*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

#### **Figure 4.**

*Geographic representation of dry tropical Forest and the sampling points in Terrero de la labor Ejido, in the municipality of Calvillo, Aguascalientes.*

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

1800–1900 0 0 0 0 1 0 1 0 3 0 5 1901–2000 0 0 0 0 0 1 1 3 5 1 11 2001–2100 0 0 0 0 0 1 1 2 2 0 6 >2100 0 0 0 0 0 1 1 0 4 0 4 Total inventories 0 0 0 0 1 0 0 5 14 1 26

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

**Topographic position Total**

**Concave coverage (%) Convex coverage (%) <10 11–30 31–50 51–70 >71 <10 11–30 31–50 51–70 >71**

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

Cover Cð Þ¼ Σ length of individuals of species i*=*total length of intersections X 100*:*

Frequency Fð Þ¼ Σ of number of times that individuals of the species

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

sites and for each altitudinal level using the Richness and diversity species® software, considering that there could be variation in diversity according to the change in environmental conditions in temperature and precipitation as mentioned in the

) was calculated for each of the

*pi* log <sup>2</sup>*pi* (5)

intercepted by the line*=*Σ total species intercepted X 100*:*

Standard Atmospheric Index (decrease of 0.6°C/100 m altitude).

*<sup>H</sup>*<sup>0</sup> ¼ �<sup>X</sup> *S*

*i*¼1

known as Rarefaction [22].

**Meters above sea level**

*Ecology of Plant Communities in Central Mexico DOI: http://dx.doi.org/10.5772/intechopen.95629*

**(masl)**

**Table 2.**

of Aguascalientes (HUAA).

following formula was used:

*2.2.5 Data analysis*

**23**

To estimate frequencies, we used the formula:

*Distribution of samplings sites according to the proposed design.*

analysis. The Shannon-Wienner alpha diversity (*H*<sup>0</sup>

The formula of the Shannon index is:

#### **Figure 5.**

*Ipsographic model of Ejido Terrero de la labor Ejido polygon, and distribution of the sampling points in the DTF.*

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.

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 m<sup>2</sup> in each inventory, using the criteria of the minimum area [16]. 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


**Table 2.**

*Distribution of samplings sites according to the proposed design.*

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 [22].

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:

Cover Cð Þ¼ Σ length of individuals of species i*=*total length of intersections X 100*:*

To estimate frequencies, we used the formula:

Frequency Fð Þ¼ Σ of number of times that individuals of the species intercepted by the line*=*Σ total species intercepted X 100*:*
