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Botanical Sciences

versión On-line ISSN 2007-4476versión impresa ISSN 2007-4298

Bot. sci vol.93 no.3 México sep. 2015

http://dx.doi.org/10.17129/botsci.159 

Florística y taxonomía

 

Floristic diversity and notes on the vegetation of Bahía Magdalena area, Baja California Sur, Mexico

 

Diversidad florística y notas de la vegetación del área de Bahía Magdalena, Baja California Sur, México

 

José Luis León–de la Luz1, Alfonso Medel–Narváez and Raymundo Domínguez–Cadena

 

Herbario HCIB, Centro de Investigaciones Biológicas del Noroeste, La Paz, Baja California México.

Corresponding autor: jlleon04@cibnor.mx

 

Received: March 19th, 2014.
Accepted: August 6th, 2014.

 

Abstract

The Bahía Magdalena region of the Baja California Peninsula is floristically part of the southern Sonoran Desert. It has several particular geographical features, of particular importance the influence of the cold California Current. We present a floristic study of the higher plants on a land area covering 5,158 km2. The flora has 506 taxa: five ferns, two gymnosperms, 438 magnoliopsida (dicotyledons), and 60 liliopsida (monocotyledons), representing 85 families and 280 genera. The plant families with major species diversity are: Asteraceae, Poaceae, Euphorbiaceae, and Fabaceae. The flora was categorized into nine life forms: perennial herbs (127) and annuals (143) represent 54 % of all species. The flora occupies eight plant communities, which are described; the most extensive community is the fog sarcocaulescent scrubland. From a biological perspective, Margarita and Magdalena islands are important areas to preserve as core zones in a future management plan because they support 19 endemic species. Mangroves and sea grass marshes are important areas serving as nursery habitats for marine fauna. Mexican environmental authorities are considering establishing a reserve to protect biodiversity in the region.

Key words: Pacific coastal islands, plant diversity, sarcocaulescent scrubs, Sonoran Desert.

 

Resumen

La región de Bahía Magdalena de la Península de Baja California ha sido considerada florísticamente parte del sur del Desierto Sonorense. Presenta diversos rasgos geográficos importantes, tales como la influencia de la fría corriente de California. Este trabajo presenta un estudio florístico de los vegetales superiores que abarca 5,158 km2 de territorio. La flora consiste de 506 taxa de plantas vasculares: cinco de helechos, dos de gimnoespermas, 438 magnoliopsida (dicotiledóneas) y 60 liliopsida (monocotiledóneas), agrupadas en 85 familias y 280 géneros. Las familias con mayor diversidad de especies son: Asteraceae, Poaceae, Euphorbiaceae y Fabaceae. La flora resultante fue categorizada en nueve formas de crecimiento: Las herbáceas perennes (127) y las anuales (143) representan el 54 % de todas las especies. La flora ocupa ocho comunidades vegetales; la más extendida es el matorral sarcocaule de neblina. Desde una perspectiva biológica, las islas Margarita y Magdalena son las áreas más importantes para considerarlas dentro de una zona núcleo en un futuro plan de manejo, porque estas incluyen a 19 especies endémicas. Los manglares y los pastizales marinos son áreas importantes para la crianza de la vida marina. Las autoridades ambientales mexicanas consideran actualmente su inclusión como área de protección a su biodiversidad.

Palabras clave: desierto Sonorense, diversidad de plantas, islas del Pacífico, matorral sarcocaule.

 

Biodiversity is commonly understood as the variety of life in all its forms, levels, and combinations; more specifically, it is the degree of variation of all life forms and their ecological processes within a given ecosystem (Wilson, 1988). Biodiversity encompasses many levels of life organization such as species, ecosystems, and genetics, but the easiest category to visualize, and that is most commonly used, is the number of species in a particular region.

Biological inventories have traditionally been considered the most extensive means of documenting species biodiversity. Diversity of flowering plants of the Baja California Peninsula was cataloged by Wiggins (1980), with 2,958 taxa (species and varieties/subspecies) with 23 % being endemic. Rebman and Roberts (2013) state than that number could reach 4,000 taxa when field work of the entire peninsula is completed and precise taxonomic work performed.

