Bioclimatic belts of Sierra Madre Occidental (Mexico):A preliminary ...

International Journal of Geobotanical Research, Vol. nº 3. 2013. pp. 19-35

Bioclimatic belts of Sierra Madre Occidental (México): A preliminary approach. Joaquín GIMÉNEZ DE AZCÁRATE (1), Miguel Ángel MACÍAS RODRÍGUEZ(2) y Fernando GOPAR MERINO(3) (1) Department of Botany. Higher Politechnic School. University of Santiago de Compostela. E-27002 Lugo. Spain.

(2) Department of Environmental Sciences. University Center of Biological and Agricultural Sciences- University of Guadalajara, Jalisco. México. (3) Centre for Research in Environmental Geography. Campus Morelia. National Autonomous University of México. Michoacán. México. Abstract A bioclimatic synthesis of the Sierra Madre Occidental (SMO) based on the diagnosis of data from 159 meteorological stations, the floral and vegetation data collected in field surveys, and in the bibliographic revision. The considered area is above the altitude of 1800 m included in the physiographical province of SMO, which entirely belongs to Tropical macrobioclimate, being represented by the bioclimates Tropical Pluviseasonal and Tropical Xeric. Broadly speaking, the first is distributed in the highlands and on the western slope of the range, while the second is sited in the dry mountain ravines and on the eastern slope. Fourteen isobioclimates were recognised, 5 for the bioclimate Tropical Xeric (Thermotropical Dry, Mesotropical Semiarid, Mesotropical Dry, Supratropical Semiarid and Supratropical Dry) and 9 for the Tropical Pluviseasonal (Thermotropical Subhumid Mesotropical Subhumid, Mesotropical Humid, Supratropical Subhumid, Supratropical Humid, Supratropical Hyperhumid, Orotropical Subhumid, Orotropical Humid and Orotropical Hyperhumid). Each one has its corresponding vegetation belt, with emphasis in the structure of its potential natural vegetation, its main bioindicators and its catenal distribution. These results are complemented by distribution maps of the bioclimates, thermotypes and ombrotypes present, and by three vegetation transects recognised along both routes made at different latitudes. Keywords: Bioindicators, distribution, isobioclimates, México, Sierra Madre Occidental.

Abreviations used AGS: Aguascalientes. CHIH: Chihuahua, CONABIO: Nacional Comision for the Knoweldege and Use of the Biodiversity. DGO: Durango. GIS: Geographical Information System. INIFAP: Instituto Nacional de Investigaciones Forestales y Agropecuarias. JAL: Jalisco. IUCN: International Union for the Natural Conservancy. NAY: Nayarit. SIN: Sinaloa. SMO: Sierra Madre Occidental. SON: Sonora. WGS: World Geodesic System ZAC: Zacatecas. Introduction The science of bioclimatology is based in the formulation of fundamentals that relate climate to the distribution of plants and their plant communities; in this way, the index and parameters used are related or delimited by living organisms and, particularly by the plants and vegetation (Müller, 1982; Breckle, 2002). Climatic dates and their parameters and index, as well as floristic and phytosociological data, are very useful tools in the analysis of those links, and allow one to draw the biogeogra-

phic boundaries in relatively homogeneous floristic territories (Tuhkanen, 1980). Over recent decades this has motivated a remarkable advance in the development of Bioclimatology. On a continental and regional level, the macrobioclimate is the main environmental regulatory factor of the distribution of global vegetation (Larcher, 2003), while the edaphic or geomorfologic factors play a secondary role. Among the most widely used approaches to relate climate and vegetation is the Walter model, with its concept of zonobioma (Breckle, op cit.); other approaches deriving from it have been proposed by Bailey (1995, 1996), Brown et al. (1998), Olson et al. (2001) y Schultz (2005). At a regional level the considerations obtained from climatic models of Gaussen (Walter & Lieth, 1960-67; Lieth et al. 1999; Rivas-Martínez, 2004, 2008), have demonstrated great effectiveness in discriminating macroclimates and bioclimates, and in relating climate and vegetation accordingly, to establish the physical parameters (thermotypes and ombrotypes). The ever-more detailed knowledge of the distribution and composition of the vegetation, along with the availability of climate data and computer tools for management and

Corresponding author: Joaquín Giménez de Azcárate. Higher Politecnic School. University of Santiago de Compostela. Spain, e-mail: Joaquin.gimenezdeazcarate@usc.es. Telf. 34 98282362 Fax: 34 982285887 ISSN: 2253-6302 (print)/ISSN: 2253-6515 (on line) ©Editaefa DOI: 10.5616/ijgr 130002

20 J. Giménez de Azcárate , M.Á. Macías Rodríguez & F. Gopar Merino

ordering, provide criteria and objectivity to the bioclimatic models and their relationship with the vegetation. This has led to bioclimatology being regarded as one of the foundations for classifying and defining the earth's ecosystems and its boundaries in a standardized, robust, predictive and practical way (Sayre et al. 2007). In addition it has been implemented in programs of study and conservation of habitats and biodiversity, in obtaining predictions for agricultural and forestry resources, and to determine future climate and vegetation scenarios (Rivas-Martínez, 2007). The geographical distribution and the orography of the Sierra Madre Occidental (afterwards referred to as SMO), favour the appearance of ecosystems with an origin and nature antagonistic as deserts, woodland, grasslands and shrublands. Its geographical position along western Mexico has determined the diversity and distribution of species and plant communities to act as a barrier to the flora and vegetation coming from high plateaux and both Coastal and Sonoran plains. It also functions as migratory corridor of the holartic species and plant communities coming from higher latitudes to Mesoamerica. Also note the remarkable importance of the endemic component in the region (Rzedowski, 1978; González-Elizondo et al. 2012), and its role as a source of environmental goods and services for society (Bye, 1995; Descroix et al. 2004). It also includes 32 of the 152 Prioriti Terrestrial Regions of México as defined by CONABIO institution (Arriaga et al. 2000). Among the dominant plant formations are pine-oak forests, adapted to a wide variety of habitats and possessing the highest floral diversity of Mexico (Rzedowski, op. cit.), encouraging the wealth of plant communities it hosts. In this sense the IUCN has recognized two hotspots of biodiversity within the SMO: the north of SMO and the upper basin of Mezquital river (Felger & Wilson, 1995; González-Elizondo, 1997). On the other hand SMO is a relevant territory for its biocultural heritage, linked to the body and vitality of the seven indigenous groups that live there (Bye, op. cit.; Boege, 2008). Studies on the vegetation and flora of the SMO correspond mostly to contributions made in particular territories of its geography; and of these the following are noteworthy: Lumholtz (1902), Gentry (1942), LeSueur (1945), White (1948), Gordon (1968), McVaugh (1987, 1989, 1992, 2001), Búrquez et al. (1992), González-Elizondo & González-Elizondo (1992), González-Elizondo et al. (1993), Laferrière (1994), Casas et al. (1995), Felger & Wilson (1995), Fisher et al. (1995), Martin et al. (1998), Reina et al. (1999), Lebgue Keleng (2002, 2005), Enríquez et al. (2003), Estrada et al. (2003), García-Arévalo & González-Elizondo (2003), García-Arévalo et al. (2004), Koleff et al. (2004), Vázquez-García et al. (2004), González-Elizondo et al. (2009, 2011), Reina & Van Devender (2005), García-Arévalo (2008), Mathiasen et al. (2008), Herrera et al. (2009), Aragón et al. (2010) and Van Devender et al. (2010). The study of GonzálezElizondo et al. (2012) stands out as an integrated synthesis of the vegetation throughout the SMO. In the field of cartography the studies showing the distribution of the vegetation units are: Anonymous (1973, 1997, 2002, 2007a); Rzedowski, 1978; Brown et al. 1998; GonzálezElizondo et al. 2007 and González-Elizondo et al. 2012.

