Correspondence analysis

Indirect ordering or correspondence analysis (AC) of the species in the six sampled transects is presented in a dendrogram (Figure 2). Three plant associations are generally appreciated, choosing 41% of the information (mean) as the cut level. The first association is of MDM (transects 1 and 2), located in the valley between the SC and Sierra Baja, at 1 600 meters above sea level north of the community

Figure 2 Dendrogram showing hierarchical relationships between sampled transects. 

La Borrega. The second type of vegetation (transects 5 and 6) is the BE, located at the foot of the SC mountain, at 1 770 meters above sea level, in the community of San Cristobal. The third group (transects 3 and 4) is the MDR, distributed in sierra baja with lomerio, at 1910 masl NE of the El Tecolote community.

In AC the multidimensional space is reduced, preserving the distances between the objects (vegetation samples), making it possible to determine the correspondence between the columns (sampling transects) and the rows (species) of the data matrix (^{de la Cruz, 1991}). Table 1 presents the proximity or correspondence values between the species and sampling transects for the first management axes.

In Figure 3 it is possible to visualize that sampling transects 1 and 2 are located very close to each other, this means that the transects share a large number of species, similarly, sampling transects 3 and 4 have a close proximity, thus like transects 5 and 6.

Figure 3 Indirect management diagram, location of sampling transects in the management space. 

Microphile desert scrub, La Borrega community

The MDM of this site is located 10 km W of SC in the western sierras and plains subprovince, in an endorheic drainage valley, made up of haulage material, mainly gravel, clay and sand. It is located at the coordinates 23° 43’ 52.5” north latitude and 101° 04’ 46.0” west longitude, at 1 600 masl, it has a PMA of 311 mm, the climate is arid-temperate (BSokw), with warm summer, TMA from 12 to 18 °C, that of the coldest month between -3 and 18 °C and that of the warmest month >18 C, summer rains, from 5 to 10% per year.

Soils have the following characteristics: pH (7.78), N (3.8 mg kg^-1), P (22.81 mg kg^-1), K (490 mg kg^-1), Ca (6 721 mg kg^-1), Fe (2.37 mg kg^-1). The soil is sandy loam (54.4% sand, 29.3% silt and 16.4% clay). The content of organic matter and the texture of the soil obtained coincide with the results of ^{CONANP (2012)}, in which it is indicated that this plain (with xerosol soils) is characterized by presenting a superficial layer of clear material, due to the low organic matter content.

In an evaluation along a permanent transect (three years), that crosses an endorheic basin in the N portion of the DC, in south-central New Mexico, it was determined that the shrub layer dominated by Larrea tridentata D. Burckhardt, it extended in the xeric portion, very low in nitrogen; perennial grasses with C4 photosynthetic path, shrubs with C3 path and herbaceous plants were co-dominant at intermediate sites along the gradient (^{Cornelius, 1991}).

This scrub is characterized by the dominance of small-leaved shrub species, crass and ephemeral plants that develop along alluvial fans, plains, slopes, valleys and soft hills, its main distribution being valleys, where the soils are textured fine, deep to relatively deep and have a layer of rocks (^{Granados, 2012}).

In this scrub 28 species distributed in 12 families were identified, of which Cactaceae and Asteraceae were the most representative, with 9 and 6 species, respectively (Table 1) of the supplementary material. Six species are endemic, one is threatened and two are under special protection.

A richness of cacti of 10 species was found, Cylindropuntia imbricata (Haw.) F. M. Knuth, Cylindropuntia leptocaulis J. Jones, Echinocactus horizonthalonius J. Daniel, Echinocactus platyacanthus R. F. del Castillo, Echinocereus enneacanthus D. Sanchez, Echinocereus pectinatus N. P. Taylor, Escobaria dasyacantha D. R. Hunt, Ferocactus hamatacanthus J. Y. Clark, F. latispinus J. H. Cota, F. pilosus N. P. Taylor, F. stainesii R. S. Wallace, Lophophora williamsii E. F. Anderson, Opuntia rastrera J. A. Reyes-Agüero and Opuntia tunicata A. C. Gibson.