In the arid and semiarid regions of northern Mexico, plants have evolved into a moderately rich and distinctive flora with specialized growth forms and a high rate of endemism (Rzedowski, 1973). The Baja California Peninsula has a long history of field studies, documentation, and interpretation of land flora. Several floristic treatments have been published for all of the territory or parts, including some taxonomic revisions (Shreve and Wiggins, 1964; Gentry, 1978; Wiggins, 1980; Gould and Moran, 1981; Daniel, 1997). Studies of plant diversity of the Baja California Peninsula have covered most of the region, but there are some regions without detailed floristic work; the most intensive collections were performed in the northern and southern sectors (Arriaga et al., 2005). An example of an area that has not received as much attention is the Bahía Magdalena region. This land has remained under low pressure from the local population, but the marine environment has been severely exploited in the last two decades (Funes–Rodríguez et al., 2007). Consequently, some civil organizations have shown interest and have proposed legal protection of the area to government environmental authorities (Danemann, 2010).

The objective of present research was to assess the diversity and distribution of higher plants found in the vicinity of Bahía Magdalena to propose short term management schemes for providing continued biodiversity, conservation, and legal protection of the area.

 

Methods

Study area. Magdalena Bay area, including the islands of Margarita, Magdalena, and Creciente are an ample surface considered by the Comisión Nacional para el Conocimiento y uso de la Biodiversidad (CONABIO) as both Mexican terrestrial and marine regions important for conservation, known as the Planicies de Bahía Magdalena (terrestrial) and Bahía Magdalena (marine), with 5,648 km2 and 17,578 km2 respectively (Arriaga et al., 2000a, b; Bizarro, 2008). Shreve and Wiggins (1964) and Wiggins (1960) recognized this area as the Magdalena Plains subdivision of the Sonoran Desert.

Figure 1 outlines our study area, which is almost the same than the area designated by CONABIO as the Planicies de Bahía Magdalena. The study area is a polygon that comprises only terrestrial land from Highway 1 to the Pacific coast and bounded on the north at 25° 52' N and on the south at 24° 22' N. This area includes 5,018 km2 of country, from which about 750 km2 is devoted to farming (Valle de Santo Domingo), which is excluded from this study. Around 1,200 km2 are occupied by lagoons, which includes mangrove stands and prairies of marine phanerogams. Most of the study area is within the Municipality of Comondú and only a bit of the southernmost sector is in the Municipality of La Paz, both are within the jurisdiction of the State of Baja California Sur.

Climate. According to García (1973) most of the area is hot and very dry (BWh in Köppen climate classification), with at least 80 % of the rainfall occurring in summer. The few weather stations in the area record annual averages as follows: Poza Grande 64 mm, Puerto Magdalena 83 mm, Puerto Adolfo López Mateos 89 mm, Puerto San Carlos 98 mm, and Santo Domingo 104 mm; but eventually, when strong hurricanes impact the area as one did in 2009, up to 200 mm is recorded in hours. Mean annual average temperature in those same stations is by 21 °C, with January having a mean temperature of 17 °C and August and September have a mean of 28 °C. Tropical storms that originate in the tropical eastern Pacific are the main source of moisture. From 2001 through 2013, four hurricanes and five tropical storms directly affected the area (Unisys Co., 2013). In addition to the rain events, an important physical factor that characterizes this region is the low and thin morning advection fog, resulting from westerly moist Pacific air, moving across the cold California Current. The wind drives saturated air inland (up to 25 km) most of the year; typically fog is present from the first hours to 9–10 in the morning, being heaviest from January to May.

Vegetation and soils. Vegetation on most part of the peninsula, including the study area, is designated as sarcocaulescent scrubland (matorral sarcocaule) according to the official Mexican chart of vegetation (DEGETENAL, 1981). This is a variation of the well–recognized desertic or xerophyllous scrubland (matorral xerófilo), a generic name for the vegetation of arid and semi–arid regions in Mexico. Rzedowski (1978, 1979) proposes that such scrubland is the expression of the transition between the southern dry–tropical vegetation and the northerly desertic in the Baja California peninsula, functionally equivalent in mainland Mexico to the thorn scrub, thorn forest, or Sinaloan deciduous forest (Brown, 1994) that characterize the same transition along the coastal plains of northwestern Mexico in southern Sonora.

A coastal strip of land on the Pacific side of the peninsula, from 26° 25' N to 24° 18' N, is named in the Mexican official chart (DEGETENAL, 1981) as fog sarco–crassicaulescent scrubland (matorral sarco–crasicaule de neblina), which is the most widespread vegetation in the study area.