The absence of geobotanical and bioclimatic studies in this territory has driven this study, the aim being to identify the bioclimatic belts and establish their relationship with the vegetation through the implementation of a Global Bioclimatics model (Rivas-Martínez, 2008, 2011a). Here we try to support a consistent, predictive and universal classification system of terrestrial ecosystems of SMO, with bioclimatology as the cornerstone for its description and systematization of differents scales (Sayre et al. 2008, 2009; Cress et al. 2009; Rivas-Martínez et al. 2011b). Background In North America the geobotanical contributions for ordering and describing the plant communities, based on bioclimatology and phytosociology, were first used almost 20 years ago (Peinado et al., 1994a, 1997a, 1997b; Rivas-Martínez, 1997; Peinado et al. 1998; Rivas-Martínez et al., 1999; Peinado et al., 2006, 2007, 2010, 2011). In México the geobotanical research that incorporates bioclimatology as a tool for geospatial analysis of ecosystems, have been developed in specific territories and had little impact. This is due to the criteria traditionally followed in the studies of vegetation, based on a physiognomyc and ecological diagnosis, supported by Köppen´s classification amended by García (Miranda & Hernández X., 1963; González-Quintero, 1974; García, 1973, 2004; González-Medrano, 2003; Anonymous, 2013), and to the taxonomic challenges surrounding a very diverse flora (Rzedowski, 1978, 1991; Mittermeier & Goettsch, 1992; Villaseñor, 2004). All of the above have prevented the adoption of a hierarchical and systematic proposal. Despite this, some studies have been carried out which contribute to the knowledge of the relationships among vegetation, distribution and climate, such as those carried out in Northwest México (Peinado et al. 1994b, 1995, 2008, 2010), Transmexican Volcanic Belt (Almeida et al., 1994, 2004; Escamilla et al. 1998, 2002; Giménez de Azcárate & Escamilla, 1999; Giménez de Azcárate et al., 1997, 2003; Giménez de Azcárate & Ramírez, 2004; Medina et al., 2012), Potosino Highplateau (Giménez de Azcárate & González-Costilla, 2011) and Yucatán Península (Barber & Crespo, 2001). Territory description The SMO is the longest mountain range in México starting at the border with Arizona and New Mexico (USA) to the north (30º 35' N) and running SSE to Jalisco where it meets with the Mexican Volcanic Belt (21º 00' N). It runs through the states of SON, CHIH, DGO, SIN, ZAC, NAY, AGS and JAL. The main peaks are Cerro Gordo (3,347 m), Barajas (3,310 m), Mohinora (3,307 m), Huehuento (3,262 m) and “Cerro de las Antenas” (3,224 m), all of them located in Durango except Mohinora located in the SW of Chihuahua; other minor summits and ranges have an altitude between 2500 m and 3000 m. The rivers from it drain into the Pacific Ocean and toward the Mexican Highplateau. In this last instance some streams flow to endorheic basins and others to the Mexican Gulf through the rivers Conchos and Bravo. Both sides of the watershed show a marked asymmetry; thus the western aspect is scoured by deep ravines that can reach 1,800 m in depth: the Urique, Cobre, Sinforosa Tamazula, San Lorenzo, Presidio, Ba-

Bioclimatic belts of Sierra Madre Occidental (México):A preliminary approach

luarte, Mezquital, Santiago and Bolaños being the most outstanding. Its eastern flank, however, presents a gradual transition to the Mexican Highplateau. The extent of the SMO coupled with its floral and physiographic complexity, has motivated this study concerning analysis of the territories belonging to the SMO which are located above 1,800m (Figure 1). This area covers 177,593 km2 and corresponds to 50.1% of the total area of the physiographic province of the SMO; this unit shares borders with the following provinces: “Llanura Sonorense” and “Planicie Costera del Pacífico” to the west, “Sierras y Llanuras del Norte” to the east and north, “Mesa del Centro” to the south-east, and “Eje Neovolcánico” to the south (Cervantes et al. 1990; CONABIO, 1997). The territories excluded from the present study are undergoing preliminary analysis and will be the subject of a future publication.

21

From a biogeographic point of view, the area considered belongs mostly to the floral province of SMO, which is included in the region “Mesoamericana de montaña”. This region is a transitional zone between the Holartic and Neotropical kingdoms, characterized by the convergence of boreal affinity woody flora, tropical affinity herbaceous flora and endemic elements. (Rzedowski, 1978, 1991, 1996; Rzedowski & Reina, 1990). The remainder area belongs to the Altiplanicie floral province (Xerofitica-Mexicana region). Following the biogeographic classification of RivasMartínez et al. (1999), the territory is included in the Madreana Occidental province (Madreana region); in specific areas of its periphery, taxa coming from the Mexicano-Xerofítica region, and particularly in the northern area of study from the Gran Cuenca region, are present.

