Study area

The San Judas-Agua del Toro micro-watershed is located at the coordinates 25°34’25 N and 100°27’43 W, at the northern end of the Great Fold Mountains, in the Eastern Sierra Madre, southwest of the city of Monterrey, in the municipality of Santa Catarina, Nuevo León (Figure 1). Access to this micro-watershed is difficult, as there are no roads for any type of vehicle, and the terrain is very steep.

From the hydrological point of view, the area is a tributary micro-watershed to the sub-basin of the Santa Catarina river, which is part of the Bravo - San Juan River Basin (^{IAENL, 2011}); its surface area is 28.2 km², along which recharge areas for the three hydrogeological units of the Campo Buenos Aires aquifer -the main source of underground water for Monterrey and its metropolitan area- are located (^{De León-Gómez et al., 1998}; ^{Masuch-Oesterreich et al., 2001}; ^{Conagua, 2009}; ^{ITESM, 2009}; ^{Conagua, 2011}); it is characterized by exhibiting a deficit over its mean annual recharge (^{Conagua, 2011}).

The Great Fold Mountains are an orographic system formed by sedimentary rocks of the Mesozoic era corresponding to geological formations that have their origin in the ocean of the higher Jurassic and Cretaceous periods (^{SGM, 2008}); there are, besides, continental alluvial deposits of the Tertiary and Quaternary periods of the Cenozoic era.

The Campo Buenos Aires aquifer consists of three hydrogeological units located in certain sets of those geological formations: the Cretaceous Unit, in the limestone of the Cupido, Aurora and Cuesta del Cura formations; the Jurassic Unit, in the limestone of the Zuloaga formation, and the Alluvial Unit, on the gravel of the alluvial deposits (^{Conagua, 2009}). Outcrops of the Cupido and Zuloaga formations and alluvial deposits were observed in the studied micro-watershed; therefore, it is a recharge area for the three aquifer units (Figure 2).

The area has two different types of climate: BS₀hw, dry semi-warm, and BS₁kw, semi-dry temperate; the mean annual precipitation ranges between 432.3 mm and 688.6 mm, and the mean annual temperature, between 14.0 and 20.7 °C (^{SMN, 2012}, ²⁰¹³). The predominant vegetation is the pine-oak forest, followed by submontane and xerophytic shrubs (^{Pronatura Noreste A. C., 2011}) (Figure 3). The occurring soil types are lithosols, regosols, rendzines and xerosols (^{Inegi, 1986}).

^{Stoll’s method (2007}) was chosen to estimate the hemeroby index, due to the similarities in topography and vegetation type (mountain canyon and temperate forest) between the area studied by Stoll and the present area, as well as because it is the most simplified among the methods reviewed in the literature.

Directed stratified sampling was used, according to three criteria: geological formation (Zuloaga Formation), vegetation (mixed pine-oak forest) and altitude (1 460 m, 1 660 m, 1 860 m, and 1 960 m). Four sampling sites were established (figures 1, 2 and 3) encompassing the total longitude and four different altitudes in the mixed forest strip located in the Zuloaga Formation.

A 20 m x 10 rectangular plot established longitudinally to the curves of the terrain was used to characterize the vegetation in each sampling site. This sampling intensity was due to limitations in time and mobility resulting from the difficulty to access the work area. However, when possible, representative sites of the physiognomic variations observed in the forest strip of interest were carefully selected. The arboreal stratum in the total area of each plot (200 m²), the shrub stratum in two cater-cornered inner 25 m² subplots, and the herbaceous stratum in five 1 m² inner subplots were considered. All individuals that were not branched from the base were regarded as belonging to the arboreal stratum. The following attributes were registered: species, height and crown diameter, for trees and shrubs, and species and cover only, for the herbaceous vegetation. The frequency, density, cover and importance values were estimated for each taxon; these data were then used to calculate the Shannon-Wiener diversity and equity index, the Margalef´s richness index, and Simpson’s dominance (^{Mostacedo and Fredericksen, 2000}; ^{Melo and Vargas, 2002}).

The hemeroby index was estimated for each sampling site through the aggregation and average of eight criteria or markers assessed per site (^{Stoll, 2007}): the composition of the arboreal stratum (1), the composition of the shrub and herbaceous strata (2), the type of regeneration of tree species (3), the use of the vegetation (4), the use of the environment (5), the stratification (6), the diversity of species of the arboreal stratus (7), and the diversity of species of the shrub and herbaceous strata (8).

The comparisons between the assessed vegetation and the corresponding natural potential vegetation for the various criteria were made based on the descriptions of the mixed pine-oak forest community cited in descriptive studies carried out in the region (^{Rojas-Mendoza, 1965}; ^{Marroquín, 1968}; ^{Rzedowski, 1978}; ^{Baca-Venegas, 2000}; ^{Jiménez et al., 2001}).

Criterion 1 (composition of the arboreal stratus). The current and potential covers of each species were compared; the presence or absence of taxa, the expected potential cover, and the cover of neophyte species were included.

