Number of samplings performed at different altitudinal levels, topographic positions and solar exposures, derived from the sampling system.
In Central Mexico converge three biogeographic provinces: Altiplano sur, Sierra Madre Occidental and Costa del Pacífico. Each one of them is composed by different plant communities: Thorn Forest, Temperate Mountain Forest and Dry Tropical Forest respectively. Our objective is to show, through phytoecological analysis, the species richness, diversity and the structure of the plant communities from the Temperate Mountain Forest and from the Tropical Dry Forest. In the Temperate Mountain Forest, 50 forest species were recorded, with a Shannon Wiener diversity index H´ = 1.63 on altitudes from 2400 to 2600 m. The Whittaker β index is Bw = 7.22. In the tropical dry forest, we identified 79 plants species with a mean diversity index H´ = 3.49 on altitudes from 1951 to 2100 m. In this ecosystem the Bw index is 8.12. This study offers important information for the establishment of management practices, considering the protection status from the areas in which this vegetation type is distributed.
- Sierra Fria
- Temperate Mountain Forest
- Tropical Dry Forest
- biogeographic provinces
Mexico is one of the five countries with the greatest biological diversity in the world, due, in part to the confluence of the Neartic (North America) and Neotropical biogeographic zones (Mexico, Central and south America). As well as, the species evolutionary processes in its territory . The Mexican territory represent only 1% of the earth’s surface; nevertheless, Mexico belongs to the select group of the five countries considered megadiverse, along with Brazil, Colombia, China and Indonesia [2, 3]. Due to its geographic locations and its multiple landscapes, a large number and diversity of ecosystems converge in the national territory. For that reason, Mexico is ranked 12th in terms of global forest area . Even though, multiple efforts have been made for the forest conservation during the last decade of the XXI century, on a global scale, forest have been transformed to other uses at a rate of 1.3x106 million ha/yr. or they have been affected by natural disturbances that have partially or totally changed their structure. This amount represents a 19% decrease in comparison to the exchange rate registered in the last decade of the 20th century (1.6X106 million ha/yr) . Temperate forests in Mexico are found mostly, although not exclusively, in the mountainous areas along the Sierra Madre Occidental (the area with the highest concentration of forest ecosystems in the country), the mountains of Sierra Madre Oriental, the Sierra Norte de Oaxaca and the Altos de Chiapas, as well as in different mountain ranges and isolated mountains in the Altiplano and intermingled in the tropical plains . The conifer and oak forest in Mexico represent the most extensive vegetation cover in terms of vegetation types dominated by woody species, this species covers 16.4% of the total surface of the country, being only surpassed by the xeric shrubland which is the vegetation type which has the largest extension . These ecosystems are important both economically and ecologically, since they support productive activities, harbor great biological diversity and serves as a refuge for wildlife. Likewise, forest provide essential environmental goods and services for the human society subsistence [7, 8].
The State of Aguascalientes has a total extension of 555, 867.4 hectares, of which 291,792.4 hectares equivalent to 52.5% present some forest type . According to the classifications issued by different sources [10, 11], the State of Aguascalientes is made up by three large ecoregions (biogeographic regions), the Temperate Mountain Forest, the Tropical Dry Forest (also known as lowland deciduous forest) and the thorn forest (including crasicaule shrubland and xeric shrubland). The first ecosystem type is mainly distributed in la Sierra Fria, Sierra del Laurel, Sierra de Tepezalá and Cerro de Juan el Grande in El Llano municipality (Figure 1). The largest area covered by Temperate Mountain Forest vegetation in Aguascalientes is located in an area locally known as Sierra Fria, this site is a Protected Natural Area by state and federal decree which covers close to 107,000 ha . In the Temperate Mountain Forest, the plant communities the most common vegetation types are oak forests (
The largest area occupied by the tropical dry forest is mainly located in the Calvillo municipality, although, there are relics of vegetation indicative of this ecosystem in the Jesus Maria, San Jose de Gracia and Aguascalientes municipalities, which suggests a larger presence of this vegetation type in the past. In the tropical dry forest, forest structures made up of shrubs and trees between 2 and 8 m high and some relics of medium tropical forest. In Aguascalientes, this is one of the ecosystems with the highest species richness . The most representative vegetation in this ecoregion corresponds mainly to the
Our objective was to provide an overview of some ecological aspects (species richness, diversity and distribution) of woody species natural communities in the most representative ecosystems of the State of Aguascalientes, assuming that there would be a high similarity degree with the vegetation of neighboring sites, considering both the environmental and physiographic characteristics from this State.