Figure 1: Location of the study area

The factors that determine the climatic traits and the diversity and distribution of the climates in the territory are conditioned by the geographical position between the subtropical high pressure belt and the intertropical convergence zone (latitudinal bands eutropical and subtropical); the exposure to the entry of warm and humid Pacific ocean winds; the broad altitudinal and latitudinal range; and the physiographical complex that determines an asimmetry of slopes due to the effect of the orographic barrier. All of this determines the thermic regime, with average monthly values getting a low or medium fluctuation, and ombric with a markedly summer character determined by the convective mass causing the socalled "Mexican Front", along the western aspect of the SMO (Mosiño-Alemán & García, 1974; Douglas et al. 1993). Moreover, the climate is going to be affected by tropical storms and hurricanes occurring in the Pacific toward the end of summer and autumn (Hastings & Turner, 1965). The rough slopes on the western side play a decisive role in the range of precipitation and temperature. These climatic features coupled with its biogeographic position between the Neartic and Neotropical

kingdoms (Rzedowski, 1978), make the region of study an exceptional location to put into practice the methodology of bioclimatic analysis. Methods The diagnosis has followed the proposal of Global Bioclimatic classification system (Rivas-Martínez, 2007, 2008; Rivas-Martínez et al. 2011a) based on the reciprocal relationship between the values of climate models and the distribution of vegetation, explained through the altitudinal zonation of the bioclimatic belts. The system includes basic aspects related to the limitations that the climate creates on vascular plants and plant communities. The basic parameters and index used in the analysis have been: Ti, mi, Mi, m, M, Tp, Pi, Pp, It, Io, Iod2 and Ic; their definitions can be found in previous papers. As climatic information was taken into account, data coming from meteorological stations located in the area of study and published in the Basic Climatological Statistics of INIFAP for the states of: AGS (Medina et al., 2006b), CHIH (Medina et al., 2006a), DGO (Medina et


al., 2005), JAL (Ruiz et al., 2003); SIN (Ruiz et al., 2005) and ZAC (Medina & Ruiz, 2004). NAY and SON have not been considered because their meteorological stations are located below the benchmark followed in this study. To complete the territorial representation information from others stations available in the National Meteorological Service of Mexico (Anonymous, 2011) was incorporated. In total there were 159 stations whose breakdown by states is: AGS 14, CHIH 48, DGO 46, JAL 7, SIN 1 and ZAC 43. For each one, in addition to considering its geographic location, basic climate information averaged monthly from the different parameters of precipitation and temperature was used. Based on this information the values of the different bioclimatic index were obtained using the Bioclima program (Alcaraz, 2013) which generates the diagnosis and the bioclimogram for each station. These results were supplemented with those obtained in the field trips carried out during the years 2009, 2010 and 2011, in which the climatevegetation relationships were checked and adjusted considering the dynamic-catenal phytosociological concepts (Rivas-Martínez, 2005). The field work was based on the selection of representative and well preserved patches of natural potential vegetation from all over the territory. Also the recommendations of Beard (1973) were taken into account focused on the analysis of the structure and the physical appearance of the vegetation, and on the presence of forest indicator species. In addition, when complexity of flora allowed it, phytosociological releves were carried out (Braun-Blanquet, 1979; Westhoff & van der Maarel, 1980), in order to accurately back up the field work. In a complementary way three transects of potential natural vegetation were established along the range (Box, 1981; Rivas-Martínez, 2007). For floristic determinations, regional and taxonomical floras were consulted, as well as floristic lists (Farjon et al. 1997; González-Elizondo et al. 1991, 2007; GonzálezElizondo & González-Elizondo, 1995; García-Arévalo & González-Elizondo, 2003; González-Villareal, 1986, 1990; McVaugh, 1987, 1989, 1992, 2001; Vazquez-García et al. 2004). The Tropicos database (2013) was used as the nomenclatural reference model. The spatial context used in the description of the vegetation units was based on geographical and physiographical aspects (Cervantes et al. 1990; CONABIO, 1997). To carry out the mapping, the information included in the Digital Climatic Atlas of México (version 2.0) was used for reference; monthly and annual averages of maximum temperature, minimum temperature and precipitation were considered (Hijmans et al. 2005; Fernández-Eguiarte et al., 2012). The layers of climatic information are available in raster format with pixels around 1 Km2. The period considered was 109 years (1902 2012). These layers were analyzed in a GIS (ArcGIS 9.3) thus obtaining the bioclimatic index values. Based on these results and through the process of algebraic mapping, a raster series with the different values obtained for each pixel was made, which gave rise to the corresponding maps of bioclimates, thermotypes and ombrotypes. The overlap of these layers allowed us to identify the isobioclimates present in the territory. To homogenize the information contained in the raster layers, these were

converted in vector format giving the corresponding polygons. The maps were made to a scale 1:500,000 and a Lambert conical projection and WGS-84 datum were considered. The minimum scale for cartography was 2 mm2 on the map, so those polygon areas smaller than 100 Ha were eliminated. The bioclimatic characterization was supplemented with the analysis of potential natural vegetation. For this, the data collected in the field and the vegetation and land use vegetation map (Anonymous 2007b) were taken into account. From this last map only the types related to climatophylous vegetation were considered, in order to link them with their corresponding bioclimatic belt. It also took into account the descriptions of vegetation related in literature (RzedowskI 1978; González-Elizondo et al. 2007, 2012). Finally bioclimatic contributions reported in neighbouring territories were considered (Peinado et al. 1994b, Rivas-Martínez et al. 1999; Macías, 2009; Peinado et al. 2010, 2011; Giménez de Azcárate & González-Costilla, 2011). Results This diagnosis places the SMO within the macrobioclimate Tropical with the presence of a marked rainy season (June to October) coinciding with the warmer half of the year. With regard to bioclimate, are represented the Tropical Xeric (1 ≤ Io ≤ 3.6) and the Tropical Pluviseasonal (Io ≥ 3.6; Iod2 ≤ 2.5); its distributions are showed in Map 1. Within the first, five bioclimatic belts are recognized (in brackets the number of stations assigned): thermotropical dry (0), mesotropical semiarid (7), mesotropical dry (90), supratropical semiarid (0) and supratropical dry (7). With respect to the Bioclimate Tropical Pluviseasonal presents five bioclimatic belts: thermotropical subhumid (0), mesotropical subhumid (8), mesotropical humid (1), supratropical subhumid (21), supratropical humid (18), supratropical hyperhumid (2), orotropical subhumid (0), orotropical humid (5) and orotropical hyperhumid (0). The presence of bioclimatic belts without representative meteorological stations is justified on the basis of the results of the extrapolations made for map making and the analysis of the vegetation. The distribution of thermotypes and ombrotypes is reflected in the maps 2 and 3 respectively. Supratropical subhumid and Supratropical humid are the bioclimatic belts better represented within Tropical pluviseasonal bioclimate, mainly along the highlands of SMO. Within Tropical xeric boclimate, the more representative belt is Mesotropical dry, mainly toward the southeastern portion (ZAC); also Supratropical dry has an important presence in the northeastern area, bordering the Chihuahuan Desert. The Thermotropical and Orotropical thermotypes, and the Arid and Hyperhumid ombrotypes are both virtually unrepresented due to problems of scale. The correspondence between bioclimates (isobioclimates) and the 15 types of climatophilous vegetation types, selected from INEGI Serie IV (Anonymous 2007b), is showed in Table 1. Other vegetation types like edaphoxerophylous, hygrophilous or secondary vegetation types, are not considered within the table, as they are not condicioned by weather, but by other factors such as the lithological, soil nature or anthropical influence.


