure and general physiognomy of each one of the vegetal associations is described as follows.

Pinus cembroides-Juniperus flaccida forest. This vegetal association is dominated at the canopy by Pinus cembroides and Juniperus flaccida, of which the first species is the most common, most abundant and dominant by basal area; includes few companions arboreal individuals, such as Rhus standleyi F. A. Barkley and Acacia schaffneri (S. Watson) F. J. Herm. (Figure 4).

Figure 4 Danserogram and physiognomical profile of the Pinus cembroides Zucc.-Juniperus flaccida Schltdl. forest. 1. Pinus cembroides Zucc., 2. Juniperus flaccida Schltdl., 7. Agave salmiana Otto ex Salm-Dyck, 20. Rhus standleyi F. A. Barkley, 22. Opuntia tomentosa Salm-Dyck, 24. Dodonaea viscosa (L.) Jacq. 

The vertical and horizontal structure may be observed in the physiognomical profile and the danserogram of Figures 4 and ⁶; the upper stratum is made up by P. cembroides and J. flaccida, with a height of 7 to 1 m and an open cover; the middle stratum, from 3 to 6 m high, gathers, mainly, individuals of Acacia schaffneri and young pinyon pine trees. The shrub stratum is made up by Agave salmiana Otto ex Salm-Dyck, Echinocereus pentalophus (DC.) Lem., Mammilaria uncinata Zucc. ex Pfeiff., Croton incanus Kunth, Dodonaea viscosa (L.) Jacq., while in the herb stratus, can be found Senna mexicana (Jacq.) H. S. Irwin & Barneby, Krameria ixine L., Condalia mexicana Schltdl., Amelanchier denticulata (Kunth) K. Koch and Castilleja tenuifolia Benth., as the most important species.

This association was recorded in three localities, Río Blanco 1 y 2 and Puerto Hondo, all with a semidry warm climate and soils of the Calcareous castañozem type. The prevailing exposition of these sites is W, with average slopes of 35 %. The least altitude in all forests for these associations is 2 01 - 2 071 m, which favors the high representation of the xerophyllic elements.

Pinus cembroides and P. cembroides-Pinus pinceana forests. This association is characterized by the total dominance of Pinus cembroides in the tree layer or even codominant with Pinus pinceana, but in both cases the floristic composition has great similarity, and that is why they form a single floristic group. Some of the accompanying tree species in these communities are J. flaccida, J. deppeana Steud. and R. standleyi.

Eight localities share similarity in species composition: San Joaquín, Cadereyta 1 and 2, Sombrerete, San Javier, where P. cembroides is dominant and Vizarrón, Maguey Verde 1 and 2 in codominance with Pinus pinceana. The latter sites have steep slopes (~ 42 %), with NW and W expositions. The altitudinal range of these communities is 2 070-2 820 m, with a predominant temperate subhumid climate.

P. cembroides and P. pinceana dominate the canopy in the upper layer with a height of 8 to 1 m and the lower layer of 4 to 7 m is represented mainly by Juniperus flaccida and J. deppeana (Figure 5).

Figure 5 Danserogram and physiognomical profile of the Pinus cemborides Zucc. forest. 1. Pinus cembroides Zucc., 2. Juniperus flaccida Schltdl., 14. Cylindropuntia imbricata (Haw.) F. M. Kunth, 19. Acacia schaffneri (S. Watson) F. J. Herm., 26. Fouquieria splendens Engelm., 27. Euphorbia antisyphilitica Zucc., 28. Coryphanta erecta (Lem. ex Pfeiff.) Lem., 29. Agave gentryi B. Ullrich, 30. Opuntia cantabrigiensis Lynch. 

The herbaceous layer is made up by Castilleja tenuifolia, Stevia salicifolia Cav., Geranium latum Small, Melampodium sp., Dalea lutea (Cav.) Willd., Echinocactus grusonii Hilm. and Opuntia rastrera F.A.C. Weber; the associated shrubby flora consists mainly of Lantana velutina M. Martens & Galeotti, Dodonea viscosa, Amelanchier denticulata, Tecoma stans (L.) Juss. ex Kunth, Brongniartia intermedia Moric. and Salvia ballotiflora Benth. In addition, epiphytic Tillandsia recurvata and T. usneoides are plentiful.

