Introduction

The xerophilous scrub is the best represented type of vegetation in northern Mexico (^{Rzedowski, 2006}). In fact, in some entities it occupies the largest vegetation cover, such as the state of Nuevo León, where the community occupies 54.6 % of its total surface (^{Palacio et al., 2000}). This plant formation is represented by others such as submontane scrubland, Tamaulipan thornscrubland, microphyllous desert scrubland and rosette desert scrubland (^{González, 2004}).

The southern region of Nuevo León comprises an area of 1 056 046 ha on which the vegetation of arid zones grows (^{UFNL, 2012}). It displays a diversity of ecosystems whithin which the micropylle desert scrub highlights, as a source of habitat and refuge for numerous animal species and for the potential of provision of timber and non-timber products that it represents.

The desert scrubland brings together communities where the plants that print the physiognomic character of the vegetation correspond to small-leaf shrub elements, which almost always include Larrea tridentata (Sessé & Moc. ex DC.) Coville and Flourensia cernua DC. (^{Rzedowski, 2006}). These groups occupy most of the arid regions of Mexico. However, at present, those in the south of Nuevo León State have a clear deterioration, mainly due to the change in land use, in particular Galeana municipality with 15 578.94 ha (^{UFNL, 2012}).

In spite of the environmental impact that has been exerted, little attention has been paid to know the negative effects on the flora, much less to recover the forest areas that have been lost or degraded (^{Conafor, 2009}).

The characterization of the vegetation is important because the distribution of the species is not similar and the appearance of a species can be null or abundant, due to its limitations in front of some edaphic or climatic factor. In this way, the information generated can be a means to make projections for conservation or regeneration in small areas or fragments of scrub vegetation (^{Rosenzweigh, 1995}).

Based on the above, the objectives of the study described below consisted of an analysis on the floristic composition and characterization of the microphyllous desert scrubland of an area of Galeana municipality. Their results will contribute to the knowledge of the community structure for proper management of plant resources and the establishment of conservation projects to counteract the harmful effects of soil degradation processes.

Materials and Methods

Field work was made in an area of 1 988 ha of the microphyllous desert scrubland of the Northeast of Mexico, to an altitude of 1 880 m in the Galeana municipality. Two sampling sites were selected: the first in Llano La Soledad between 24°52’32” N and 100°43’33” W and the second at La Hediondilla between 24°58’45” N and 100° 41’49” W (Figure 1). The local climate corresponds to the dry or temperate type BSok(x’), subtype of the driest with a P / T ratio lower than 22.9; Average annual temperature between 12 °C and 18 °C, average temperature of the coldest month between 3 °C and 18 °C, average temperature of the hottest month less than 22 °C; percentage of winter precipitation with respect to the annual total, greater than 18 %. The dominant soils are of the Haplic xerosol type and rich in calcium that support a vegetation resistant to high concentrations of gypsum (^{Estrada-Castillón et al., 2010}).

Figure 1 Location of the sampling sites. 

The study was carried out from March to December 2015. Sixteen 1 024 m² sampling units each were set up and displayed in a divided plot in a randomized block design, where the large plots accounted for the two sampling sites, La Hediondilla and Llano La Soledad, and the small ones to the 16 sampling units (8 at each site). The design of the latter was 32 x 32 m square and separated from each other by a distance of approximately 1 000 m (^{Bautista and Palacio, 2011}).

From the 8 sampling units per site, four were established in areas with disturbance and the other four in the absence of disturbance. The criteria used for locating units with or without disturbance were based on the information provided by the ejido commissariat and the field trips on the use they have for agricultural activities and livestock or their exclusion.

For the characterization of the vegetation present in the sampling sites, the identity of the species and their synonyms were verified through bibliographic information of ^{Brummitt and Powell (1992)} and ^{Correll and Johnston (1970)}, field trips and the collection of botanical specimens of all species, which were identified and deposited in the herbarium of the Facultad de Ciencias Biológicas de la Universidad Autónoma de Nuevo León (UANL)

For each sampling unit, al plant forms (trees, shrubs and herbaceous) were measured and each individual were measured with a diameter tape (Forestry Suppliers Mod. 283D / 10M), height and crown area were measured by the prohected space of the crown in north-south and east-west directions.

The richness of species and the floristic diversity of each sampling unit were determined by the quantification of different indexes such as Shannon-Weiner, Simpson, Margalef and Evenness, based on the recommendations of ^{Müeller-Dombois (1974)}, ^{Franco (1989)}, ^{Rosenzweigh (1995)} and ^{Mora (2014)}.

For each species the folowing structural variables were calculated: abundance, dominance, frequency and importance value, applying the following equations:

The Margalef index (DMg) and the Shannon-Wiener index (H’) (^{Magurran, 2004}) were used to estimate the α diversity of the communities using the equations:

Evenness or the proportion that are kept by the relative abundances of the species of the community was determined by the Shannon-Wiener (E) Evenness Index, according to ^{Magurran (2004)}.

The obtained data were submitted to an analysis of hierarchical clusters or cluster, in order to achieve the maximum homogeneity or to identify the difference between the sampling units.

