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Bioclimate-vegetation interrelations in northwestern Mexico.

As the primary independent factor of the environment, climate generally is employed as the principal basis for classification of vegetation. Since the times of Humboldt and following pioneer conceptual works (e.g., Merriam, 1898; Dokuchaev, 1899), most efforts to divide the world into life zones and ecological regions have been based primarily on distribution of climate-vegetation zones. The area of plant ecology that relates structure and distribution of vegetation (including seasonal variations) to climatic variation is bioclimatology. Bioclimatic classification schemes attempt to determine relationships between average values of air temperature and precipitation and geographic distribution of living organisms, mainly individual species or communities of plants (Muller, 1982; Walter, 1985).

World zones showing marked climatic gradients can be the best laboratory to assess whether distribution of vegetation follows a climatic pattern, as reflected by meteorological data. Of particular ecological and bioclimatic interest is the strip of western continental Mexico along the Gulf of California and Pacific Ocean, crossing the states of Sonora, Sinaloa, Nayarit, Jalisco, and Colima. In a north-south direction, this region crosses the transition from the most arid deserts of North America to tropical forests (Wiseman, 1980), and it is here that tropical deciduous forests reach their northern distributional limit in the Western Hemisphere (Reichenbacher et al., 1998; Van Devender et al., 2000). The flora and vegetation of this region have not been examined in detail (Martinez-Yrizar et al., 2000; Rzedowski, 2006). Extensive botanical surveys have been conducted only in the Rio Mayo Basin (Gentry, 1942; Yetman et al., 1995; Martin et al., 1998; Van Devender et al., 2000); most work at the ecosystem level has been conducted at the Estacion de Biologia Chamela in Jalisco (Martinez-Yrizar et al., 2000).

Previously, we related climax vegetation of western North America to bioclimatic variation (Peinado et al., 1997a), established limits of Pacific zonobiomes using vegetation as an indicator (Peinado et al., 1994a, 1997b, 2007), and identified climatic similarities that give rise to almost identical models of zonation in salt-marshes of California, Baja California, and the European Mediterranean (Peinado et al., 1995c), as well as in Pantropical mangroves (Peinado et al., 1995a). Herein, we report results of a phytosociological study in the area between the delta of the Colorado River and the Colima-Michoacan border. This study was designed to determine whether available predictive models that relate climate and vegetation serve to define observed changes in vegetation in an area whose north-south arrangement allows for analysis of several climatic zones within a latitudinal gradient representative of the main zonal types of vegetation that grow in the northern tropics.

MATERIALS AND METHODS--Bordering the Pacific Ocean and Gulf of California, the study area extends 2,044 km from San Luis del Rio Colorado (32[degrees]29'N) to the southern tip of Colima (18[degrees]44'N). Longitudinally, the area reaches its western limit in Riito, Sonora, at 114[degrees]55'W, whereas the easternmost site surveyed was 103[degrees]40'W, in Callejones, Colima.

The study area lies west of two of the largest mountain systems of continental Mexico, the Sierra Madre Occidental and the Sierra Madre del Sur. The Sierra Madre Occidental, the longest and most continuous mountain system in Mesoamerica, whose west-to-east gradsect is the richest in flora and fauna on the Pacific Coast of North America (Martinez-Yrizar et al., 2000), runs parallel to this coast from the northern limit of our study area southward to northern Jalisco, where it merges with the Neovolcanic Axis. The Sierra Madre del Sur runs west-southeast close to the ocean from southern Jalisco, at about Cabo Corrientes, where it meets the Neovolcanic Axis, southward as far as the Isthmus of Tehuantepec. Both sierras shelter the tropical biota of our study area, warding off incursions of continental polar air masses, which in some winters sweep south from Siberia and Canada to freeze tropical vegetation across the Atlantic Basin as far south as Yucatan (Martin and Yetman, 2000).

Between the delta of the Colorado River and the southern tip of the Sierra Madre Occidental is the Northwestern Coastal Plain Physiographic Province, a wide lowland area, characterized by extensive intermountain plains occupying the western one-half of Sonora, and gradually narrowing to a belt 40-60 km wide throughout the length of Sinaloa. Coastal plains are interrupted in Nayarit by the Neovolcanic Axis. Interspersed with high mountains and isolated volcanoes, this axis is characterized by east-west valleys that are wide paths for tropical, moist-air masses from the Gulf of Mexico. South of the Neovolcanic Axis, from Cabo Corrientes to Chiapas, there is no coastal plain because the Sierra Madre del Sur merges directly with coastal beaches.

The position of the Baja California Peninsula between the mouth of the Colorado River and Mazatlan, with its mountain chains >3,000 m in elevation, blocks against the cold Californian current, attenuating humidifying effects of coastal fogs, which are characteristic of Pacific lowlands from southern California to western Baja California at similar latitudes (Peinado et al., 2005).