2. Materials and methods
Three studies were conducted individually. During 2008–2015, the natural communities of the temperate mountain forest in the area commonly known as Sierra Fria, in the northwest of the State of Aguascalientes, as well as the main disturbances that have affected them in the past and present were analyzed [10, 13]. Likewise, during the period 2011–2015 a study was carried out to determine the diversity, dynamics and functioning of the tropical dry forest in the Calvillo municipality [14, 15].
2.1 Temperate Mountain Forests
2.1.1 Study area and sampling design
This study was carried out in to the Sierra Fria Protected Natural Area (SF-PNA) which is 106,114.6 hectares in size and is located in the northwest of the Aguascalientes State. This area has an altitude ranging between 2,100 and 3,050 masl. The study area comprised 25 thousand hectares, in a polygon located between the coordinates 102°31′31″ to 102°37′44″ west longitude and 22°05′47″ at 22°14′03″ north latitude, assuming that the conditions both geographic, ecological and climatic are representative of the entire ANP (See Figure 2).
A stratified sampling strategy was developed . The sampling strata were delimited based on the altitude, solar exposure, and geoform of the site (flat, concave and convex terrain). The first stratum was defined using a Digital Elevation Model (DEM) of the ANP SF, elaborating a spatial grid according to five altitudinal categories: i) 2,000-2,200, ii) 2,200-2,400, iii) 2,400-2,600, iv) 2,600-2,800, and v) >2 800 masl.
To stablish the altitudinal strata, the level curves from study site were defined using the DEM. The solar exposure was approached using an exposure map made with a SPOT 2010® imagine on which the DEM of the site was superimposed. Subsequently, a mesh map was prepared using the ArcGis 10.2. The geoform was obtained based on the slope, where flat terrain = sites with a slope ≤ 10%, concave t. = slope ≥ 10 and ≤ 25% and convex t. = slope ≥ 25%.
2.1.2 Identification, distribution and abundance of forest species
To identify the tree and shrub diversity in the study area, we conducted 60 phytoecological inventories in 60 different sites distributed randomly using the sampling scheme already described (Table 1).
|Altitude levels||Topographic position|
The field samplings were performed in rectangular plots of 600 m2, with a central line 100 m in length and two lateral lines with three m of separation. In each inventory, the frequency of the tree and shrub species present were determined, as well as the site environmental variables. Individuals with DBH ≥ 5 cm and height ≥ 1.50 m were considered as trees. Individuals below these categories were considered as juveniles and shrubs. The variables recorded in the site were: altitude, slope (in %), solar exposure (N, S, E, W), physiography (flat land, hillock, plateau, middle slope, high slope, ravine bottom, creek), coverage (c1 = ≤10%; c2 = 11–30%; c3 = 31–50%; c4 = 51–70% y c5 = ≥70%) and geoform. Management variables related to land use (no use, forest exploitation, wildlife management, grazing, agriculture and conservation) were considered as well as intensity of use (null, moderate, over-exploited and not determinable). Each one of the sampling points were geographically located by Transverse Mercator Units (UTM).
In order to identify the oak and conifer species in the field, keys generated by De la Cerda  and Siqueiros , respectively, were used. The unknown species were collected in botanical presses and identified at the Autonomous University of Aguascalientes herbarium (HUAA). To leave evidence of the new species records in the ANP SF, specimens were deposited in the HUAA.
2.1.3 Distribution and abundance of species
To estimate the distribution of tree and shrub forest species, the presence of each of the species found in each of the 60 sampling sites was quantified. In the case of species considered as restricted distribution (eg.