The analysis of relative importance value (VIR) is presented in Table 2, where the species with the highest VIR are seen: L. tridentata, followed by C. leptocaulis, Jatropha dioica B. Dehgan, O. tunicata, F. cernua M. O. Dillon and E. enneacanthus. A photo of the community and a semi-realistic physiognomic profile are shown in Figure 4a and 4b).

Figure 4 a) species of the MDM in the La Borrega community; b) semi-realistic profile; c) Mezquitales of the municipality of Catorce; d) semi-realistic profile, San Cristobal community; e) MDR, El Tecolote community, municipality of Catorce; and f) Semi-realistic profile of the MDR. 

The results obtained are analogous to the description made in the analysis of the vascular flora, carried out in the SC and adjacent territories (^{González-Costilla, 2007}), in which this area is described as a formation dominated by the species Larrea tridentata, Parthenium incanum G. K. Brizicky, Zinnia acerosa A. M. Torres, Flourensia cernua, Opuntia imbricata, Opuntia leptocaulis, Prosopis laevigata M. C. Johnston and Fouquieria splendens M. Baker. According to ^{Marroquin (1964)}, the dominant layer in this type of scrub is the shrub layer and frequently includes L. tridentata and F. cernua.

Thorny forest, San Cristobal community

The BE is located on the edge of the Sierra Madre Oriental and Mesa del Centro physiographic provinces. Located at the foot of the scrubland, 6 km west of the SC, in valleys and intermountain plains with endoreic drainage, made up of structures of forced accumulation. This is where the so-called sandy substrate beach usually occurs at 1770 meters above sea level. The TMA is 16.6 °C, with a maximum of 37 °C and a minimum of -7 °C. The PMA is 311 mm.

The climate is considered temperate-dry (BS₀ hx’), the driest of the BS, semi-warm with intermediate rainfall, cool winter and winter precipitation greater than 18% (^{SEDUVOP, 2006}). This area is classified as a fertile surface with agricultural potential, with the availability of underground water, whose salinity is high and low in sodium. However, it is also classified as an open mesquite site with a high degree of erosion, due to the action of the wind, and sandy soil. It is located on the northern limit of the ANP, so its agricultural expansion compromises the conservation of the site.

Soil have the following characteristics: pH (7.31), N (26.3 mg kg^-1), P (31.9 mg kg^-1), K (2240 mg kg^-1), Ca (6206 mg kg^-1), Fe (3.4 mg kg^-1). The soil is loamy in texture (46.4% sand, 39.3% silt and 14.4% clay). The high content of P, K, and OM in the soil of this site (compared to the MDM) is perhaps due to the presence of Prosopis sp., since studies in arid areas demonstrate that mesquite increases N and organic matter in the soil (^{Herrera-Arreola et al., 2007}).

Authors such as ^{Reyes-Reyes (2003)} report that C mineralization in mesquite leaves is greater than in huizache leaves, also demonstrates that the release of N from the leaves is greater when the organic matter content of the soil is less , organic matter, microorganisms and nutrients accumulate under the mesquite canopy. 22 species from 11 families were identified; Asraceae and Fabaceae were the most representative (Table 2) of the supplementary material.

Two endemic species were also found. In this area, the presence of small-leaved shrub species and ephemeral plants characteristic of the MDM is frequent. However, this site is notable for the presence of shrubby and arboreal phreatophilic species, mainly Prosopis L. and Acacia farnesiana (L.) Willd. Its main distribution is influenced by underground water bodies, which is why its presence in the foothills is abundant, it develops along alluvial fans in which it shares dominance with Lycium berlandieri (T. Fukuda), L. tridentata, F. cernua and Rhus microphylla (J. S. Lassetter).