According to the zonobiome classification scheme described by Walter (1985), our study area spans two zonobiomes (tropical summer-rain with deciduous forests and subtropical deserts) and two zonoecotones (semievergreen forests and thornforest and climate savannas). From a phytogeographical standpoint, tropical summer-rain with deciduous forests corresponds to the Xerophytic-Mexican region and tropical summer-rain with deciduous forests to the Caribbean region, both within the Neotropical kingdom (Thorne, 1993; Rzedowski, 2006).

Before conducting fieldwork, we examined data from meteorological stations in our study area. To avoid large deviations due to continentality effect, an essential criterion in the final selection process was that every station had to be <100 km away from the sea. We selected 254 meteorological stations, and we compiled additional data for each station from several sources (Ruiz et al., 2003, for Jalisco; Ruiz et al., 2005b, for Sinaloa; Ruiz et al., 2005a, for Sonora; Vizcaino et al., 2007, for Colima). Data for Nayarit were obtained directly from the Mexican National Meteorological Service (http://smn.cna.gob.mx). For each station, several climatic variables and indices were obtained or calculated. To determine which classification system best fit vegetational changes observed in our field work, we assessed six bioclimatic systems: Koppen (1931), Troll (1966), Trewartha (1968), Walter (1985), Bailey (1996), and Rivas-Martinez (2007). Climatograms were constructed using the program Bioclima (F. Alcaraz, pers. comm.), which generates climatograms according to the model of Walter and Lieth (1960-1967).

Each meteorological station was bioclimatically classified and assigned to a particular potential type of vegetation using satellite images and available maps of vegetation (Leopold, 1950; Anonymous, 1995, 1997, 2002). Field work was performed at sites near each station by examining satellite images (http://earth.google.com) to ensure assessment of potential vegetation that was relatively well preserved. Predicting vegetation based on structure or physiognomy of a community is the basic method of treating vegetation on a broad scale in relation to climate (Beard, 1973). Accordingly, the aim of our field work was to make lists of dominant vascular plants and identify structure of vegetation and dominance of species at each site. Sites at meteorological stations were visited before and after rainy seasons in 2002-2006 in an effort to detect physiognomic changes in dominant communities. When vegetation was well-preserved, this field work was undertaken around each station. When this was not the case, data were collected from the nearest sites to each station with an accessible potential vegetation that was relatively well-preserved. Environmental data collected from each site were elevation, slope, aspect, type of soil, and geological substratum. Edaphic and geologic data were obtained from Anonymous (1995, 2001). Data on vegetation were obtained from censuses of vascular plants recorded at each site. Sites were selected according to homogeneity of physical features, structure of vegetation, and dominance of species, and their sizes were based on the minimum area of relatively uniform stands (Westhoff and van der Maarel, 1973). At each site, we recorded dominant species and their corresponding ecophysiognomic characters, following the six categories described and summarized by Box (1981).

Distribution of each taxon was determined using bibliographical sources and maps obtained from databases at the Missouri Botanical Garden (http://mobot.mobot.org/W3T/Search/vast.html) and the United States Department of Agriculture (http://plants.usda.gov). Plants were identified using regional floras and monographs (Gentry, 1942; Shreve and Wiggins, 1964; Wiggins, 1980; Felger, 2000; Pennington and Sarukhan, 2005). Nomenclature follows the United States Department of Agriculture or the Missouri Botanical Garden.

RESULTS--Data from all 254 stations are available as additional supporting information at http://www2.uah.es/ambiente/bioclimate. Climatic data indicate several latitudinal gradients. First, there is a southward rising temperature gradient (increasing mean yearly temperature, mean monthly maximum temperature of coldest month, mean monthly minimum temperature of coldest month, sum of mean monthly temperatures of months whose mean temperature is >0[degrees]C, and thermicity index; for climatic definitions and indices throughout the text see Rivas-Martinez, 2007). In contrast, continentality indexes, which reflect differences between maximum and minimum temperatures, decrease latitudinally because thermal stability increases toward the equator.

Annual precipitation was 56-2,226 mm. Although precipitation at each station was highly influenced by local factors such as elevation, aspect, and orography, there was a increase in rainfall from station 1 southward as far as Bahia Banderas (stations 191-194). From there, a southward decrease in rainfall occurred, which was interrupted only by values recorded at some mountain stations benefiting from orographic precipitations (stations 214, 223, 224, 236, 240). This decrease in rainfall may be sudden, e.g., between stations 196 and 198, at the same elevation and separated only by four latitudinal minutes (ca. 17 km) there is a difference of >400 mm in precipitation.

