Introduction

Arid and semi-arid ecosystems cover more than 50 % of the surface area of Mexico; they are especially prone to natural or anthropogenic disturbances, such as clearing, the expansion of the agricultural frontier, the extraction of useful species, and overgrazing (^{Flores et al., 2006}; ^{Montaño et al., 2006}). Researchers point out that 50 000 hectares of semi-arid vegetation are modified each year in Mexico, which causes disturbances and deterioration of the resources and values present in those ecosystems (^{Flores et al., 2006}; ^{Montaño et al., 2006}).

Besides the change in the architecture and composition of the vegetal species, the ecosystem has modifications in the biogeochemical cycles, resulting in the loss of the habitat of various animal taxa and microorganisms; increased erosion, reduced fertility, and disturbances in the flow of water and its retention time in the soil (^{Flores et al., 2006}). Three fourths of the territory of the state of Zacatecas (56 463 km2) correspond to arid zones or shrubs; of these, 14 % offer favorable conditions for agriculture; 79 %, for livestock breeding, and 7% are covered with timber and non-timber forests (^{Inafed, 2014}).

The Mimosa genus is distributed through the American continent, from the south of the United States of America (^{USDA, 2014}; ^{TNPD, 2014}) to Argentina; it is also found in Africa, Asia and Oceania (^{Camargo et al., 2001}). In Mexico it is represented by approximately 104 to 10 species (^{Camargo et al., 2001}) present in tropical, subtropical arid and semi-arid regions.

These species are an important component of the xerophile shrubs of the Altiplanicie Mexicana; they occupy large areas of central Mexico, particularly in San Luis Potosí, north-central Jalisco, Aguascalientes, northern Guanajuato and southern Zacatecas; as a dominant species, it is found in patches of up to 8 000 individuals ha-1. It is known as gatuño, garabatillo, garruño, uña de gato (catclaw), etc., according to the area where it grows (^{Pavón et al., 2001}).

Two species -Mimosa dysocarpa Benth. and Mimosa aculeaticarpa Ortega- grow at the Sierra de Órganos National Park (PNSO). The latter species has a broader distribution within the park (^{Enríquez et al., 2003}). Both are considered to be invasive of agricultural lands and pastures; in general, farmers and livestock breeders consider them to be a nuisance and a problem in the plots where they carry out their activities.

However, invasive plants are those introduced species that can produce offspring, often in large numbers and at a considerable distance of their progenitors; therefore, they have the potential to extend across a large territory (^{Richardson et al., 2000}). Furthermore, the ^{IUCN (2000)} regards these plants as likely to produce negative impacts for biodiversity.

Legumes (Mimosae) such as catclaw play important roles in the ecosystem, as they form and retain the soils, fix nitrogen, are key pieces in the ecological succession and in degraded ecosystems (^{Miranda et al., 2004}), and even act as nurse plants (^{Flores et al., 2006}; ^{Pavón et al., 2011}).

At PNSO, all productive activities except ecotourism are forbidden since 2008; this, along with the soil restoration work, has favored the ecosystem’s recovery.

The purpose of this work was to propose a forestry arrangement of the main catclaw populations in stands using the index of Füldner and Gadow for the mixture of species, and Pommerening’ index to determine the dimensional structure. The results are expected to contribute to strengthen the bases that determine the relevance of the species in the vegetal succession process in the area, as well as the fulfillment of the goals of the Park.

Materials and Methods

The Sierra de Órganos National Park has a surface area of 1 124.65 hectares; it is located in the west of the state of Zacatecas, in the municipality of Sombrerete. It is part of the Sierra Santa Lucía, a branch of the Sierra Madre Occidental, bordering with the Altiplanicie Mexicana, between the coordinates 23°44’58’’ and 23°48’29’’ north, and 103°45’51’’ and 103°49’36’’ west, and reaching altitudes of 2 170 to 2 560 masl (^{Conap, 2014}; ^{Enríquez et al., 2003}).