Soils are classified into four major types:

(1) Isla Magdalena and Isla Margarita have hills up to 550 m composed of Triassic–Jurassic rocks that are among the oldest of the peninsula, and are one of the 20 areas in Mexico with serpentine rocks (Ortíz–Hernández et al., 2006). Soils that develop from the high content of magnesium are extremely limiting to the variety of plants that can tolerate these conditions (Brady et al., 2005).

(2) Parts of the islands (Margarita, Magdalena, and Creciente), as well parts of the coast, are sandy, forming active or stabilized dune fields of Holocene age (Murillo et al., 1994); the fog sarco–crassicaulescent scrubland typically occupies stabilized dunes located as far as 25 km inland, up to 50 m above sea level.

(3) Gley (waterlogged) clay soils are of Pleistocene and Holocene age; mostly associated with mangroves (manglares) and salt flats (salitrales) along the shores of the lagoons (González–Zamorano et al., 2011).

(4) Flat areas are composed of alluvial soils derived from run–off carrying fine sands and silts, these occupy the lowest elevations of endorheic watersheds. They are closed basins that retain runoff for weeks to months, and when dry, leave silty deposits at the surface.

Botanical history. The first botanical expedition in this and surrounding areas occurred in 1837, when the English ship HMS Sulphur sailed up the west coast of the North American continent and elsewhere (Bentham, 1844). During the voyage, botanists collected a large number of plants surrounding Cabo San Lucas and Bahía Magdalena. This expedition yielded more than 100 new species for the peninsular and Mexican flora, which nowadays carries the species authority Benth. (George Bentham). Between 1889 and 1902, the Californian botanist Townshend Stith Brandegee made several trips to the southern Baja California Peninsula; the first trip in January and February 1889 included exploration in the Bahía Magdalena area (Brandegee, 1889), where he collected 158 taxa, including 12 new species, some of them endemic to Margarita and Magdalena islands. Several expeditions to these islands during the 20th century were leaded by Californian botanists (Shreve and Wiggins, 1964; Wiggins, 1980), but no novel species were reported except the description of a new species collected by Howard Scott Gentry in 1939, Tuctoria fragilis (Sw.) Reeder in the Hyrai plains.

Logistic. In September 2009, with support from CONABIO, we began fieldwork in this area just after the landfall of Hurricane Jimena in this area (Category 4 on the Saffir–Simpson scale). Within the boundaries of the Bahía Magdalena area (Figure 1), about 190 to 210 km of dirt roads provide access by two– and four–wheel drive vehicles. The islands were reached by small motor boats (pangas). Specimens were collected, preserved, and archived by traditional procedures (Lot and Chiang, 1986).

All collected specimens are in the CIBNOR herbarium in La Paz, BCS (HCIB). Some duplicates were exchanged with the San Diego Natural History Museum (SD). The basic references for identification and nomenclature were Wiggins (1980) and Shreve and Wiggins (1964). Some additional references were consulted (Bravo–Hollis, 1978; Bravo–Hollis and Sánchez–Mejorada, 1991a, b; Gentry, 1978; Gould and Moran, 1981; Hickman et al., 1993; and Turner et al., 1995). Subsequently a checklist was prepared consulting Spears (2006) and Tropicos webpage (2013) of the Missouri Botanical Garden for updating of nomenclature which basically follows APG III (Angiosperm Phylogeny Group) classification.

 

Results

The flora. Over 38 months we visited 116 collection sites, which are shown in Figure 1. A database of all collections from the study area was assembled that includes 2,294 specimens, which can be consulted online at CONABIO webpage by request (http://www.conabio.gob.mx/institucion/cgi–bin/datos.cgi?Letras=HJ&Numero=2). Of these, 1,835 are the contribution from our field collections, 158 more are reported by Brandegee (1889), and the remaining 301 are included in the BajaFlora database (www.bajaflora.org), which represents a consortium of seven herbariums that collected and archived plant information from the Baja California Peninsula through most of the 20th century, such database is operated by the San Diego Natural History Museum (SDNHM). Table 1a synthesizes the main taxonomic groups of the 506 taxa of vascular plants in the study area. The attached Appendix 1 shows the floristic checklist, which contains five taxa (species and infraspecies) of ferns, two of gymnosperms, and 498 of Magnoliophyta (angiosperms) consisting of 438 magnoliopsida (dicotyledons) and 60 liliopsida (monocotyledons). The flora is represented in 86 families and 280 genera.