Another north-south gradient was number of humid months (months in which precipitation [greater than or equal to] twice mean monthly temperature), i.e., the value is zero for northernmost stations and increased southward to the northern side of Bahia de Banderas, where the mean value of 5 humid months stabilizes. Seasonal pattern of rainfall also displayed a longitudinal gradient. If stations are arranged by longitude, then the greater the longitude, the lower the percentage of rainfall in summer. Although the easternmost station is ca. 700 km from the cyclogenic Gulf of Mexico, the westernmost station is 1,650 km from there. Lower percentages of rainfall in summer were at northernmost stations and minimum percentage was at northwestern most stations.

Gradients in precipitation and temperature caused a similar bioclimatic gradient. Of the six classification schemes used in our study, the one designed by Rivas-Martinez (2007) best revealed shifts in distribution of potential vegetation. According to his scheme, all stations we examined can be included in the tropical macrobioclimate and assigned to three bioclimates, three thermotypes, and six ombrotypes. To analyze potential vegetation, it is useful to separate horizons (lower and upper) within thermotypes and ombrotypes using thermicity and ombrothermal indices, respectively. Northernmost arid stations are in the tropical-desert bioclimate, which spreads continuously between San Luis del Rio Colorado and the northern Yaqui River Basin near Guaymas, at ca. 28[degrees]30'N. South of here, a gradient of increasing rainfall begins, giving rise to stations with tropical-xeric conditions. On the coast of Nayarit, where the Neovolcanic Axis merges into the Pacific Basin, maximum rainfall occurs at stations with a tropical-pluviseasonal bioclimate.

DISCUSSION--Changes in rainfall and temperature in our study area are determined by latitudinal, regional, and local factors. Nonetheless, our study area is virtually frost-free and potential vegetation is conditioned more by rainfall than by cold temperatures. General distribution of vegetation follows the global macroclimatic pattern of the Earth linked to latitude, although it is affected by regional impacts of mountain ranges. The northern part of our study area is within the anticyclonic subtropical belt, an area ca. 30[degrees] north and south of the equator in which descending air masses, associated with high atmospheric pressure and clear skies, prevail. Extreme heat and lack of rainfall is typical of the desert climate, which is common in this subtropical zone. It is in this zone that the North American deserts occur (Brouillet and Whetstone, 1993). The tropical desert bioclimate is within this belt, spreading across 650 km within the Sonoran Desert phytogeographical division (Turner and Brown, 1982).

Between the subtropical belt and the intertropical convergence zone (an area of relatively low atmospheric pressure centered ca. 10[degrees] north and south of the equator), trade winds blow northeasterly in the Northern Hemisphere. Regions affected by trade winds are much drier than the convergence zone, but they receive more rainfall than deserts. The Gulf of Mexico, Caribbean Sea, and subtropical western Atlantic are sources for the moist, warm, tropical air masses that affect the climate of Mexico south of the subtropical anticyclone, south of ca. 28[degrees]N (Bryson and Hare, 1974). This latitudinal fringe is dominated by easterlies or trade winds that blow in summer across the Neovolcanic Axis moving moist, warm, air masses westward as far as the Pacific Coast, where they merge with cool maritime masses to give rise to the Mexican monsoon along western slopes of both Sierra Madres (Douglas et al., 1993). Hence, the farther a site is from the Gulf of Mexico, the weaker the cyclogenic influence and the lower the amount of rainfall. Because of their latitudinal position along the rainy corridor of the Neovolcanic Axis, several stations at sea level (stations 150-196) north of Bahia Banderas recorded greatest rainfalls.

Accordingly, across most of our study area, a wet-dry tropical climate prevails because of seasonal alternation between dominance by dry air masses of the subtropical-high belt during winter solstice, and moist air from zones with easterlies during summer solstice. The time of year with humid months occurs near summer solstice in May-September. An essential criterion to define the tropical climate is rainfall pattern or seasonal precipitation rhythm. In the tropics, most rainfall occurs 4 months after summer solstice. Of the 247 stations with this condition, 239 exhibited typical tropical rhythm (percentage of summer-rainfall + precipitation in autumn > precipitation in winter + precipitation in spring). For eight stations, the pattern was percentage of summer-rainfall > precipitation in winter > precipitation in autumn > precipitation in spring. For stations 1, 2, and 4, it was autumn > winter > summer > spring, and only for station 8 was the pattern winter > autumn > summer > spring. Given that these four are the northwestern stations, their rainfall rhythm is affected by the southern arm of the winter Sonoran route, which carries part of the moist, mild, maritime Pacific air masses inland (Bryson and Hare, 1974; Brouillet and Whetstone, 1993), and their remoteness protects them from influence of disturbances from the Gulf of Mexico in summer. These stations have a biseasonal precipitation regimen typical of the Sonoran Desert (Walter, 1985). However, even at those four stations, >50% of rainfall occurs after summer solstice (percentage of summer + autumn precipitations is >50%).