The frequency of the species found was determined on 100 m transect at ground level, observing 100 separate points every meter. The species found at each point were recorded (when there was more than one vegetation layer), counting the number of times that each species appeared (absolute frequency)  over the whole transect. Relative frequency was calculated using the Equation :
Frequency of the species x = absolute frequency obtained from each site sampling.
Subsequently, an abundance index was calculated using the equation:
Spp.ai = Identified Species abundance index.
With this data, distribution and abundance graphs of the main arboreal-shrub forest species were created. The phytoecological analysis was used to calculate the species richness and the Shannon index diversity (
S = species richness; Pi = proportion of the individuals of species i with respect to the total number of individuals; ni = number of individuals of species i
S = Species richness and S = mean richness of the site.
2.2 Dry Tropical Forest (DTF)
2.2.1 Study area
Although there are some studies that suggest the existence of relics of Dry Tropical Forest (DTF) vegetation in some municipalities of the Aguascalientes State [15, 20], this ecosystem has a greater representation both in surface area and in its conservation status in Calvillo municipality. The study was conducted in 26 sites with DTF vegetation cover in Terrero de la Labor ejido, located within the Sierra Fria Protected Natural Area, in the Municipality of Calvillo, State of Aguascalientes, in Central Mexico. The ejido polygon comprises an area of 5,861 ha. , of which, the DTF occupies 45% of its total area (Figure 3). It is located within the extreme coordinates: 102°43′58.88“ West Longitude and 22°6’4.78” North Latitude and at the Southeast end 102°41′24.95“ West Longitude and 21°44’27.61” North Latitude.
2.2.2 Selection of the study sites and sampling design
We used a stratified sampling design system . Sampling strata were delimited based on geoforms, slope, exposure and altitude. To characterize geoforms, three criteria were used: concave, convex and flat terrain. A concave geoform was defined when the slope ranged between 10 and 25%, which usually corresponded to ravines and small depressions. When the sites had a slope between 25 and 60% they were characterized as convex sites. Flat terrains had slopes <10%. Solar exposure was defined using an exposure map made with a Geographic Information System from a 2008 Spot® satellite image and a digital elevation model (MDE). Only the main cardinal points (North, South, East and West) were considered. To locate the altitudinal strata, the contours of the zone defined from the MDE were used. Subsequently, a grid map was developed for the identification of the sampling areas (See Figure 4).
2.2.3 Selection and characterization of sites to quantify of the composition and abundance of woody species
We established 26 sites to quantify phytoecological inventories, distributed in the landscape according to the above mentioned sampling system. At each point, the projected coordinates of the site were taken with GPS Garmin 48 XL line in UTM format, zone 13 North and with reference Datum WGS84 and with accuracies of 5 to 12 m with differential kinematic adjustment (WAAS). Subsequently, the points were placed on a SPOT 2010 satellite image (Figure 5). Site variables considered were the slope (%), solar exposure, physiography of the terrain, intensity and type of use and canopy coverage.
Slope at each sampling site was obtained by direct field measurement with a Bruntton clinometer with a precision of +/− 2° of variation for each 100 meters of length. This data in turn was contrasted with the data obtained from the digital elevation model with precision of 1 to 2 meters in the Z value. Five classes were used to define the slope: i) <10%, ii) 11–30, iii) 31–50, iv) 51–70 and v) > 70. Exposure to solar radiation was estimated considering the cardinal points North (N), South (S), East (E) and West (O), as well as their combinations.
The altitude of each site was obtained directly in the field with the support of a GPS with barometric adjustment to reduce the effect of mathematical variation of the Geoid model and with precision of 1 to 3 meters. This was compared with the data obtained from the prospecting of points against elevation level curves obtained from the digital elevation model to reduce the potential errors of direct measurements.