Map 1: Bioclimates of Sierrra Madre Occidental

Map 2: Thermotypes of Sierrra Madre Occidental

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ISOBIOCLIMATES



x

Supratropical Hyperumid

Orotropical Hyperumid

Supratropical Humid

Mesotropical Humid

Orotropical Subhumid

x

Bosque de Cedro

Orotropical Humid

Bosque de Ayarín

Supratropical Subhumid

Mesotropical Subhumid

TROPICAL PLUVISEASONAL Thermotropical Subhumid

Supratropical Dry

Mesotropical Dry

Thermotropical Dry

Supratropical Semiarid

VEGETATION TYPE

Mesotropical Semiarid

TROPICAL XERIC

x



x



X

Bosque de Encino

x

x

x





x



x

x



x

x

Bosque de Encino-Pino

x

x

x

















x



x



x





































Bosque de Oyamel Bosque de Pino

x

x



Bosque de Pino-Encino

x

x







Bosque de Tascate

x

 

Bosque Mesófilo de Montaña 



x

x



x

x



x

x

Chaparral

x

Matorral Crassicaule





Matorral Desértico Micrófilo



Matorral Desértico Rosetófilo



Selva Baja Caducifolia

x

Selva Baja Subcaducifolia

x







x

x x

x x



x

x

x



x

x

Table 1. Correspondence among vegetation types selected of INEGI (Anonymous 2007b) and the isobioclimates recogniced. The intersections ✔ link the vegetation types with the isobioclima (s); if that intersection is marked with x, shows that the vegetation type has an azonal or marginal nature.

For each isobioclimate recognized a selection of meteorological stations with its corresponding diagnosis is shown in Table 2. In this regard Figure 2 shows six bioclimatic diagrams of some of these stations; a complete diagnosis of which is presented in Table 3. Correspondence with vegetation belts The extent of the area and its geomorphological and physiographic complexity condition the biota growing there, which also presents an enormous taxonomic and syntaxonomical difficulties. Therefore, the proposition and description of the phytocoenoses that correspond to each situation is very complicated and is outside of the scope of this paper. Nevertheless, taking into account the bioclimates existing in the study area (Tropical xeric and Tropical pluviseasonal) and their bioclimatic belts, the more representative types of potential natural vegetation have been defined and characterized, without going into the peculiarities of the phytocoenoses hosting. For this reason the diagnosis is focused on the dominant structure and physiognomy, its main bioindicators, its distribution and catenal position; when there was information available, dynamic and ecological aspects are discussed in order to complement the analysis. It should be added that a number of the bioindicators recognized for each belt are not present in the whole of it, especially in the largest ones; also some of those bioindicators are able to fluctuate towards neighboring horizons by physiographic and edaphic compensatory accommodations.

Tropical Xeric Bioclimate Thermotropical Dry It is distributed mainly below the benchmark considered here (1,800 m), although it can extend above this level when local physiographic and climatic conditions are favourable, as in the case of the warmest and most sheltered ravines. The ombrotype Dry is the most widespread, and is located on the lower slopes of the western of SMO, as well as in the deeper intermontane ravines crossing it (Yaqui, Mayo, Fuerte, Mezquitic, Acaponeta, Chapalagana, Bolaños, etc); through these ravines its associated woodland may ascend to around the 2,000m level. (González-Elizondo et al. 2012). The structure of the potential vegetation is a plurispecific deciduous microforest, tropical deciduous forest sensu Rzedowski (1978). Its floral composition shows no clearly dominant species, being shared among the representative bioindicators of this bioclimatic belt: Amphipterygium adstringens, Bursera benthamii, B. fagaroides, B. graveolens, B. multijuga, B. schlechtendalli, Cedrela odorata, Ceiba acuminata, C. aesculifolia, Cochlospermum vitifolium, Cordia alliodora, Haematoxyllum brasiletto, Leucaena esculenta, L. lanceolata, Lonchocarpus spp., Lysiloma spp., Pachycereus pecten-aboriginum, Pseudobombax palmeri, Stenocereus queretaroensis, S. turberi, Tabebuia impetiginosa, etc. Disruption of this forest leads to the development of a replacement plant community which holds some of the previous trees and secondary species like Erythrina flabelliformis, Guazuma ulmifolia, Ipomoea arborescens, I. murucoides, Lysiloma acapulcense, Plumeria acutifolia, Tecoma stans, Vachellia spp


Figure 2: Bioclimatic diagrams of six representative stations of the isobioclimates recogniced: Campo nº 5 (CHIH), La Ciudad (DGO), Vascogil (DGO), Cuahutemoc (CHIH),

25


Figure 2 (cont.): Bioclimatic diagrams of six representative stations of the isobioclimates recogniced: Malpaso (ZAC) and El Palmito (SIN). The steepness and irregularity of the thermotropical areas favours the contact with the different Pinus spp. and / or Quercus spp plant formations found in the upper bioclimatic belt (Mesotropical), which is reflected in contrasting mosaics of vegetation. Sometimes these forests can appear in the thermotropical belt, where they are linked to exceptional soil conditions such as stony outcrops, nutrient poverty and abundance of heavy metals (Penington & Sarukan, 2005; Macías, 2009). As we further away from the SMO to the Chihuahuan or Sonoran Deserts the aridity increases leading to the presence of Arid and Semiarid ombrotypes (Rivas-Martínez et al.1999; Peinado et al. 2010). Mesotropical Semiarid Its location is restricted to the driest places in the intramountain valleys and eastern foothills of the adjacent physiographical subprovinces: “Sierras y Llanuras de Durango”, “Gran Meseta y Cañones Duranguenses”, “Mesetas y Cañadas del Sur” and “Sierras y Valles Zacatecanos”. Its potential vegetation corresponds to brush communities abundant in microphyllous and thorny species, growing mainly on well-drained alluvial soils. As representative bioindicators, Celtis iguannea, Eysendhardtia polystachya, Flourensia cernua, Fouquieria splendens, Forestiera angustifolia, Koeberlinia spinosa, Lantana camara, Larrea tridentata, Prosopis laevigata, Yucca decipiens, Y. torey and Y. rigida, can all be recognised. Toward the more southern and warmer areas the appearance of Bursera fagaroides, B. bipinnata, Ipomoea intrapilosa and I. murucoides is more noticeable. Its alteration or transformation by livestock rearing favours the domain of brush with mimosas (Mimosa aculeticarpa, Mimosa biuncifera, M. dysocarpa, M. monancistra), acacias (Vachellia constricta, V. neovernicosa, V. schaffneri), magueyes (Agave asperrima, A. lechuguilla) prickly pears (Cylindropuntia imbricata, Opuntia duranguensis, O. leucotricha, O. streptacantha) and sotoles