The Sierra Madre Occidental and the Sierra Madre del Sur act as barriers and provoke intense rainshadows that distort the general distribution model on the regional scale. Influence of the Sierra Madre Occidental becomes noticeable north of the Neovolcanic Axis, while influence of the Sierra Madre del Sur can be observed south of the axis. Smaller mountain ranges also locally affect distribution of vegetation. Latitudinal increase in rainfall from station 1 southward is locally or regionally interrupted. Locally, because some stations occurred in the rainshadow of small coastal mountains (Sierra Vallejo: stations 98, 102, 103, 104, 107; Sierra Zapotan: station 106), and regionally, because north of ca. 23[degrees] most stations lie in the rainshadow of the Sierra Madre Occidental. Decrease in rainfall from Bahia Banderas southward is because most stations occurred in the rainshadow of Sierra Madre del Sur.

Tropical-desert bioclimate defines a region that also could be called the Larrea tridentata-Parkinsonia microphylla region, because both species are the most widespread and representative vascular plants. In the rainiest deserts (upper-arid ombroclimate), the foothill paloverde (P. microphylla) is a common member of the zonal vegetation (arbosuffrutescent and crassicaulescent deserts) along with several columnar cacti, whereas in hyperarid and lower-arid areas, it is restricted to rocky places (sarcocaulescent desert) and wet soils (vegetation on drainage ways). Larrea tridentata, the most representative of the hot-desert evergreen shrub biotype (Box, 1981), dominates or co-dominates every zonal community, and its distribution marks both the northern border between the warm Sonoran Desert and the cold Great Basin Desert (MacMahon, 1988; Peinado et al., 1995b), and the southernmost limits of the tropical-desert bioclimate at the transition with tropical-xeric areas. The southern limit of L. tridentata appears a few kilometers south of Guaymas (Shreve and Wiggins, 1964; Rzedowsky, 2006), where the tropical-xeric bioclimate begins and where the transition from arid desert vegetation to semiarid thornscrubs occurs (Burquez et al., 1999).

Tropical-desert zones with hyperarid and lower-arid ombroclimates correspond to the lower Colorado River Valley subdivision of the Sonoran Desert (Shreve and Wiggins, 1964; Turner and Brown, 1982) and belong to the Colorado province, the most arid region of North America (Peinado et al., 2006). The most widespread zonal vegetation is that of the microphyllous desert, a desertscrub that flourishes on regosols and on Quaternary coarse alluvial plains and lower bajadas. Vegetation of this type, which spreads across thousands of alluvial hectares of the Colorado province (Peinado et al., 1995b), are physiognomically similar, with L. tridentata and Ambrosia dumosa dominating an open scrubland with the ocotillo (Fouquieria splendens) among many bush-stem succulents, mostly Cylindropuntia. As the sand fraction increases, F. splendens decreases in importance and psammophilous species become dominant shrubs of the sandy microphyllous desert. Under intense evaporation provoked by the hyperarid ombroclimate, the vegetation shows a simple structure (hiperarid desert), whose two dominant and fairly exclusive species, Tidestromia oblongifolia and Atriplex hymenelytra, have the capacity to enhance needs for water by using dew (Babu and Went, 1978). In areas of flash floods and arroyos crossing plains, the number of small trees is much greater, to the extent that they form an azonal type of vegetation (vegetation on drainage ways). Alongside the Colorado River, dense riparian thickets of desert-willows (Chilopsis linearis) grow, along with other trees and arborescent shrubs, which are being displaced by introduced Tamarix.

In arid deserts, fissured, rocky ground provides the wettest habitat (Walter, 1985). Hence, on lithosols and regosols derived from acidic and basic igneous rocks, microphyllous and sandy microphyllous deserts are replaced by sarcocaulescent desert, an open woodland physiognomically dominated by columnar cacti and many deciduous small trees that have fleshy stems of swollen appearance (sarcocaulescent of Shreve and Wiggins, 1964). Although these plants are distinctive of this type of vegetation, they are outnumbered by broad-raingreen small trees and deciduous or evergreen shrubs.

From near Puerto Libertad southward to Guaymas, there are low mountains and hills comprised of Cretaceous acidic-igneous rocks. Because these rocks are common on the opposite Gulf Coast, within the Angelino-Loretano sector of the Baja California Peninsula (Peinado et al., 1995b), this Sonoran fringe of the Gulf Coast is rich in Angelino-Loretano disjunct plants, which are nowhere else on the mainland yet occur widely in Baja California, a peninsula that commenced separation from the continent in the late Tertiary ca. 3,600,000 years ago (Peinado et al., 2007). Alkaline desert also is on the Gulf Coast, but on solonchaks and solonetzs. This vegetation is widely distributed along the Pacific Coast of Baja California (Peinado et al., 2008), whose only locations outside Baja are in this Sonoran coastal fringe.