The richness of cacti at this site is low, only Cylindropuntia leptocaulis, also present in MDM and MDR, was found. Associated species at this site were: Ephedra compact, Bouteloua scorpioides J. T. Columbus, Artemisia filifolia E. D. McArthur, Bidens odorata W. C. Yang, Zinnia acerosa, Artemisia ludoviciana E. D. McArthur, Flourensia cernua, Gymnosperma glutinosum J. Ruffin, Koeberlinia spinosa W. C. Holmes and Larrea tridentata.

The number of species in this area is the lowest of the three sites, similar to the study carried out in southern DC (^{Montaña, 1990}), with the lowest species richness on the sandy substrate beach. In the analysis of relative importance value (VIR) (Table 3), the species with the highest VIR are seen: Lycium berlandieri, followed by Prosopis sp., L. tridentata, F. cernua, R. microphylla and C. leptocaulis.

^{Rzedowski’s (1994)} description of MDM of DC is similar to these results, with the possible appearance of communities of Prosopis sp., Koeberlinia spinosa or Fouquieria splendens, which extend for hundreds of kilometers in DC ^{Granados (2012)} refers to salty shrubs in open mesquite sites, as a widely distributed community present in soils of fine texture and eroded by the action of the wind and sandy soil; both sources report Parthenium incanum, P. argentatum S. Kumar, Viguiera stenoloba E. E. Schilling, Zinnia acerosa, Dyssodia sp. and Jatropha dioica. In Figure 4a and 4d a photo of this type of community and its semi-realistic scheme are shown.

Rosetophile desert scrub, El Tecolote community

The MDR of this site is located 25 km W of the SC, in the subprovince valleys and western plains (23° 42’ 19.2” north latitude and 101° 07’ 28.8” west longitude. It develops at 1 910 masl. The climate it is very arid-temperate (BWkw), TMA from 12° to 18 °C, that of the coldest month between -3° and 18 °C and that of the warmest month > 18 °C, summer rain regime of 5 to 10.2% annually.

The site is located in a low folded mountain range with a hill in the N-S direction of extrusive igneous rock; the soils are leptosols (^{CONANP, 2012}). The soils have the following characteristics: pH (7.30), N (57.8 mg kg^-1), P (58.86 mg kg^-1), K (2 060 mg kg^-1), Ca (7814 mg kg^-1), Fe (26.84 mg kg^-1). The soil is loamy in texture (42.4% sand, 35.3% silt and 22.4% clay). According to ^{CONANP (2012)}, the type of soil at this site is characteristic of geomorphological units of mountainous systems with convex slopes with rounded tops of limestone-sandstone, these surfaces are used for livestock and corn production.

According to the soil taxonomy used by the Department of Agriculture of the United States of America, the lithosols in this area are Entisols or Aridisols, soils with little evidence of development of pedogenic horizons and that, in general, only have an A-horizon ochric. They are found on slopes that undergo strong erosive processes, can withstand different types of plant formations and have a mineral nature

The scrub that develops at this site is characterized by the abundance of individuals of species with thick, narrow and elongated leaves, sometimes thorny like lechuguilla (Agave lechuguilla H. S. Gentry) or sotol (Dasylirion cedrosanum E. E. Castillon) or well-formed but fibrous (Yucca carnerosana A. M. Minnis), arranged in a rosette. Among these individuals there are basically two types of development: 1). Those species that display an elongated stem and manifest how (Yucca sp. and Dasylirion sp.); and 2) those that lack a visible stem and develop basilar leaves, such as (Agave sp. and Hechtia sp.) (^{Granados, 2012}). In the municipality of Catorce, large valleys of hills and low mountain ranges host species such as Agave lechuguilla, Yucca filifera E. E. Castillon, Yucca decipiens M. Ricker, Parthenium argentatum and Dasylirion cedrosanum A. Robles-Esparza.

In the eastern slopes of the Sierra Baja (N-S direction) is frequent development of Opuntia rastrera A. Muñoz-Urias and Opuntia stenopetala A. Garcia-Velazquez, species such as A. lechuguilla, A. striata D. J. Bogler and Hechtia glomerata K. Burt -Utley, they develop in colonies of large masses that delineate the landscape across large areas with the eventual appearance of colonies of Fouquieria splendens E. C. Bate-Smith.