The Mimosa populations present at PNSO are distributed in patches that are easily identifiable by their reddish gray hue during the rainy season, and gray during the dry season.

The main catclaw populations were arranged in stands using a Garmin Dakota® GPS to georeference the sites (Figure 1).

Figure 1 Location of the catclaw populations and the sampling sites at PNSO. 

Based on the surface area of the stands, a 5 percent sampling was carried out (^{Graciano, 2001}). Fifty-three 500 m2 circular sites (SM) were established. The registered dasometric parameters were genus, height, diameter of the base and crown cover.

The relative frequency (RF) and the absolute density were estimate using the formulas suggested by ^{Zarco et al. (2010}):

The tallest catclaw in each site was selected as the center for estimating the indices of ^{Füldner and Gadow (1994}) and of ^{Pommerening (2002}):

Where:
V_j = Discrete binary variable takes on a “0” value

when the jth tree belongs to the same species as the reference shrub i, and a “1” value when it belongs to a different species

S_j = Distance between the ith tree and its nearest neighbor

4 = Number of neighbors

The conditions of the soil cover were determined in each site. For this purpose, 100 mutually equidistant points were drawn -50 from north to south, and 50 from east to west. In these, the cover for every 0.5 m was determined, taking into account the dead leaves”, “naked soil” and “rock” variables.

The database was captured in specific formats according to the case, and the Excel^® software was used for the statistical analysis of the data.

Results and Discussion

84 stands were delimited within the PNSO; most were located in the lower parts of the Park, at an altitude range between 2 279 and 2 357 masl. These sites are characterized by being located, above all, in open areas, often in mild slopes or close to the intermittent streams and roads of the Park. The total surface area occupied by catclaw stands was 53.651has, i.e. 4.76 % of the total surface area of the PNSO.

^{Enríquez et al. (2003}) carried out a study of the flora and vegetation of the PNSO; they noted that the Mimosa- Opuntia vegetation unit is found in scattered patches on eroded soils and on sedimentary rock substrates, as was also observed in the present work. However, they document a larger surface area (78.19 has) than estimated by our research. This may be due to the different methodologies utilized, or to the fact that, when ^{Enríquez et al. (2003)} carried out their work, the Park was more damaged, as grazing had not yet been totally forbidden in the areas occupied by the catclaws.

The delimited stands were located at various exposures; however, they prefer the arid lands facing east, as 30 % of catclaw plants grow at this exposure, as pointed out by ^{Enríquez et al. (2003)}.

Table 1 summarizes the dasometric characteristics of the identified genera. The Juniperus (cedars), Pinus (pines), and Yucca (palm trees) stand out for their height and diameter. The catclaws are located in the middle part of the canopy, with an average height of 103.63 cm and a diameter of 2.1 cm, with individuals of heights ranging between 15 cm and 4 m. These values agree with those indicated for the Mimosa-Opuntia vegetation type (^{Enríquez et al., 2003}), the average height pointed out by these researchers ranges between 1 and 1.5 m.

Table 1 Genera of woody plants, their dasometric and ecological parameters, crown covers and their cover (%) in the sampling sites at PNSO. 

* Pinus FR= Reforested specimens.

Mimosa, Opuntia and Acacia stood out for their frequency (%). These results may indicate that catclaws act as nurses for the nopal cacti, as most of the latter were very young. ^{Jiménez- Sierra and Jiménez-González (2002)} register a more abundant regeneration of cactaceae and agaves beneath the crowns of the trees and bushes, as the protection provided by their crowns favors certain necessary factors during the first stages of regeneration. ^{Bravo et al. (2007)} studied the relationship between the size of the prickles of Neobuxbaumia tetetzo (F.A.C. Weber ex K. Schum.) Backeb. beneath the canopy of Mimosa luisana Brandegee at different distances from its center, and estimated that the size of the plants differs according to their distance from Mimosa luisiana.

An approximate density of 3 120 plants ha-¹ was estimated, of which 79 % corresponded to catclaws, growing in sites with difficult edaphic conditions.