Tropical-desert zones with an upper-arid ombroclimate at higher elevations show the mesotropical thermotype and phytogeographically correspond to the Arizona upland subdivision of the Sonoran Desert, whose most characteristic vegetation comprises crassicaulescent desert (Shreve and Wiggins, 1964). Vegetation is low woodland of raingreen trees and columnar cacti, with intervening spaces occupied by xeric dwarf-shrubs, and many bush cacti largely confined to this desert, some of which are local endemics (crassicaulescent desert). Upper-arid stations close to the coast and those at lower elevations correspond to the plains of Sonora subdivision of the Sonoran Desert (Shreve and Wiggins, 1964), which is dominated by the arbosuffrutescent desert. Landscape physiognomy is woodland dominated by small, low-branching trees that are restricted to streamways and wet places in lower-arid areas. Larrea tridentata occupies sunny places and the driest steep slopes. The dominant shrub is Encelia farinosa. Other xeric dwarf-shrubs, some large columnar cacti, along with scattered Carnegiea gigantea and four species of bush-cacti (Cylindropuntia) are the most conspicuous plants. Most areas originally occupied by these woodlands are now irrigated and almost covered by agricultural crops.

As in other zones of the world in which there is a gradual transition from tropical deciduous forests to deserts, thornforests intermix with deciduous forests and impenetrable thornscrubs merge with deserts. In effect, other regions of the world with a flourishing thorny vegetation are regarded as zonoecotones (Walter, 1985). The transition from tropical-desert to tropical-xeric conditions may be observed in types of habitat in which vegetation of both bioclimates occur close to ca. 28[degrees] near Guaymas, i.e., where the easterlies begin to produce their effects. Thus, from Guaymas northward to Rio Sonora, outlying bodies of tropical-xeric vegetation (thornforest) with some outstanding species such as Acacia cymbispina, Ceiba acuminata, Haematoxylum brasiletto, Ipomoea arborescens, Pachycereus pecten-aboriginum, and Pithecellobium sonorae are confined to streamways and their bordering depressions. South of Guaymas, A. dumosa, Atamisquea emarginata, P. microphylla, Fouquieria diguetii, F. splendens, L. tridentata, Pachycereus pringlei, and many other typical desert plants vanish, thornscrubs and trees become dominant, and small areas of desert vegetation are in the driest situations of the prevailing tropical-xeric vegetation. Tropical-xeric vegetation will be the dominant type southward in the coastal plain and foothill regions from here to northern Nayarit, spreading along a coastal fringe ca. 800 km long.

Pachycereus pecten-aboriginum and Lysiloma divaricatum are the best bioindicators of the tropical-xeric region, in which three main types of vegetation can be distinguished; thornscrubs, thornforests on zonal soils and azonal mesquite forests on alluvial plains, and deciduous forests. These show the shrubby structure of Hiemifruticeta (lower-semiarid ombroclimate thornscrubs), the mixed shrubby-arborescent structure of Hiemifruticeta-Hiemisilva (upper-semiarid ombroclimate thornforests and wet-soils mesquite forests), or in the rainiest areas with a dry ombroclimate, the characteristic forest of Hiemisilva (deciduous forests). Whereas, in both thornscrubs and thornforests, dominant plants are raingreen thornscrubs or small trees (thornforests, mesquite forests), most of them are leguminous. In deciduous forests, larger broadleaved trees dominate and there is greater diversity of tropical families and life-forms. Neotropical deciduous forests show a greater biodiversity than other tropical ecosystems (Bullock et al., 1995). Of 20 species of trees in deciduous forests of Sonora, 90% had broad leaves (Van Devender et al., 2000).

From a structural or physiognomic point of view, thornscrubs and thornforests resemble the caatinga of northeastern Brazil and the cactus-thornbush semidesert of Venezuela (Walter, 1985), two types of vegetation that occur under similar climatic conditions and are dominated by low-growing trees, shrubs, columnar cacti, and terrestrial bromeliads. Thornscrubs are also climax vegetation in semiarid areas of Madagascar (Takhtajan, 1985) where arborescent-succulents of the genera Euphorbia and Didiera are vicariants of Neotropical columnar cacti. Dominance of Acacia, Caesalpinia, Cassia, Mimosa, and other leguminous trees and shrubs reveals pantropical relationships of thorny communities of the New and Old worlds related to paleoclimatic and paleogeographical events (M. Peinado et al., in litt.). Our analysis of the Cape Region of Baja California, revealed strong floristic links between thorny communities and deciduous forests, such that they were assigned their own phytosociological class; Pachycereo pecten-aborigini-Lysolometea divaricati (Peinado et al., 2008).