(Dasylirion duranguense), in addition to numerous grasses (Aristida spp., Bouteloua spp. Chloris gayana, Cynodon dactylon, Megathyrsus maximus, Melinis repens, Pennisetum ciliare, etc). These grasses are present as well in other belts of Xeric bioclimate. In the flat valleys with deep alluvial soils, the potential plant formation prevailing is a thorny and phreatophilous microforest dominated by Prosopis laevigata, which is often found alongside Cercidium praecox, Eysenhardtya polystachya, Ipomoea spp., and Vachellia spp., among other species. Mesotropical Dry It extends to almost all the physiographic subprovinces of SMO, especially in the intramontaine valleys, in the eastern slopes and in their adjacent plains. Its potential vegetation is made up of different semi-open microforest dominated by sunshaded and stunted evergreen or deciduous oaks (Quercus chihuahuensis, Q. eduardii, Q. emory, Q. grisea, Q. radiata, Q. resinosa, Q. viminea, etc. ), which often incorporates other trees and shrubs, sometimes with a secondary role, as Juniperus deppeana, J. duranguensis, J. erythrocarpa, J. flaccida, Opuntia spp., Lycium berlandieri, Rhus microphylla, R. virens, Vachellia constricta, V. pennatula, V. schaffneri, etc. At its upper limit frequently we find the presence of pines (Pinus leiophylla var. chihuahuana or P. cembroides mainly), that came from Supratropical Dry belt. At its lower limit, bordering the intramontaine dry ravines, may appear species suited to thermophillous conditions which grow up from the bottom of the valleys, like Bursera copalifera, B. fagaroides, B. multijuga, Croton sp., Ipomoea intrapilosa, I. murucoides, Lysiloma divaricatum or Randia sp. among others species. In habitats altered by fire or overgrazing a secondary thicket of Dasylirion duranguensis and Dodonaea viscosa has expanded, whose extension has been increasing in recent years aided by this degraded environment.


Meteorological stations

Parameters and indexes

27

Isobioclimate

Alt

T

P

It

Io

Bioclimate

Thermotype

Ombrotype

Malpaso (ZAC)

2135

16.9

367.5

418

1.82

TrXe

Mtr Low

Sar Up

Santa Anita (CHIH)

1891

16.7

381.1

324

1.92

TrXe

Mtr Up

Sar Up

Venadero (AGS)

2026

17.8

538.7

444

2.52

TrXe

Mtr Low

Dry Low

Balleza (CHIH)

1920

17.8

491.0

377

2.30

TrXe

Mtr Up

Dry Low

Durango (DGO)

1885

17.1

475.6

390

2.32

TrXe

Mtr Up

Dry Low

Canatlán (DGO)

2000

15.8

536.0

363

2.82

TrXe

Mtr Up

Dry Up

Aguazarca (AGS)

2417

15.5

582.0

388

3.13

TrXe

Mtr Up

Dry Up

Cd. Cuauhtémoc (CHIH)

2010

14.2

478.1

280

2.80

TrXe

Str Low

Dry Low

Rosario (DGO)

1800

15.1

458.3

311

2.53

TrXe

Str Low

Dry Low

Monte Escobedo (ZAC)

2190

15.4

719.2

370

3.88

TrPs

Mtr Up

Shu Low

El Palmito (SIN)

1875

16.6

1181.6

428

5.92

TrPs

Mtr Low

Shu Up

El Cantil (DGO)

2035

15.1

1469.4

368

8.11

TrPs

Mtr Up

Hum Low

Otinapa (DGO)

2400

12.9

666.0

271

4.31

TrPs

Str Low

Shu Low

Madera (CHIH)

2092

10.7

734.6

168

5.70

TrPs

Str Up

Shu Up

Guachochi (CHIH)

2420

10.2

780.0

185

6.36

TrPs

Str Up

Hum Low

Vascogil (DGO)

2400

11.3

1366.8

236

10.05

TrPs

Str Up

Hum Up

La Ciudad (DGO)

2580

10.0

1531

208

12.83

TrPs

Str Up

Hhu Low

O. de Camellones (DGO)

2180

10.7

1603.0

219

12.52

TrPs

Str Up

Hhu Low

Campo No. 5 (CHIH)

2700

9.5

978.7

144

8.65

TrPs

Otr Low

Hum Low

Tres Ojitos (DGO)

2600

8.4

981.0

134

9.76

TrPs

Otr Low

Hum Up

Table 2: Meteorological stations with their parameters associated, bioclimatic indexes and diagnosis. Acronyms used: Alt: Altitude (meters). T: Average annual temperature (° C). P: average annual precipitation (mm). It: Thermicity index. Io: Annual ombrothermic index. TrXe: Tropical Zeric. TrPs: Tropical Pluviseasonal. Mtr: Mesotropical. Str: Supratropical. Otr: Orotropical. Sar: Semiarid. Shu: Subhumid. Hum: Humid. Hhu: Hyperhumid. Low: Lower. Up: Upper


Station Index Campo N° 5

La Ciudad

Vascogil

Cd. Cuahutémoc

Malpaso

El Palmito

M

8.4

14.2

14.6

16.3

21.3

18

m

-3.4

-3.3

-2.3

-2.3

3.7

8.2

It

144

208

236

280

418

428

Ic

14.1

8.7

9.9

14.2

7.9

6.2

Id

15

19.8

19.6

20.6

18.3

11.9

Io

8.65

12.83

10.05

2.8

1.82

5.92

Ios1

4.3

13.3

8.2

2.0

2.3

6.3

Ios2

8.48

19.6

13.34

4.05

3.64

10.45

Ios3

9.58

20.69

14.15

4.66

3.69

11.25

Ios4

7.83

16.69

11.57

3.88

2.88

8.61

Tp

1131

1193

1359

1704

2023

1995

Ts

479

419

477

614

589

563

Pp

978

1531

1366

478

367

1181

EuOc Up

ShOc Up

ShOc Up

EuOc Up

EuhOc Low

EuhOc Low

Cont

Table 3: Bioclimatic diagnosis of the six stations of reference selected. Acronyms used: M: Average temperature of the maximums of the coldest month. m: Average temperature of the minimums of the coldest month. It: Thermicity index. Ic: Simple continentality index. Id: Diurnality index or daily thermal interval. Io: Annual ombrothermic index. Ios1: Ombrothermic index of the hottest month of the summer quarter. Ios2: Ombrothermic index of the hottest two month of the summer quarter. Ios3: Ombrothermic index of the summer quarter. Ios4: Ombrothermic index of the four month period resulting from adding the summer quarter and the month inmediately preceding it. Tp: Positive annual temperature. Ts: Average temperature of summer quarter. Pp: Positive annual precipitation. Cont: Continentality. EuhOc: Euhyperoceanic ShOc: Subhiperoceanic. EuOc: Euoceanic. Low: Lower. Up: Upper.