These species develop on the site with mainly shrub and sub-shrub species, the arboreal ones are scarce, there are no winter herbaceous plants, no germination or flowering in winter and most of the species are evergreen. Some of the most abundant species at this site were Agave lechuguilla, Euphorbia antisyphilitica E. H. Muldavin, Zinnia acerosa, Parthenium argentatum and Agave striata H. S. Gentry.

In this community, 61 species distributed in 16 families were identified, of which the Cactaceae, Asparagaceae and Asteraceae families were the most representative (Table 3) of the supplementary material. Twenty endemic species were found, four threatened, six under special protection and three in danger.

The richness of cacti at this site is very high, 30 species, including Cylindropuntia leptocaulis, Cylindropuntia imbricata, Echinocereus stramineus M. Baker, Coryphantha poselgeriana M. A. Baker, Opuntia engelmannii J. G. Jones, O. leucotricha S. Samah, O. stenopetala M. Fuentes-Perez, Ferocactus pilosus H. M. Hernandez and Echinocactus platyacanthus.

The DC region stands out for having the greatest diversity of cacti on the planet, this family is the most threatened in the ecoregion. Throughout DC there are 324 species of cacti, of 39 genera. The Mammillaria, Opuntia, Coryphantha and Echinocereuus genera represent 58% of the total diversity of the region; 17 (43.6%) of the 39 registered cactus genera are restricted to DC. Likewise, a total of 229 species (70%) are endemic to the region (^{Hernández, 1994}).

Of the total species registered in the DC, 826 (67.8%) are of endemic distribution (^{Villarreal, 2017}). With regard to the richness of cacti and endemism, the state of SLP is the most important, due to the fact that the total number of cactus species in 141 (44% of species in the entire DC) and the number of endemic species of the status is 17 (^{Hunt, 1999}). On the other hand, ^{Jimenez (2011)} recognizes SLP as the state with the greatest diversity of species, with 151.

The analysis of relative importance value (VIR) is presented in Table 4, where the species with the highest VIR are seen: Yucca filifera followed by A. lechuguilla, Y. carnerosana, E. platyacanthus, F. pilosus and O. rastrera. Photo and semi-realistic profile of this community are shown in Figure 4e and 4f.

The dominant stratum at the site is shrubby and subbushy, which could influence the chemical composition of the soil. Because compared to the soil of MDM and BE, that of this site with differences in concentration of inorganic N, P, Fe and Ca. As shown by ^{Schlesinger (1998)}, the distribution of N, P and K in the soil it is strongly associated with the presence of shrubs in desert habitats. Shrubs concentrate the biogeochemical cycle of these elements on ‘fertility islands’ under their canopy, while in adjacent sterile spaces they are comparatively devoid of biotic activity as reported by ^{Sánchez-González (2007)}.

Canonical correspondence analysis

In relation to ACC, the characteristic roots for the first two management axes (Table 5) suggest that the composition of species among plant associations is related to environmental factors of the first two axes of variation; in other words, the analysis is satisfactory, since with the data corresponding to the first two axes of variation, the observed distribution can be explained and, possibly, the physicochemical characteristics are sufficient to explain (^{Granados et al., 2013}).

That soil porosity plays a key role in the distribution of vegetation, it was also observed along with altitude, slope and orientation, as the most relevant in its management in the SC. The classification of communities obtained in the AC is confirmed with the ACC diagram (Figure 5), since the associations of the diagrams (Figures 2 and 3) agree on their order.

Figure 5 ACC of the transects in the municipality of Catorce, SLP. 78 species and 10 environmental variables were included. 1 and 2 (MDM), 3 and 4 (MDR) and 5 and 6 (BE). 

Uses of plants

The dominant layer is the shrub and sub-shrub (56%). The main uses of the identified species are medicinal (25%), forage (21%) and food (12%).