Between Guaisimas Bay (south of Guaymas) and northern Sinaloa (stations 49-116) are extensive alluvial plains, comprising deltas of the Yaqui, Batacosa, Mayo, and Fuerte rivers. Much of this area is now cultivated and produces substantial agricultural crops. The biggest change to the landscape has resulted from introduction of the African buffelgrass (Pennisetum ciliaris; Burquez et al., 2002). The original azonal vegetation of these alluvial plains was like that of the parts that remain uncultivated, a closed or open phreatophytic thornforest of mesquite (Prosopis glandulosa var. torreyana), with Parkinsonia praecox and several species of Bursera forming a conspicuous layer of small trees (mesquite forests). Phreatophyllic behavior of these mesquite thornforests is most marked in April, when mesquites are in full leaf and form bright-green spots at bottoms of valleys, contrasting with parched zonal vegetation (thornscrubs or thornforests) on arid hills.

Beyond the alluvial plains, potential vegetation in southern Sonora and part of Sinaloa is Sinaloan thornscrub, thornforest, and tropical thorn woodland. Under the lower-semiarid ombroclimate, dominant structure is thornscrub, whereas the thornforest, which is the potential vegetation in upper-semiarid areas, follows floodplains of rivers and margins of smaller streams. Basic structure and composition of both types is dominated by xeromorphic, drought-deciduous, often thorny, pinnate-leaved, multitrunked trees or shrubs 2-10 m in height, with a strong component of succulents and thorny plants, many of them shared with arbosuffrutescent desert and crassicaulescent desert. Typical shrubs show multiple stems arising from a common base at or near ground level (raingreen thorn scrubs). In thornforests, height of trees is greater and some shrub-sized plants in thornscrubs become trees in thornforests (e.g., Erythrina flabelliformis, Haematoxylum brasiletto, and Jacquinia pungens). The joint canopy of xeric trees have an irregular height of 4 (thornscrub) to 10 m (thornforest), with microphyllous trees as dominants, mostly Leguminosae. Two species of columnar cacti, Stenocereus thurberi and Pachycereus pecten-aboriginum, rise above upper levels of thornscrub and level the canopy of the thornforest. Some raingreen broad-leaved trees become co-dominant when structure of the thornforest has been attained.

Dominant vegetation in dry ombroclimate is tropical summer-rain deciduous forest (deciduous forests). Tropical deciduous forests are in Africa, on both sides of the equator, in southeastern Asia and India (dry-monsoon forests), and in northernmost Australia (Walter, 1985). On the Pacific Coast of Mesoamerica, deciduous forests grow as a continuous ribbon from the Mayo River in southern Sonora southward to Costa Rica (Reichenbacher et al., 1998); its average width being only 50 km (Gentry, 1995). From the Pacific Basin, deciduous forests spread across central Mexico and disjunctly in the Sanlucan province of Baja California (Peinado et al., 1997a), as well as in three zones of the Mexican Atlantic Basin (Anonymous, 1997; Rzedowski, 2006). In South America, tropical deciduous forests that share co-dominant trees cover a large area south of the Amazon Basin and smaller areas extending beyond 20[degrees]N in Mesoamerica (Leopold, 1950; Walter, 1985; Lott, 1993).

Although destruction of deciduous forests in favor of African buffelgrass has accelerated (Burquez et al., 2002), it endures near its northern limits thanks to extraordinary roughness of foothills, cliffs, and gorges of the Sierra Madre Occidental. There, structure of deciduous forests vary enormously from site to site (Murphy and Lugo, 1986). Composition of species also is diverse and there are many variants due to its wide area and to differences in aspect, slope, elevation, substrate, depth of soil, exposure, and local factors. This mosaic is tightly related to availability of water for establishment and growth of plants (Barbera et al., 2002). However, most deciduous forests in our study area were dominated by Lysiloma divaricatum. In contrast, in Chamela Bay where the infratropical thermotype begins, deciduous forests were dominated by leguminous trees of the genus Lonchocarpus (12 species; Lott, 1993) rather than L. divaricatum. Because of the wide distribution of deciduous forests across the New World, most (60) of the 170 species recorded in our samples were Neotropical elements, most of which are shared with other Mesoamerican and South American deciduous forests (Bullock et al., 1995); 23 species span from southern Sonora to the Isthmus of Tehuantepec, 22 are Mesoamerican taxa, and the other 22 are Mexican endemisms. Relative isolation of the northern Pacific deciduous forests with regard to their Neotropical counterparts reflects the large number of endemisms: 38 taxa, mostly trees, are endemic to our study area; 15 of these are endemic in the southern part, from Bahia Banderas southward; and a further five species are restricted to north of Bahia Banderas. These data are similar to reports at the genus level indicating more regionally endemic genera in deciduous forests of western Mexico than in those of Central America (Gentry, 1995; Trejo and Dirzo, 2002).