Supratropical Semiarid It has a very small area, restricted to the physiographic subprovince of “Sierras y Llanuras Tarahumaras”, in the northeastern SMO. It is usually found on underdeveloped soils characterised as xerosoils, responsible, together with the climate, for the xerophytic character of the vegetation. Its potential vegetation is dominated by rosetophyllous and microphyllous scrublands, which sometimes to go with crasicaule species, most of them endemics of the Chihuahuan Desert. The height of these shrubs ranges between 1 and 3m, with variable densities as a function of grazing intensity. These plant formations are dominated by Agave lechuguilla, Cercocarpus montanus, Euphorbia antisiphylitica, Garrya wrightii, Larrea tridentata, Opuntia spp. Parthenium argentatum, Prosopis sp., Quercus depressipes, Vachellia spp. and Yucca carnerosana. The herbaceous stratus is composed of grasses, mainly of Bouteloua genus. Supratropical Dry It roughly corresponds with the area of mid mountain located in the northern portion of SMO with a marked effect of rain shadow because of its location to leeward of the humid winds. It is distributed discontinuously throughout the subprovinces of “Sierras y Cañadas del Norte”, “Sierras y Llanuras Tarahumaras”, “Gran Meseta y Cañones Chihuahuenses”, “Sierras y Llanuras de Durango”, “Gran Meseta y Cañones Duranguenses” and “Mesetas y Cañadas del Sur”. Its potential plant formations are preferably formed by mixed coniferous and deciduous (oak) microforests; sometimes a particular

group of them can dominate the plant formation. The most conspicuous indicator elements are: Garrya wrightii, Juniperus deppeana, Pinus cembroides, Quercus chihuahuensis, Q. emoryi and Q. grisea. Towards its upper horizon the presence of Arbutus arizonica, Cupressus arizonica, Pinus engelmannii, P. leiophylla var. chihuahuana, Quercus arizonica, Q. eduardii, Q. durifolia or Q. laeta becomes more noticeable. As serial elements associated with the disturbance by livestock Forestiera angustifolia, Lindleya mespiloides, Mimosa aculeaticarpa, M. biuncifera, Quercus eduardii and Vachellia schaffnerii usually occur, as well as different grasses. Tropical pluviseasonal Thermotropical Subhumid Its presence is restricted to small scattered areas of the Pacific slope (subprovince of “Pié de la Sierra”) in the states of Sinaloa, Nayarit and Durango, bordering on the Thermotropical dry belt, which has a wider area of distribution. Its potential vegetation consists of subdeciduous mesoforests dominated by Astronium gravelolens, Brosimium allicastrum, Bursera simarouba, Enterolobium cyclocarpum, Swietenia humilis among others. Occasionally appear elements from pine-oak forest making up patches more or less pure with the previous species; between them can be presents Pinus devonian, P. duouglasiana, P. herrerae, P. lumholtzii, P. luzmariae, P. maximinoi, P. oocarpa, and other species such as Arbutus madrensis, A. tesallata, A. xalapensis, Clethra rosei, Quercus crassifolia, Q. praineana, Q. resinosa,


etc. The floristic composition and the proportion of trees changes depending on various environmental factors such as orientation, altitude, soil type, etc. The alteration of these potential forests favours the appearance of secondary deciduous forest with elements coming from Thermotropical dry belt and its serial stages, taking advantage of the new xeric and heliophilous conditions for its development. Mesotropical Subhumid This belt is present extensively throughout the study area, mainly occupying mid-mountain areas both on the western slopes of the range and on the internal slopes without a marked effect of rain shadow. This broad distribution is the reason that the potential vegetation corresponds with different mixed conifer and oak mesoforests which often incorporate other broadleaf trees. As important bioindicators of the belt are: Arbutus xalapensis, Bocconia arborea, Juniperus deppeana, Pinus devoniana, P. engelmannii, P. lumholtzii, P. luzmariae, P. oocarpa, Quercus coccolobifolia, Q. gentryi, Q. magnoliifolia, Q. oblongifolia, Q. resinosa, Q. rugosa and Q. viminea. The replacement serial brush incorporates some thorny elements in its lower horizon (Vachellia spp., and Mimosa spp.), shrubs of Compositae plant family like Baccharis, Eupatorium, Roldana, Senecio, Stevia, Verbesina, and ericaceaous shrubs as Arctostaphyllos pungens, Befaria mexicana and Vaccinium caespitosum. Many of these species are often introduced into the undergrowth showing indications of disturbance, mainly associated with the movement of livestock and fire damage. Mesotropical Humid Its distribution area is restricted to specific patches of the physiographic subprovinces “Gran Meseta y Cañones Duranguenses” and “Mesetas y Cañadas del Sur”, located around 2,000 masl in ravines and steep slopes facing the humid winds and therefore with high precipitation records. The potential vegetation corresponds with mixed subperennial macroforests. The Mesotropical lower horizon are dominating by mesophytic forests, where are frequent Alnus acuminata, Arbutus xalapensis, Brahea aculeata, Carpinus caeroliniana, Cedrela odorata, Cleyera integrifolia, Ceanothus depressus, Clethra spp., Cornus disciflora, Garrya laurifolia, Ilex quercetorum, Litsea glaucescens, Magnolia pacifica ssp. tarahumara Oreopanax xalapensis, Ostrya virginiana, Oreopanax spp., Persea liebmannii, Tilia americana, and pines and oaks as Pinus herrerae, P. maximinoi, P. oocarpa, Quercus candicans, Q. castanea, Q. diversifolia, Q. obtusata, Q. splendens and Q. subespathulata. Toward the upper horizon some of these species, especially the most thermophillous, tend to disappear; at the same time as other species appear such as Abies neoduranguensis, Clethra rosei, Pinus ayacahuite, P. douglasiana, P. pseudostrobus, Quercus scytophylla and Styrax ramirezii, acquiring the plant formation more typical of pine-oak forests. The alteration of these forests boost the development of a dense border substitution brush with Cercocarpus macrophyllus, Coriaria ruscifolia, Monnina wrightii, Prunus spp., Rhamnus betulifolia, Rhus aromatica, Ternstroemia lineata, Triumfetta discolor, Verbesina spp. and Waltheria indica.