The physiography of the terrain was characterized considering flat terrain (slope < 10%), steep (without slope), medium slope (10–25%) and high slope (>60%). The exposure of the sites was quantified with a compass and the magnetic north was taken as reference for its definition in the previously defined ranges. Exposure for each stand of the sampling site was also analyzed along with the digital model of exposures generated from the digital elevation model. The Table 2 shown the sample points distributed in the landscape of the Dry Tropical Forest.
|Meters above sea level (masl)||Topographic position||Total|
|Concave coverage (%)||Convex coverage (%)|
Other characteristics considered in the description of the sites were the degree of modification (i.e. transformation of geographical space, introduction of species), its intensity (light, medium and overexploited), as well as the type of use by local inhabitants (hunting, grazing, gathering, etc.).
2.2.4 Species richness
To describe species composition, we used a sampling design based on nested plots in an area of 1024 m2 in each inventory, using the criteria of the minimum area . We started with a plot of 1 x 1 m in a direction perpendicular to the slope in which all present species were recorded, and subsequently, the plot. Subsequently, the plot was increased in size to 2 X 1, 2 X 2, 2 X 4, 4 X 4 m etc. registering the new species for each increment in the area of the squares until reaching the maximum extension (i.e.: 32 x 32 m = 1024 m2), to obtain an area/species curve. We then identified the area in which the present species stabilized. This sampling method increased the probability of finding rare species as the area increased, an effect known as Rarefaction .
Identification of species was estimated in the field by morphological characters described in previous studies. Specimens that could not be identified in the field were collected and later identified in the Herbarium of the Autonomous University of Aguascalientes (HUAA).
We used the linear intercept survey method (Canfield line). A 100 m long line was perpendicular to the slope, starting at the GPS coordinates of the sampling site, then intersection lines were defined were individuals of DTF species were counted at constant intervals of 1 meter. Shrub and tree individuals were categorized into five heights classes 0–1 m, 1.1–2 m, 2.1–4 m, 4–8 m, 8–15 m and > 16 m. For each class we measured canopy cover of each species by measuring the perpendicular projection of the crown and the frequency of species. To estimate crown, cover the following formula was used:
To estimate frequencies, we used the formula:
2.2.5 Data analysis
Species composition was estimated through the identification of the species found in each of the sampling plots. To find a limit on the number of samples and to reduce the possibility of under- or over-sampling, we conducted a rarefaction analysis. The Shannon-Wienner alpha diversity (
The formula of the Shannon index is:
– Total number of species (species richness)
– Proportion of individuals of species i in respect to total of individuals (i.e.: relative abundance of species i):
– number of individuals of species i
– Total number of individuals of all species
The index considers the number of species present in the study area (species richness), and the relative number of individuals of each of those species (abundance).
To estimate replacement rates of species Whitakker’s β diversity was computed, using the diversity found for each altitudinal level analyzed as reference.
Where: β = Whitakker’s β diversity.
S = Total number of species in samples.
α = Mean number of species in samples.
3.1 Temperate Mountain Forest
3.1.1 Richness and diversity species
In the 60 sites, 50 species were recorded, corresponding to 20 families and 27 genera (Table 3), of which, due to their structure, 47% (n = 24) were considered trees (height ≥ 3.5 m) and 53% (n = 27) shrubs and juveniles. The best represented families were
|Species||Key||Common name*||Family||Forest classification**||Use¥||Report***|
|Sangre de grado||Sh||Nu||Y|
|Gatuño de la sierra||Sh||Nu||Y|
On average, the highest
In the Figure 6, we shown an example of dominant vegetation in Temperate Mountain Forest (in conifers,
3.1.2 Distribution and abundance of species
The most widely distributed species belong to the genus
Out of 50 recorded species, 6 are the ones with the highest abundance indexes.