In transition zones, thornforests and deciduous forests intermix, although deciduous forests can be distinguished by its greater height (unbroken canopy is 10-20 m above ground), larger leafage (macrophyllous and mesophyllous taxa dominate contrasting with the microphyllous shrubs and trees that dominate thornforests), the reduced dominance of thorny and succulent plants, and by strong penetration of species, genera, and families showing a tropical distribution, including epiphytic orchids and bromeliads. Trees and understories are leafless during the dry season in which flowering and maturation of fruit occur. Unlike thornforests, arborescent cacti (Pachycereus pecten-aboriginum, Stenocereus chrysocarpus, Cephalocereus purpusii, and some tall-arborescent Opuntia), which mainly prosper in xeric habitats, are shorter than tree-tops and stay hidden under dense leafage during the rainy season. Two medium-sized palms, Acrocomia aculeata and Sabal rosei, always absent from the semiarid region, are reliable bioindicators of potential deciduous forests because they often inhabit burnt or disturbed places near human settlements.

Along flood channels of rivers, around reservoirs, and on floodplains of major rivers of this dry zone, several riparian communities appear (riparian forests) dominated either by massive sabino or bald cypress (Taxodium distichum var. mucronatum), or by two enormous deciduous trees, Populus mexicana ssp. dimorpha and Salix bonplandiana, that require reliable access to water. Although sabinos and willows are the most conspicuous trees in frequently flooded areas along margins of rivers, the cottonwood (Populus) is more typically on floodplains. Most tall cottonwood forests appear now as open parklands cleared for agricultural crops and grazing, although some isolated massive trees have been spared to provide shade for human settlements. Freshwater lagoons, highly limited because of restricted seasonal freshwater, occur at mouths of rivers with clumps of Phragmites australis and Typha domingensis at edges of streams, which often are choked by Eichhornia crassipes and Pistia stratiotes in deeper stagnant waters and irrigation channels.

Stands of taller evergreen trees become more dominant when rainfall increases. A huge thick-trunked tree (Enterolobium cyclocarpum), along with Brosimum alicastrum, are the best indicators of tropical-pluviseasonal vegetation, the semi-evergreen tropical rainforest, which is the most complex and best-structured vegetation in our study area. This forest has a structure intermediate between deciduous forests and tropical evergreen forest, the latter being exclusive to tropical-pluvial areas with humid and hyperhumid ombroclimates. Well-known semi-evergreen tropical rainforests are the humid-monsoon forests of India, but seasonal rainforests also occur in Mesoamerica, South America, and Africa (Walter, 1985). Because the Atlantic Basin is rainier and its potential vegetation consists of evergreen rainforests, semi-evergreen tropical rainforests are more widespread in the Mexican Pacific Basin, where they occur nearly continuously from southern Sinaloa southward to coastal Chiapas (Leopold, 1950; Rzedowski, 2006). The broad distribution of the Pacific semi-evergreen tropical rainforest reflects its floristic composition; most plants are Neotropical (56% of species) and Mesoamerican (27%) elements.

Given its transitional nature, evergreen and raingreen broad-leaved trees coexist in semi-evergreen tropical rainforest. The tree stratum reaches a height of 30-40 m, and three stories sometimes are recognizable at better-preserved sites. Upper tree story consists of isolated and emergent megaphanerophytes such as E. cyclocarpum and Conzattia multiflora rising far above the other trees. Middle and lower stories form a dense leaf canopy. The proportion of deciduous trees with large leaves is greater in the middle story (20-30 m height), whereas lower stories (10-15 m) are mostly small-leaved evergreens. Lianas, including some strangler-trees of the genus Ficus, are numerous and may form thick tangles in gaps left by old tree falls. Most epiphytes are Tillandsia. Because of shady conditions under the closed canopy, the herbaceous stratum is scarce or almost nonexistent. In lowlands, the large palm Attalea cohune shares dominance in the emergent story, and probably once formed dense palm forests on well-drained sandy soils that have been replaced with plantations of Coccos nuccifera. Like other tropical palms, A. cohune is fire-resistant and forms secondary successional communities in highly disturbed areas.

On the upper horizon of the thermotropical-subhumid belt, on ultramafic soils (ultramafic; Kruckeberg, 2002), communities dominated by deciduous oaks replace semi-evergreen tropical rainforest. Oak-pine forests are the largest climax vegetation of the Mexican mountains (Rzedowski, 2006), yet in the Pacific Basin, they thrive at lower elevations (Rzedowski and McVaugh, 1966), rendering communities of oaks that lack pines in the upper-thermotropical belt. These oak communities are oak scrubs and oak woodlands, and flourish in several regions of continental Mexico (Leopold, 1950; Rzedowski, 2006), at Sierra de la Laguna in Baja California Sur (Peinado et al., 1994b) and, in our study area, on the western slopes of both Sierra Madres from Sonora to Colima. Diversity of the genus Quercus in Mexico is almost 200 species (Rzedowsky, 2006). In the Sierra Madre Occidental of Nayarit, there are 23 species of Quercus (Tellez, 1994). In coastal oak woodlands, the number of species is less. The upper and open story (8-12 m) is mainly Q. magnoliifolia along with other scattered species of oaks, the middle story (4-6 m) has the same oaks along with a few other trees, and a third subordinated story is low shrubs (1-1.5 m) and herbs (mainly Poaceae), producing an oak-savanna appearance in many places.