According to ^{Granados and Tapia (1990)}, the shade produced by the crown cover of trees and shrubs modifies the soil microclimates, which in turn favors the establishment of plantlets of annual or perennial vegetal species. This proves the importance of the cover.

Table 1 shows the canopy cover in the sampling sites; 40.1 % of the area was estimated to be covered by some kind of woody plants, and the highest values corresponded to catclaws and pines. The percentage of cover per genus, in descending order, comprised catclaws, pines, huisaches and cedar trees.

An average value of 0.264 was estimated for the index of Füldner and Gadow, which takes on values close to 1 when the neighbors belong to genera other than the central catclaw; in this case, this value indicates that the stands are even, with the presence of few genera.

The average value for Pommerening’s index was 1.282, which implies that there is little space between the stands. Table 2 shows that the average distances between the neighbors and the center (catclaw) range from 89.5 cm for Mammillaria spp. to 220 cm for Forestiera. Opuntia stands out with 120.3 cm, mostly with young specimens, as mentioned above, as cited by ^{Bravo et al. (2007)}, who point out a potential nursing effect.

Table 2 Distance between the near neighbors and the center of the genera. 

The average soil covers consisted of 65.2 % “dead leaves”, 32.9 % “naked soil”, and 1.8 % “rock”. Notably, most of the soil is covered, protected from the rain and from the wind by the fallen leaves and branches, as well as by the canopies of the occurring plants, especially of catclaws.

Similar results are documented by ^{Montaño et al. (2006}), who assessed the relationship between the presence of Prosopis and the properties of the soil in two sites at Valle del Mezquital, Mexico, through a period of 10 years; in the sites with the largest cover, the diversity and amount of organic matter were larger. This shows that the contributions of organic matter by their type of plants enrich the soil and favor the establishment of other vegetals.

The correlation between the “dead leaves’” soil coverage and the percentage of the aerial cover is shown in Figure 2. The low value of R may be due to the fact that a high density of plants exists in certain sites within a portion of the 500 m² surface area, from where they spread to the rest of this area.

Figure 2 Correlation between the soil cover and the surface area cover. 

^{Flores et al. (2006}) studied the relationship between the presence of Mimosa monansistra and dry matter production, the amount and depth of water infiltration at two points: beneath and outside the canopy of the leguminous plant. According to them, the presence of catclaw does indeed exert a positive influence on the biomass production of herbaceous plants under their canopy, as well as on the amount of water infiltrated during the established period, as well as on its depth.

Conclusions

The catclaw populations are located mostly in the lower areas of the Park, at altitudes ranging between 2 200 and 2 360 masl in open areas; up to 30 % of the plants are located at an eastern exposure. The delimited stands are even, with little spacing between them and with an average density of 3 120 plants ha-1, of which 79 % are Mimosa. In average, 40.1 % of the surface area of the stands is covered with plants. The average height and diameter of mimosas were 103.63 cm and 2.1 cm, respectively.

15 genera were identified, among which Opuntia, Acacia and Pinus turned out to be prominent. In the case of young nopal cacti, there may be some degree of nursing by the mimosas.

It was determined that Mimosa dysocarpa and Mimosa aculeaticarpa at PNSO are non-invasive plants; because they are not exotic plants, they cover only 4.7 % of the total surface area of PNSO and form well-defined compact patches, besides occupying areas where other types of vegetation find it hard to survive, such as mountainsides, the banks of intermittent streams and areas subjected to overgrazing, where they fulfill a soil-forming and soil-protecting role: therefore, they are not harmful to biodiversity.

Conflict of interests

The authors declare no conflict of interest.

Contribution by author

Ricardo Ismael Mata-Páez: development of the manuscript; Pedro Antonio Domínguez-Calleros: development and revisión of the manuscript; José Rodolfo Goche-Télles: elaboration of graphs and figures, as well as support in statistical analysis; Manuel Antonio Díaz-Vásquez: elaboration of graphs and figures, as well as support in statistical analysis.