Composition, structure and diversity of a mesquite in Pesquería (Northeastern Mexico)

Area of study

The present research was carried out in the conservation area of the Ternium Industrial Center of México, which has an area of 46 hectares and is located within the municipality of Pesquería, Nuevo León, in Northeastern Mexico (Fig. 1). It is located between latitude 25° 45’ N and longitude 99° 58’ W, at an average altitude of 306 masl, which belongs to the physiographic region of the North Gulf Coastal Plain [6]. The predominant climate is very dry and semi-warm (Bwhw), with an average annual temperature in the range from 20 to 21 °C. The types of soils present in the majority of cases are xerosol, castañozem, feozem, regosol and in the minority of cases are fluvisol, vertisol and rendzine. The average annual rainfall is 550 mm. The vegetation of the area corresponds to a mesquital with a history of cattle and hunting use and is currently disturbed [14].

Analysis of the vegetation

In order to fulfill the objective, the “c” sub basin of the Pesquería River with in a low part of the San Juan River Basin (Rio Bravo, Rio Bravo) was selected, where the conservation area is located and where intermittent surface runoff occurs. Vegetation evaluation was carried out using three randomly distributed sampling sites. The dimensions of each site was 40 m × 40 m (1,600 m²). A census was taken of all shrub and tree species with a basal diameter (d_0.10) ≥ 5 cm. The species of each individual was identified and recorded, taking the measurements of total height (h) and crown diameter (d_crown) in north-south and east-west directions. For the nomenclature of families, orders and species we followed the APG III [15], and the scientific names and families were corroborated in the database of Tropicos [16].

Data analysis

Composition

Community structure

The abundance of the evaluated plant species was 375 N/ha. The most abundant species within the community were Acacia amentacea with 85 N/ha, Havardia pallens with 73 N/ha, and Prosopis glandulosa with 42 N/ha. When added these three species equal 200 N/ha, representing 53.33% of the total abundance of the evaluated plant community.

The evaluated community crown area was 6,600 m²/ha, representing 66.00% coverage. The species that presented the greatest dominance were Prosopis glandulosa with 26.30%, Acacia farnesiana with 17.42%, and Havardia pallens with 13.00%, amounting to 56.72% of the total coverage. The most frequent species had some presence in the three evaluated sites, and were Acacia amentacea, Cordia boissieri, Havardia pallens, Prosopis glandulosa, and Sideroxylon celastrinum.

Importance value

Vertical structure

The total height of the evaluated individuals fluctuated between 2.3 m and 9.00 m. The highest abundance of individuals is represented in the height class 5–7 m with 173 N/ha which is equivalent to 46% of the evaluated population, followed by class 3–5 m with 118 N/ha representing 31% of the evaluated population (Fig. 2).

According to the classification proposed by Pretzsch, where three high strata are established, the high stratum is composed of 60 N/ha (16.11% of the population) belonging to five species, the mean stratum by 221 N/ha (58.89% of the population) of 12 species, and the low stratum by 94 N/ha (25% of the population) with 13 species.

The species found in the three strata of height are Acacia amentacea, Acacia farnesiana, Havardia pallens, Parkinsonia texana, and Prosopis glandulosa. Those species that are well represented in the vertical structure of the community are also those that presented the highest IVI values in the horizontal structure.

Species diversity

The evaluated plant community had a Margalef Diversity Index value of D _Mg  = 2.50 and a Shannon Index value of H′ = 2.28.

Species rank abundance curves

The known models were fitted and the best was a geometric model (Fig. 3), followed by the Poisson LogNormal model (AIC = 123.86, dAIC = 2.34, x ² = 7.5581, df = 13, p = 0.87), which is followed by the Neutral model (AIC = 131.25, dAIC = 9.72, x ² = 184.89; df = 13, p < 0.001).

Composition

The composition observed in our study resembles that reported by Briones and Villarreal [11], in the thorny bush of Prosopis-Acacia. According to these authors, the composition and physiognomy correspond to the extra-desert mesquite described by Rezedowski [4] for the state of San Luis Potosí, and the spiny and evergreen forest [7]. However, the composition differs from other studies in which Prosopis sp. (mesquite) dominates the arboreal strata in a mono-specific manner [8]. There are associations similar to those reported in our study, but with lower heights, which are described by Briones and Villarreal [11], such as the medium thorny scrub of Prosopis-Acacia.

The most representative family of the mesquite was Fabaceae, and the others families only presented one species. This record is similar to that obtained by Montaño et al. [28], who report the same number of families in spite of having a greater number of species, also with the most representative being the Fabaceae, a characteristic situation for the xerophilous scrub of this region. The most representative species (Prosopis glandulosa, Acacia amentacea, Havardia pallens, and Acacia farnesiana) in this study are reported as being characteristic of this type of community [29]. The mesquite woodland includes forests dominated by Prosopis glandulosa var. torreyana and var. glandulosa, which are recorded in areas that have been disturbed [30], and these are frequently associated with species such as Forestiera angustifolia, Leucophyllum frutescens, Ziziphus obtusifolia, Acacia rigidula (= A. amentacea), A. berlandieri, Celtis pallida, and Karwinskia humboldtiana [13], the same species as recorded in the present investigation.