Mangrove vegetation, or tidal woodland, begins to appear in the thermotropical belt of the Gulf of California at 29[degrees]05', at its Neotropical northern limit (Peinado et al., 2007). Accordingly, in our study area, mangroves thrive everywhere except on the northwestern mesotropical coast, where they are replaced by saltmarsh communities of Limonio californici-Frankenietea salinae (Peinado et al., 1995c). In our analyses of mangroves, zonation was similar to that in other North American and Mesomerican areas; red mangrove (Rhizophora mangle) forms the pioneer fringe and main seaward belt, black mangrove (Avicennia germinans) typically grows at levels covered at high tide and exposed at low tide, and buttonwood (Conocarpus erecta) forms a third fringe that is flooded by the highest spring tides or only by storm tides. White mangrove (Laguncularia racemosa) may appear within each fringe, but its ecological optimum seems to be in the inner fringe. Saline soils of depressions close to the sea, which are flooded by higher tides or whose soils show effects of gleyzation produced by brackish water, are the habitat of mangrove shrubs ([less than or equal to] 1.5 m tall) of the association Allenrolfeo occidentalis-Maytenetum phyllanthoidis (Peinado et al., 2008). Beyond edges of mangrove swamps in seasonally inundated flats or depressions, there are Acrostichum danaeifolium, Annona glabra, Coccoloba barbadensis, Hibiscus pernambucensis, Hippomane mancinella, and Pithecellobium lanceolatum. For additional information on phytosociology of mangroves see Peinado et al. (1995a, 2008). Coastal dunes occur along much of our study area. Gradual geographic replacement, or shift, in flora of beaches and dunes from north to south is similar to that on the Gulf Ccoast of the Baja California Peninsula (Peinado et al., 2008).

CONCLUSIONS--Bioclimatic classification schemes attempt to determine relationships between average values of air temperature and precipitation, and geographic distribution of living organisms, mainly of single species or communities. There have been few bioclimatic classifications and systems proposed for global use. Our study was designed to identify relationships between the distribution of vegetation observed in field studies and climates determined using bioclimatic classification systems. Among the systems available, we used six that can be applied on a worldwide scale. Of these six schemes, the zonobiome system of Walter (1985) is efficient at predicting changes in communities based on macroclimate. The bioclimatic classification by Rivas-Martinez (2007) reveals shifts in distribution of potential vegetation on a continental, regional, and local scale. This determines that bioclimatic units (bioclimates, thermotypes, ombrotypes, and vegetation belts) can be correlated closely with types of vegetation in our study area, which have been traditionally based on physiognomic descriptions. The on-line nature of this bioclimatic system, which only requires elementary climatic data available at any meteorological station (http://www.globalbioclimatics.org/form/online.htm), makes this a useful tool to predict types of vegetation in any region of the world. The results we obtained, reported herein and previously (Peinado et al., 2007), confirm the good predictive value of this bioclimatic classification across all of North America.

This study is part of doctoral research by M. A. Macias, who was granted scholarships from the Secretaria de Educacion Publica (Mexico) and the universities of Alcala and Guadalajara. Research was made possible by an agreement between the universidad de Alcala and universidad Autonoma de Baja California, and supported by grants from the Agencia Espanola de Cooperation Internacional para el Desarrollo (A/024250/09). The authors thank E. Lopez-Alcocer from the universidad de Guadalajara for supporting our fieldwork, and A. Ruiz-Corral from the same university for providing meteorological data. We also thank F. Alcaraz from the Universidad de Murcia for the use of his computer program Bioclima, which has been valuable in this study, and A. Burton for help with the English translation.

Submitted 26 November 2007. Accepted 14 October 2009.

Associate Editor was David B. Wester.

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MANUEL PEINADO, * MIGUEL A. MACIAS, JUAN L. AGUIRRE, AND JOSE DELGADILLO RODRIGUEZ

Departamento de Biologia Vegetal, Universidad de Alcala, Madrid, Spain (MP)

Departamento de Ciencias Ambientales, Universidad de Guadalajara, Guadalajara, Jalisco, Mexico (MAM)

Catedra de Medio Ambiente, Universidad de Alcala, Madrid, Spain (JLA)

Facultad de Ciencias, Universidad Autonoma de Baja California, Ensenada, Baja California, Mexico (JDR)

* Correspondent: manuel.lorca@uah.es
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