Almost 50 % of the flora occurs in 13 families, the best–represented are Asteraceae (68 taxa), Poaceae (42), Euphorbiaceae (39), and Fabaceae s.l. (35). Forty–three families are represented by a single species. Euphorbia is the most diverse genus (23 taxa), others are Atriplex (11), Ambrosia (10), Lycium (9), Perityle (7), Cryptantha (6), Sphaeralcea (6), Physalis (6), Bahiopsis (5), Boerhavia (5), Encelia (5), Ipomoea (5), Leptochloa (5), and Suaeda (5).

As in other dry tropical areas in Mexico, families Asteraceae, Poaceae, Fabaceae s.l., and Euphorbiaceae predominate in the floristic list; as in southwestern Sonora, bordering the Sonoran Desert (Martin et al., 1998; Van Devender, 2000), and the southern part of the peninsula (León de la Luz et al., 2008).

The resulting flora was categorized as nine life forms (Table 1b) whose classification is compatible with the proposals of Shreve (1951), Crosswhite and Crosswhite (1984), and Medina (1999) for the Sonoran Desert and dry tropical floras. Single–stemmed herbaceous plants, perennials (127) and annuals (143), represent 53 % of all species. Trees are poorly represented (6), but there are significant shrubby forms (91). Hydrophytes (27) include aquatic and marine plants and those growing in moist soils, mostly in mangroves, vernal pools, springs, and at the only oasis in the area (el médano). Succulents (53) are a complex group that includes rossete–stemmed and leaf succulents that are members of the Agavaceae, Aizoaceae, Bromeliaceae, Cactaceae, Crassulaceae, and Portulacaceae families; also some semi–succulents forms in the Anacardiaceae, Burseraceae, and Capparaceae families were here included. Parasites (5) include dodder (Cuscuta) and mistletoe (Psittacanthus and Phoradendron). Vines (21) belong to Asclepiadaceae, Convolvulaceae, Cucurbitaceae, Passifloraceae, Sapindaceae, and other families. Prostrate or low ground–cover plants (32) include annual or perennial herbs with decumbent and procumbent growth and those that form mats, these typically inhabit sandy soils. Some species can be present in two or three categories, but the most prominent character was considered in this classification.

In the flora checklist, families in each major group are arranged in alphabetical order according modern classification (Spears, 2006), and then by genus, species, and infraspecies. Well known non–native and invasive taxa are indicated with an asterisk (*). After the author name, we include, in some cases, inside square brackets, alternative names for these taxa, especially when there are changes in nomenclature from Wiggins (1980), our primary floristic reference, these names were consulted in Tropicos (2011) webpage. For all entries, we include, after the scientific name and author, a two–letter code for the life forms listed in Table 1b. Finally, a common name for the taxon, if applied in the area, is included.

Plant communities. Figure 2 shows the spatial distribution of the vegetation types or plant associations, which were modified from the INEGI image (CONABIO, 2002). The sarcocaulescent scrubland is the most extensive vegetation type of the peninsula; in the study area, it occupies the innermost inland surface between 50 m and 70 m above sea level. Subtle variants of this type, considered in the older and recent INEGI charts, are the crassi–caulescent, sarco–crassi–caulescent, and desert microphyllous scrublands. These designations depend on the physiognomically dominant plants (cacti, semi–succulents, and shrubby legumes, respectively). Such scrubland covers almost 700 km2 within the study area, and the physiognomy of this vegetation includes stemmed succulent forms, such as Pachycereus pringlei, Lophocereus schottii var. schottii, Stenocereus thurberi var. thurberi, S. gummosus, Cylindropuntia cholla, and semi–succulent plants, such as Jatropha cinerea, J. cuneata, Bursera microphylla, Fouquieria diguetii; shrubby plants with microphylous leaves, such as Atriplex linearis, Ambrosia magdalenae, Larrea tridentata, and leguminous shrubs and trees, such as Acacia constricta, Mimosa purpurascens, Parkinsonia florida, P. praecox, and Prosopis palmeri.

A special type of sarcocaulescent scrubland covers a strip of land up to 15 to 25 km wide parallel to the coast of Bahía Magdalena, which contains sandy soils derived from marine sediment, an ancient beach or paleodune gradually rises inland above sea level to reach up to 50 m. The most common are Fouquieria diguetii, Jatropha cinerea, J. cuneata, Pachycereus pringlei, and Stenocereus gummosus. This vegetation is the most extensive in the study area, occupying 3,300 km2. The fog supports a rich lichen flora dominated by Rocella decipiens Darb., R. gracilis Bory, R. peruensis (Krempelh.) Darb., R. portentosa (Gay) Darb., Lecanora caesiorubella Ach., Niebla cephalota (Tuck.) Rundell and Bowler, and N. ceruchis Rundell and Bowler. These grow on shrub foliage (Nash III et al., 2002); the bromeliad Tillandsia recurvata is also common.