A basic table of cover categories (Table 1) was used to establish potential cover categories according to the consulted information (Table 2). Based on the cover obtained in field, a current cover category was assigned to each registered arboreal taxon. The categories obtained from the comparison were later correlated with a matrix (Table 3).

The correlation value for each cover category was added to obtain the value of Criterion 1.

Where:

C1 = Criterion 1

RV _n = Retention value of species “n”

Criterion 2 (composition of the shrub and herbaceous strata). The presence and relative cover of species indicating disturbance in both strata were assessed; the neophyte and native species that are foreign to the assessed vegetal community were considered, as were the species that indicate changes in the cover of the respective stratum, based on the information available in the literature. Specific categories were established for the comparison (Table 4). A cover index was obtained for each of the taxa belonging to one of the categories, based on a matrix (Table 5).

The disturbance index was then calculated using the following formula:

Where:

Di = Disturbance index

Dp = Disturbance probability

Ci = Cover index

Finally, the value of the criterion was determined by correlating the disturbance index with the values shown in Table 6.

Criterion 3 (type of regeneration of arboreal species). The relative regeneration cover of each present arboreal species was estimated, by counting the young individuals and the seedlings. According to the classification shown in Table 7, each taxon was assigned a representative factor.

The value of the criterion is the sum of the products of the relative covers of each species in regeneration and the corresponding factor, as shown in the following formula:

Criterion 4 (use of vegetation). The intensity, type and use history of the vegetation in the study area were estimated based on personal communications with the owner of the plot and direct in-field observation of the factors involved in each use type. For example, in order to evaluate the intensity in the use type “grazing”, the presence and number of cattle, the perceptible compacting of the soil, the appearance of the vegetation and the disturbance marker species brought about by overgrazing. The correspondence factors were established (Table 8).

The product of the factors is the Influence Number, which corresponds with a specific criterion value (Table 9).

Criterion 5 (use of the environment). The type of use of the environment around the site at a distance of not more than 1 km was determined based on in-field observations. A criterion value corresponds to each use type (Table 10).

Criterion 6 (stratification). The observed structure was compared with the potential structure cited in the literature for the same type of vegetation, assigning it a class based on the stratification types shown in Table 11.

Criterion 7 (richness of arboreal species). The registered number of arboreal species was contrasted with the minimum number and with the most frequent number of arboreal taxa under natural conditions, as documented in the bibliography (Table 12).

Criterion 8 (richness of species of the shrub and herbaceous strata). The registered number of species in the shrub and herbaceous strata was compared with the minimum number and with the most frequent number of species under natural conditions as reported in the literature, as shown in Table 12.

Once the criteria were obtained, their values were averaged in order to estimate the index or degree of hemeroby by sampling site. The average of the degrees in the sampled sites was subsequently calculated in order to estimate the overall hemeroby index for the pine-oak forest in the study area. The index was then interpreted according to the classification shown in Table 13.

The conditions and risk factors for the vegetation in the watershed were determined through in-field observation and direct communication with the owner of the plot, who was requested information regarding the current and past activities carried out in the plot and their degree of intensity.

Characterization of the vegetation

A total of 36 species were registered, corresponding to 34 genera and 23 families (tables 14 and 15). The arboreal stratus of the vegetation is dominated by Quercus canbyi Trel. (index of importance value: 92.51), Pinus arizonica Engelm. (64.17) and Quercus laeta Liebm. (55.98); and the shrub stratum is dominated by Rhus virens Lindh. ex A.Gray (51.73), Leucaena greggii S.Watson (48.50), and Decatropis bicolor (Zucc.) Radlk. (46.53). In the herbaceous stratum, the most dominant species are Gymnosperma glutinosum Less. and Ageratina espinosarum (A.Gray) R.M.King & H.Rob. (tables 16, 17 and 18).

The ecological indices estimated by stratum for each sampling site are shown in figures 4, 5 and 6.

Conservation status of the vegetation

Figure 7 shows the criteria and estimated degrees of hemeroby per sampling site.

Based on the average of the degrees of hemeroby of all the sampling sites, an overall degree of hemeroby of 4 was estimated. This indicates that the vegetation corresponds to an oligohemerobic or almost natural site, in which there is a low anthropic influence and a clearly recognizable natural vegetation.

Risk conditions for the vegetation

Three conditions or risk factors were observed in the study area: 1) the presence of cattle, which has existed in the micro-watershed for several decades -cattle and goats in the past, but currently only cattle (Figure 8); 2) the existence of areas with a partial or total loss of the vegetation cover, hydric erosion and erosion gullies (Figure 9); the surface of these areas was estimated in more than 45 ha, distributed in a stretch of 4 km, along which a gradient was observed in the intensity of the loss of vegetation cover; 3) the presence of patches of trees of the Pinus genus, with symptoms of a likely infestation by debarker insects, Dendroctonus spp. (Figure 10).