29

Supratropical Subhumid Its distribution area include most of the physiographic subprovinces, with the exception of the most southern ones, at lower altitude. It extends along the high mountain areas in the lee of the wet Pacific winds. The potential vegetation corresponds mainly with different pine and oak mixed mesoforests, whose more noticeable species are Arbutus arizonica, A. madrensis, A. tessellata, Garrya ovata, Juniperus deppeana, Pinus engelmannii, P. leiophylla, P. teocote, Quercus arizonica, Q. castanea and Q. durifolia. In topographically favorable locations, such as hydromorphic plains, Pinus arizonica var. cooperi is often incorporated and becomes dominant in this biotope. When its undergrowth and contiguous deforested areas are affected by fire, a replacement bunchgrass, zacatonal, is settled; It is constituted by grasses of the genus Aristida, Bouteloua, Bromus, Festuca, Stipa and Muhlenbegia. In places with shallow soils or rocky outcrops are characterised by thickets dominated by Arctostaphylos pungens. Supratropical Humid and hyperumid Both types show a similar distribution to the previous belt, although they occupy smaller areas, mainly associated with ravines strongly influenced by the wet winds; this explains the high rainfall records, particularly in the hyperhumid ombrotype. Its potential vegetation is constituted for different conifer macroforests, pures or mixed, of Abies duranguensis, Hesperocyparis lusitanica var. lindley, Picea chihuahuaza, Pinus spp. and Pseudotsuga menziesii; at the lower horizon, some oaks make an appearance. The main bioindicators of these supratropical environments, as well as the previous species, are Arbutus bicolor, A. madrensis, Pinus arizonica var. cooperi, P. ayacahuite, P. leiophylla, P. duranguensis, P. strobiformis, Populus tremuloides, Prunus serotina, Quercus crassifolia, Q. macvaughii, Q. scytophylla and Q. sideroxyla. Abies duranguensis and Picea chihuahuana dominate the sciophyllous biotopes covering the wetter ravines and slopes of the range, belonging to hyperumid ombrotype. The replacement fringes of these forests are dominated by Asteraceae and Labiatae bushes with an emphasis on genus like Eupatorium, Hyptis, Roldana, Salvia, Stevia and Senecio. Orotropical Subhumid Its distribution is restricted to patches on the highest areas of the northeastern SMO, in the physiographic subprovince of “Sierras y Valles del Norte”, where there is a noticeable decrease in the influence of humidity from the Pacific Ocean, and a prevalence of xeric conditions typical of the western edge of Chihuahuan Desert. The potential vegetation corresponds with pure forests of Pinus spp. or mixed forests adding Quercus spp. and Juniperus deppeana. The main representative species encountered are Pinus arizonica, P. chihuahuana and P. teocote, some oaks like Quercus arizonica, Q. depressipes and Q. laeta, and arbutus, Arbutus bicolor. The lack of meteorological stations, coupled with the marginal geographical situation of this belt, hampers the interpretation of data and the establishment of clear criteria that would differentiate it from the other two orotropical belts found on the main summits.


Figure 3: Transect of potential natural vegetation and its relationship with thermotypes along the itinerary between Yécora (SON), Cerro Mohinora (CHIH) and Ciudad Cuahutémoc (CHIH) at 27ºN and 29ºN. 1). Thermo- and low Mesotropical,

Figure 4: Transect of potential natural vegetation and its relationship with the thermotypes along the itinerary between El Palmito (SIN), Espinazo del Diablo (DGO), Cerro Huehuento (DGO) and Durango City (DGO) at 23ºN and 25º N. 1).

Figure 5: Transect of potential natural vegetation and its relationship with thermotypes along the itinerary between Mesa del Nayar (NAY), Río Chapalagana (JAL), Siera de Cardos (ZAC) and Zacatecas city (ZAC) at 21ºN and 22ºN. 1).


Orotropical humid and hyperhumid Both belts are restricted to the highest peaks of the range, mainly over 3,000 m, as well as some high land located at the northern end of SMO in the state of CHIH. These enclaves are scattered throughout the physiographic subprovinces of “Sierras y Cañadas del Norte”, “Gran Meseta y Cañones Chihuahuenses”, “Gran Meseta y Cañones Duranguenses” and “Mesetas y Cañadas del Sur”. Its potential vegetation consists of different conifer meso-macroforests with sporadic incidence of broadleaf trees. The main biomarkers are Arbutus bicolor, Pinus ayacahuite, P. arizonica var. cooperi, P. teocote, Pseudotsuga menziesii, Quercus crassifolia, Q. depressipes and Q. sideroxyla. The harshness of the climate in these areas, along with the stony and poorly developed soils, leads to forest communities which are not very diverse and often show signs of fire damage, fallen and dead trees etc. This accelerates the open aspect of these forests and the abundance of herbaceous plants, mostly of Gramineae and Compositae families, (typical of this next stage) owns of the serial stages, as Bouteloua, Draba, Muhlenbergia, Poa, Primula, Sedum and Senecio, among others. In edaphoxerophyllous locations, around some of the summits there is a scrub community dominated by Quercus depressipes, Arctostaphylos pungens and Helianthemum glomeratum. As an integral synthesis of the previous results, three transects are displayed showing both gradients, latitudinal and altitudinal. These illustrate the arrangement of the bioclimatic belts and their corresponded potential natural vegetation, to agree with data, interpretations and extrapolations realised (Figures 3, 4 and 5). The footnotes describe the vegetation types related within each bioclimatic belt and their dominating byotipes. The asymmetry in the limits of thermotypes in western and eastern slopes are due to the moderating and refreshing action of the humid winds coming from the Pacific coast. Discussion and conclusions The location and range of the SMO determines its funcion as a biological corridor, especially for holartic affinity flora, which along with the tropical component, presents towards the foothills, has favoured a high floral and phytocenotic diversity. The large physiographic and climatic complexity has contributed greatly to the diversity and originality of its biota, and therefore its consideration as a well distinct ecoregion. From the bioclimatic point of view all the territory is Tropical, distinguishing between the Tropical Pluviseasonal and Tropical Xérico bioclimates; the former has greater distribution throughout of the range´s highlands and the western flank, where the better represented thermotypes and ombrotypes are Mesotropical, Supratropical and Orotropical, and the Subhumid, Humid and Hyperumid, respectively. The potential natural vegetation of these belts is related to different forests dominated by Pinus spp. and Quercus spp.; the preceding taxa reliably combine with other conifers (Abies, Hesperocyparis, Juniperus, Picea, Pseudotsuga). The Tropical Xeric bioclimate shows a more restricted distribution toward the eastern flank and the dry mountain ravines, where the floral influence from the Chihuahuan Desert increases. The more prevalent ther-