Pinus cembroides - P. cembroides var. bicolor forest. Los Juárez and Villitas have this kind of vegetal association, both in which the taxa with highest RIV were P. cembroides and P. cembroides var. bicolor, as they are the most frequent species with the highest density; however, the first one has the greatest average basal area (Figure 6).

Figure 6 Danserogram and physiognomical semi-realistic profile of the proper vegetation of group 3. 1. Pinus cembroides Zucc., 2. Juniperus flaccida Schltdl., 4. Bouvardia longiflora, 7. Agave salmiana, 1. Stevia saliciflora, 13. Juniperus deppeana, 24. Dodonaea viscosa, 32. Pinus cemboides var. discolor, 34. Pinguicola acuminata 

The highest level of the canopy is rather low and is 7.5 m tall and in it the co-dominant species coexist with Juniperus flaccida and Juniperus deppeana. The shrub stratum is made up of Gnaphalium sp., Lantana velutina, Dodonea viscosa, Bouvardia ternifolia, Agave salmiana, Dasylirion acrotrichum (Schiede) Zucc. and Pinguicula acuminata Benth.

This ecological group develops in the higher parts in record, from 2 850 to 2 890 masl with a mean slope of 40 %, a S exposition and a sub-humid mild-weather, mainly.

Juniperus flaccida - Pinus cembroides forest. The last association in record is dominated at the canopy by Juniperus flaccida and Pinus cembroides; the first one being a more frequent taxon, with greatest density and basal area, that is, the most important species for the ecosystem. Such association was found just in one location, Pinal de Amoles, and was the richest community in species composition of the tree stratum, with related species such as Quercus germana Schltdl. & Cham., J. deppeana, P. pinceana and Q. mexicana Bonpl.

The tree stratum gets up to 1 m high, in which P. cembroides is the most outstanding species. The shrub layer includes Baccharis conferta Kunth, Bouvardia ternifolia, Baccharis salicifolia (Ruiz & Pav.) Pers., Fuchia thymifolia Kunth, Scenecio albonervius Greenm., Symphoricarpos microphyllus HBK and Agave salamiana. The most relevant herbs that were identified were Thalictrum fendleri Engelm. ex A. Gray, Arenaria lanuginosa (Michx.) Rohrb. and Vicia ludoviciana Nutt. (Figure 7).

Figure 7 Danserogram and physiognomical semi-realistic profile of the vegetation of the site in Pinal de Amoles. . Pinus cembroides, 2. Juniperus flaccida Schltdl, 8. Pinus pinceana Gordon, 15. Quercus germana Schltdl. & Cham., 16. Quercus mexicana Bonpl., 17. Senecio sp., 18. Baccharis conferta Kunth. 

This locality shows a greater affinity with the species of mild-weather forest, which become evident in its high tree diversity; it grows in steep slopes (73 %), at 2 427 masl with a prevailing side slope towards NW and a mild-subhumid weather with summer rains.

Ordination

Some of the most relevant physical factors for the establishment of the communities were used to manage the pinyon pine forests from a canonical correspondence analysis, which detects the correlation between the environmental variables and the composition of each of the locations. The statistical values for the three first ordination axis are shown in Table 2, which suggests that the environmental factors that were analyzed are correlated with the flora associated to the pinyon forests of Querétaro and that these three axes may explain 60 % of the variation in the localities.

Table 2 Results of the for the first three ordination axes. 

In regard to the most relevant environmental variables for the ordination of data (Table 3) it may be noted that as altitude becomes higher, pH descends while iron increases; on the other hand, the increment in the amount of organic matter means the same for the potassium and calcium concentrations, both essential macronutrients for the nourishment of plants.

Table 3 Correlation matrix between the environmental variables that were assessed in the actual study.  

The correlations with significant values are in bold.

The most important variables that define the differential establishment of species in the pinyon pine communities of Querétaro are the organic matter, potassium, calcium, altitude and slope (Table 3).

The behavior of the locations, and therefore, of the vegetal associations, according to the environmental variables, may be observed in Figure 8, in which three clusters can be seen, in one of which two different associations are together: Pinus cembroides and P. cembroides and P. cembroides var. bicolor.

Figure 8 Canonical correspondence analysis of 14 sampling sites with 78 plant species and 12 environmental variables. 

Table 4 Correlation matrix between the first three axes and the 12 environmental variables. 

The correlations with significant values are in bold.