There are species such as
Out of the dominant conifer species at the SF-Natural Protected Area,
In Figure 9 we shown some species of
3.2 Dry Tropical Forest
3.2.1 Richness and diversity woody species
We identified 79 species of trees and shrubs, within 45 genera and 14 families (see Table 5). The best represented families were Fabaceae (13 genera), Asteraceae (11 genera) and Cactaceae (9 genera). The genera better represented were
|Fabaceae||Huizache o Cascalote|
|Asteraceae||Orégano de monte|
|Burseraceae||Arbol de chicle|
|Scrophulariaceae||Hierba del cancer|
|Ulmaceae||Vara en cruz|
|Boraginaceae||Amapa o Vara prieta|
|Fabaceae||Palo azulo o Varaduz|
|Asteraceae||Cola de zorra|
|Meliaceae||Cicuito o Cuero de indio|
|Amaranthaceae||Cola de zorra|
|Fabaceae||Ébano o Palo fierro Tepeguaje|
|Cactaceae||Biznaga de seda|
|Euphorbiaceae||Pata de gallo|
|Fabaceae||Gatuño o Uña de gato|
|Asteraceae||Talacao o Vara blanca|
|Cactaceae||Nopal chaveño o duraznillo|
|Apocynaceae||Flor de mayo|
|Rutaceae||Naranjo agrio o Zorrillo|
|Altitud level (masl)||Sampled sites|
|Slope range (%)||Sampled sites|
3.2.2 Distribution and abundance of woody species in the DTF
Of the 79 species identified, eight are distributed in more than 70% of the plots of Terrero de la Labor ejido. The species with the greater distribution are the
On the other extreme, the rarest species were
The Figure 12 shown some species of the dominant vegetation in tropical dry forest, in this case, of the Terrero de la labor and las Moras ejidos in the Municipality of Calvillo, Aguascalientes State.
The loss of biodiversity is one of the environmental problems that has managed to arouse broad global interest in the last two decades [4, 23]. Some of the main causes are related to human activities, either directly (overexploitation) or indirectly (habitat alteration), although there is generally an interaction between them. The communication systems have impacted in such a way that both the government and the private sector, as well as society in general, consider a priority to direct greater efforts towards conservation programs. The basis for an objective analysis of biodiversity and its change lies in its correct evaluation and monitoring.
In the Temperate Mountain Forest, the 50 woody species identified show a high species richness in comparison with other mountain regions. The best represented genera correspond to oak trees (
In the Sierra Fría, the most widely distributed and abundant species are the potosine oak (
The distribution of species such as
The species richness in BTS is generally lower than in humid tropical forests , although higher than in Temperate Mountain Forests . The BTS is dominated by relatively short trees, most of which lose all their foliage during the dry season. In this community, herbaceous life form, thin woody species, and vines are common, but epiphytes and thick lianas are less abundant and diverse than in humid forests . Diversity is generally higher without a clear dominance of any species, to the point that many of them are rare . In this type of ecosystem, it is common to identify some genera such as
The species richness found at the Terrero de la labor Ejido BTS (N = 79) is similar to that reported by Trejo (2005) , where he points out that on average the tropical dry forest in Mexico harbors around 74 species with a DBH ≥ 1 cm in 0.1 ha. However, in the study site, some species considered “rare” which are indicators of medium forest (e.g
The analysis of the diversity, distribution and abundance associated with the Tropical Dry Forest in Aguascalientes has been little addressed, so the study conducted in the BTS of the Municipality of Calvillo represents one of the first efforts to understand this ecosystem natural heritage . Previously, partial floristic studies had been carried out, studies which mainly referred to the dominant vegetation types and some important species, however on these studies there were gaps in relation to the ecology of the plant communities . On the other hand, other studies mention some factors related to the mortality of these natural communities , but there is no information on vegetation diversity which reflects the real tropical dry forest importance.
This work contributes directly to the management of the ecosystems analyzed. Knowledge about species richness and their distribution provides an overview of the territory’s conservation state, considering that both the Temperate Mountain Forest and the Tropical Dry Forest studied are part of the Sierra Fria Protected Natural Area, which is the protected area with the biggest extension in the State. On the other hand, the bases are established for the restoration of degraded ecosystems, either through active restoration or through mechanisms of ecological succession (passive restoration) .
The authors acknowledge the participation of Jesus Argumedo-Espinoza for his cartographic support. Likewise, we thank the facilities provided of the owners of the Sierra Fria, as well as Jesus Velasco Serna of the Terrero de la Labor ejido for in the gathering of field information.