31

motypes are Mesotropical and Supratropical, the Dry ombrotype being the most abundant. The potential vegetation is made up by forests and sclerophyllous bushes of Quercus spp. and Pinus spp. that can incorporate xerophitic taxa of Chihuahuan affinity (Lycium, Prosopis, Opuntia, Vachellia, Yucca) or of Neotropical Pacific affinity (Ceiba, Ipomoea, Bursera, Lysiloma).The structure, the size and the composition (family and genus) of the potential natural vegetation encountered along the range (coniferous forests, oak forests mixed forests and shrublands), fit the bioclimatic belt model in a true, recurrent and predictable way, and they are in tune with those identified in other analogous tropical territories. The meteorological stations (159) cover the territory in an heterogeneous and uneven manner. Most of them are sited between 1,800 m and 2,500 m in altitude. This did not prevent the completion of the bioclimatic diagnosis of the higher areas, through the extrapolations and algebra of maps made in the mapmaking, and the identification of the vegetation belts made in the field. In any case an adequate reference was obtained and adjusted to the scale of the work and the territory, based on the reciprocal relationships between the bioclimatic diagnosis and its corresponding climatophyllous vegetation. The territorial division of the bioclimatic units obtained, bioclimas, thermotypes and ombrothypes (Maps 1, 2 and 3), reflects greater precision in the areas surrounding the stations, whilst in remote areas without stations and with access problems, the adjustment is less accurate. It is in these areas that new field works needs undertaken to check the relationships between the bioclimatic extrapolations and the vegetation shown here. Moreover it should be considered that the scale used is determined by the extent of the territory. Thus in those territories without reference stations, the bioclimatic diagnosis is more strongly supported by the analysis of the corresponding potential vegetation. In any case the cartography obtained, together with its geobotanic diagnosis, besides providing a spatial dimension to the results of this work, constitutes a useful tool for future investigations of an ecological nature that tackle applied aspects of characterization, evolution, restoration and management of habitats and ecosystems, as well as studies of ecological and territorial management. As a complement to maps 1, 2 and 3, three transects accompany them (Figures 3, 4 and 5) generated from the bioclimatic information and the fieldwork. These transects cut transversally through the SMO at different latitudes, and they illustrated the layout of the bioclimatic belts and their corresponding potential vegetation types. The huge extent of the study area, combined with the low density of meteorological stations and their irregular distribution, with some territories lacking reference stations, the bioclimatic diagnosis must lean with more emphasis on the analysis of the corresponding vegetation potential. This is particularly important for future works of a local or regional character that deal with the methodological approach used here. The results presented must be used to establish the foundations of further investigations aimed at a better understanding of distribution, composition and ecosystem functioning, as well as the dynamics and possible response to new scenarios of global change. In spite of


the extent and complexity of the territory, this proposal advances the establishment of a bioclimatic classification based on the intimate relationship between climate and vegetation. As it becomes available a better geobotanic diagnosis of the different ecosystems will be more accurately adapted and the relationship between the dynamic, ecological and biogeographics aspects of phytocenosis and its bioclimatic belts that define them. Likewise the advances and contributions will favour the establishment of a classification system getting progressively more solid and better structured, that gives clarity and judgment to the difficult stage of the classification of the vegetation of Mexico within a global context. Acknowledgements The authors would like to thank to Ángel Pérez Zamora of the Department of Environmental Sciences (University of Guadalajara), for his help in the organization of the climatic information and in the preparation of the maps; to Socorro and Martha González-Elizondo of the Interdisciplinary Research Centre for Regional Integral Development, Durango unit (CIIDIR-IPN), for their support in the fieldwork and for their contributions to the manuscript. This work has been backed by the projects of the AECID A/012635 and A/024250/09. References Alcaraz F, 2013. Bioclimatología con R. Universidad de Murcia. Available from: http://www.um.es/docencia/geobotanica/ficheros/practica1.pdf. Almeida L, Cleef AM, Herrera A, Velázquez A, Luna I. 1994. El zacatonal alpino de la vertiente NW del volcán Popocatépetl, México y su posición en las montañas tropicales de América. Phytocoen. 22:391-346. Almeida L, Giménez de Azcárate J, Cleef AM, González-Trápaga A. 2004. Las comunidades vegetales del zacatonal alpino de los volcanes Popocatépetl y Nevado de Toluca, Región Central de México. Phytocoen. 34(1):91-132. Anonymous. 1973. International classification and mapping of vegetation. United Nations Educational, Scientific and Cultural Organization. París, Francia. 35 p. Anonymous. 1997. Regiones ecológicas de América del Norte. Hacia una perspectiva común. Comisión para la Cooperación Ambiental de América del Norte. Quebec, Canadá. 63 pp. Anonymous. 2002. Conjunto de datos vectoriales de la carta de uso del suelo y vegetación: escala 1: 250 000. Serie III (continuo nacional). Instituto Nacional de Estadística, Geografía e Informática. Aguascalientes, México. Anonymous. 2007a. Ecorregiones terrestres de México. Escala 1: 1,000,000. Instituto Nacional de Estadística, Geografía e Informática; Comisión Nacional para el Conocimiento y Uso de la Biodiversidad e Instituto Nacional de Ecología. México, D.F., México. Anonymous 2007b Conjunto de datos vectoriales de la Carta de Uso de Suelo y Vegetación, Escala 1:250,000, Serie IV. Conjunto Nacional. Instituto Nacional de Estadística, Geografía e Informática. Aguascalientes, México. Anonymous 2011. Normales climatológicas. Servicio Meteorológico Nacional y Comisión Nacional del Agua. Available from: http://www.smn.cna.gob.mx. Anonymous. 2013. Uso de Suelo y Vegetación. Instituto Nacional de Estadística y Geografía. Aguascalientes, México. Available from: http://www.inegi.org.mx/geo/contenidos/recnat/usosuelo/Default.aspx

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