Structure

The three species with high abundance were Acacia amentacea, Havardia pallens, and Prosopis glandulosa, and these results are in agreeance with those of Estrada et al. [29], who notes that these species are the most abundant. Montaño et al. [28] note that in Mexico’s xerophilous scrub, mesquite is an abundant species and possibly a key species in disturbed scrubland. Rojas-Mendoza [13], INEGI [6] and Estrada et al. [29] note that it is a species that resists the effects of disturbance and is thus associated with disturbed zones.

The evaluated community has a coverage of 66% (6,600 m²/ha), which means that 34% of the area is devoid of vegetation. This may be due to the fact that only individuals greater than 5 cm in diameter were considered, since for similar communities in Northeastern Mexico that considered smaller individuals, coverage greater than 100% (13,973 m²/ha) was recorded, which denotes a considerable overlap of crowns [31].

The most frequent species are the genus Prosopis and Acacia, similar to those reported by Montaño et al. [28], being of greater presence in the mesquites. Similarly, Mora-Donjuan et al. [32], note that the genus Prosopis has a greater frequency in the microphyllous desert scrublands.

Vertical structure

The total height of the evaluated individuals fluctuated between 2.3 m and 9.00 m. This height is considered high compared to other plant communities of the xerophilous scrub [32]. The value obtained from the Vertical Species Profile Index (A) was 3.03, with an A _max of 3.74 and an A _rel of 81.15%, indicating high vertical structural diversity. Pretzsch [33] notes out that high vertical and horizontal heterogeneity are strongly related to high diversity and ecological stability. This suggests that the species and their abundances are evenly distributed in the three height strata, and that the system is in a good state of conservation. These results are better than those presented by Alanís-Rodríguez et al. [34], Jiménez-Pérez et al. [35], Villavicencio et al. [36], and Mora-Donjuan et al. [32] who determined that the evaluated xerophilous scrub communities, many of which were previously subjected to some type and intensity of use, present only one or two highly dominant strata.

The presence of Acacia amentacea and Parkinsonia texana in the three strata shows the level of adaptation required for these environments. These species are well adapted to water stress, and they use mechanisms to avoid dehydration of their tissues, and to adjust their morpho-physiological characteristics to cope with the long season of dryness, as well as the fixation of active nitrogen [30], which also explains the possibility of their behaving as colonizing species.

Species diversity

The specific richness that was recorded differs from that documented by Montaño et al. [28], in a larger sampling area which registered 36 species, 14 of which were documented in the present study. This situation is presented by the environmental factors in each research area, Montaño et al. [28] studied a semi-arid spiny scrubland of the Mezquital Valley, where precipitation of 520 mm and temperature range of 16–24 °C were similar to the present study precipitation 550 mm and temperature range of 20–21 °C. The altitude factor positively influences the development of the species, whereby Montaño et al. [28] recorded an altitude of 2013 m.a.s.l compared to the altitude of 306 m.a.s.l in our study. This suggests that environmental factors play a positive role in the species diversity of the study area, although most of the recorded species are distributed exclusively or preferably in the arid or semi-arid zones of Mexico [37, 4].

The low number of species is associated with that the mesquites harboring the lowest number of taxa for the northern zone of the state [29], being that this community has the smaller registered presence of Caesalpiniaceae and Fabaceae. This is largely limited to edaphic features, and mesquites are distributed in the northwestern end of the state, where sandy soils dominate [29].

Regarding the diversity of the community described in the present study, the data presented in other studies do not account for the properties of the studied communities, and therefore, the comparison with other mesquite communities is not feasible. However, the evaluated area showed values of D _Mg  = 2.50 and H′ = 2.28, and these values are similar to those recorded for other xerophilic scrubs. Mora-Donjuan et al. [31] evaluated a reference area of Tamaulipan thorn scrub (TTS) and recorded D _Mg  = 2.26 and Diversity H = 1.94. They are also similar to those recorded by Jiménez-Pérez et al. [38], who evaluated the diversity of a regenerated TTS community with agricultural history in Northeastern Mexico and obtained values of D _Mg  = 2.17 and alpha diversity H′ = > 2.27. The values recorded in this study are greater than those of Pequeño-Ledezma et al. [39], who registered values of D _Mg  = 1.40 and alpha diversity of H = 1.27 in a post-livestock regeneration area in the Tamaulipan thorn scrub.

As reported in the existing mesquite studies, these communities are mostly reported as monospecific to Prosopis sp., or floristically not very diverse but mainly dominated by Prosopis and Acacia. This qualitative observation, as reported in other studies (e.g. [11, 8]), is consistent with the quantitative results obtained in our study, where high values for Prosopis glandulosa, Acacia amentacea, Havardia pallens, and Acacia farnesiana - the species that distinguish the association. The dominance of these taxa over others is also observed in the species abundance curve of the studied community which was adjusted to the geometric model.

Species abundance curves

In the present study, the fit to the geometric model suggests a typical structure of strongly stressful environments. Magurran [26] describes the species abundance model adjusted to the geometric model as the model associated with adverse environments or initial successive stages, where the limiting conditions prevail. In the case of mesquite, the environment corresponds to that of arid zone [4]. The dominant species (Prosopis glandulosa, Acacia amentacea, Havardia pallens, and Acacia farnesiana) present different adaptations to conditions of water stress and light saturation [40–42].