The Analysis of Variance showed the difference between the sampling units, through the support of the statistical package SPSS 20.0 version (^{SPSS, 2011}).

Results and Discussion

From the 30 species of trees, shrubs and herbs recorded in the 16 sampling units (Table 1), 3 875 individuals were assessed, which are grouped in 16 families, from which Asteraceae (9) and Cactaceae (6) are the most important and in the rest of the families, only 1 to 2 species were found (Figure 2). Unit 13 had the smallest number of species (1), while Unit 12 the highest with 35 especies, and both belong to La Hediondilla; these data confirm that the number of species is related to the degree of disturb of the area (Table 2).

Table 1 List of species identified in the microphyllous desert scrubland. 

Figure 2 Number of genera registered per family. 

Table 2 Richness and diversity indexes per sampling unit in the study sites. 

Abundance. In regard to the absolute abundance of each one of the elements compared to the rest of the species, two were the most important: Flourencia cernua (Asteraceae) with 1 133 individuals, which was the most abundant, followed by Larrea tridentata (Zygophyllaceae) with 824 (Table 3); the former agrees with the results of ^{González et al. (2013)} in which 86 % of the total cover came from Larrea tridentata (48 %) and Flourensia cernua (38 %).

Table 3 Structural description and vegetation diversity of the microphyllous desert scrubland 

Dominance. The taxa with the greatest dominance were: Cylindropuntia leptocaulis with 13.85 % of the coverage of the study area, followed by Koeberlinia spinosa with 12.44 %, while the opposite was verified with Machaeranthera pinnatifida with 0.02 % which is a very low value; the evaluated area has a cover of 1 497.36 m² ha^-1, which means that 85 % of the area lacks vegetation and represents values lower than those of other investigations in the same place of study. ^{González et al. (2013)} recorded 3 325 m² ha^-1, which means that there is a loss of species that contribute most to the productivity of this ecosystem (^{Arriaga, 2009}).

Frequency. The species of outstanding frequency were Flourencia cernua, Larrea tridentata, Koeberlinia spinosa, Bouteloua simplex and Tiquilia canescens, as they were present in the 16 sampling units; the opposite occurred with Berberis trifoliolata, Mamillaria heyderi and Yucca filifera, which were only recorded in one of the sites (Table 3).

Importance Value Index (IVI). The main ecological weight was calculated in the shrub stratus for Flourencia cernua (15.610 %), followed by Larrea tridentata (12.21 %), Koeberlinia spinosa (6.15 %) and herbaceous by Tiquilia canescens (6.07 %); similar values have been reported by ^{Mata et al. (2014)} and ^{Ugalde et al. (2008)}. The species with lower IVI are Mammillaria heyderi (0.28 %) and Gaura coccinea (0.49 %) (Table 3).

Margalef Index. The richness value (DMg) ranges from 2.95 to 5.85 for the sampling units under disturbance, while for those without it, ranges are from 1.72 to 3.84, which means that there is a significant difference between the species richness of the sites, with or without disturb (F = 4.44, P = 0.02) (Table 4).

Table 4 Results of the Margalef Index ANOVA. 

Shannon-Wiener Index. Sampling unit 12 has the highest value H’= 2.67, which may be due to the fact that it has a better combination of characteristics of density, frequency and coverage (^{Medina, 2015}). On average, the those of Llano La Soledad have a value of H’= 1.98 while in La Hediondilla it is 2.14, which shows that there is no difference between the specific biodiversity of the sampling units of both sites (Table 5). Some studies performed in the microphyllous desert scrubland report similar values (^{Estrada-Castillón et al. 2010}; ^{González et al., 2013}; ^{Mora et al., 2013}).

Table 5 Results of the Shannon Index ANOVA. 

Simpson Index. A calculation of D = 0.12 to D = 0.43 was made with this index, which is not always higher when the highest number of species is recorded, due to the absence of a dominant species. In this case, sampling units 6 and 13 show that, despite having the highest value in terms of dominance, it has a low species richness, while the sampling unit 12 has the lowest dominance and counts with the greatest number of species (Table 6).

Table 6 Results of the Simpson Index ANOVA. 

Evenness. Plant species present in the sampling units with and without disturbance were compared, from which it was verified that there is a similarity between them (Table 7).

Table 7 Results of the Evenness ANOVA. 

Conclusions

This research study research provides information on the floristic composition and characterization of the microphyllous desert scrubland that serves as the basis for the restriction of the use of some species, for studies of the use and change of the soil, for an adequate management of the vegetal resources and the establishment of proposals of conservation and/ or regeneration in small areas or fragments of vegetation of the most representative ecosystem of the arid zones, the microphyllous desert scrubland of Northeast of Mexico.

The number of individuals was higher in Llano La Soledad (2 632), however, sampling sites had a similar diversity. The species with the highest value index were Flourencia cernua, Larrea tridentata and Condalia wanockii, which suggests good ecological behavior in the study community and reflects the efficiency in the use of water, light, space and nutrients resources, a situation that ensures their survival.

The area evaluated has a high species richness that is related to the degree of disturbance that occurs in the sampling units caused mainly by overgrazing. There is a small number of abundant species and a large proportion of low abundance species with a clear dominance of three taxa of the shrub and herbaceous strata