A plant association called locally mezquital is dominated by the mezquite Prosopis articulata, which grows along the margins of arroyos or drainage systems on the western side of the Sierra de La Giganta. Such conditions are more common on alluvial plains toward the Pacific coast. In this relatively small plant association, which occupies no more than 35 km2, local farmers use the mezquite wood for making charcoal. The mezquital follows arroyos on the vegetation maps, but they are not continuous; the density of trees is variable, sometimes dense, but sparse in other areas. This community is ecologically important as evergreen patches of vegetation on the landscape because the indigenous animals intensively use the trees for shelter, nesting sites, and feeding. Other species in this association are Vallesia glabra, Ambrosia ambrosioides, and Acacia farnesiana, although this last seems to be introduced and dispersed by cattle that concentrate in suitable places for forage and shade. Along these same drainage ways, there are occasional small riparian wetlands or oasis with standing water, but within the study area, only one oasis was found (el médano), which is characterized by the Bonpland willow Salix bonplandiana.

There are sparse and small areas that are irregularly and seasonally inundated. The largest in surface area is called llano de Hiray, covering some 65 km2. Adding all similar formations, this habitat could cover some 100 km2. The lowest areas of the llano de Hiray are about 2 m below sea level. After heavy rains, these low–lying areas remain flooded for weeks, or months, because there is no external drainage to the sea. When flooded, the water percolates slowly or evaporates; thus, a succession of hydrophytes occupies the flats, species that are present are adapted to specific depths of water, season, and water–holding capacity of the soil. Tuctoria fragilis is a locally endemic grass in this wetland that covers the landscape, appearing when the soil reaches moderately high water content. Other hydrophytes found in ponds or seasonal arroyos include Ammannia coccinea, Bergia texana, Eleocharis geniculata, Lythrum spp., Marsilea fournieri, Mecardonia vandellioides, Petunia parviflora, and Stemodia durantifolia. During dry seasons, some semi–halophytic species are common, including Heliotropium curassavicum and Argemone gracilenta, which are particularly abundant sometimes.

The mangrove surrounding Bahía Magdalena is the most extensive on the peninsula, covering an area of 236 km2 (González–Zamorano et al., 2011). Rizhophora mangle and Laguncularia racemosa are common near drainage channels; Avicennia germinans occurs inland, where flooding occurs during the highest tides a few times a year. Associated with the mangroves, particularly behinds stands of A. germinans are the saltwort Batis maritima and shoregrass Monanthochloe littoralis. The mangrove stands have a dynamic link with tidal conditions (± 0.85 m), usually through extensive tidal channels that flood the mangrove. Blocked channels lead to the death of ample surfaces of mangrove, starting in the shallowest depths. Fresh water supply is irregular, which usually occurs when extraordinary rains discharges through the few arroyos that reaches the coast. There is no evidence of undersea discharge of freshwater from the land.

Slightly higher than the mangrove stands, the soils are usually wet but can quickly dry out. These are clayey soils that are rich in organic matter, but highly alkaline and saline. They are limited to a few vascular plants. The vegetation is dominated by members of the Chenopodiaceae, commonly referred to as salitrales (saltflats or mudflats) that occupy 202 km2 (González–Zamorano et al., 2011). These areas flood during irregular high tides that occur from July through December or after heavy summer rainstorms. Succulent and halophytic plants are characteristic. The most common plant is Allenrolfea occidentalis, with Arthrocnemum subterminale, Sarcocornia pacifica, and Salicornia bigelovii. Bordering the saltflats are a characteristic plant association dominated by Lycium (L. andersonii, L. berlandieri, L. brevipes, L. fremontii, and L. megacarpum), Castela peninsularis, and species of Suaeda.

On the islands, two vegetation types are well established: one we designated as montane scrubland (matorral montano). This appears only on rocky mountainous areas on the islands (two on Magdalena and one on Margarita), covering an area of 250 km2. This montane scrubland is sparse and low vegetation that covers the slopes of the hills of both islands. After heavy rains, the canopy is estimated to have 40 to 60 % coverage. Some of the most common plants are Ambrosia magdalenae, Bajacalia tridentata, Euphorbia misera, Fouquieria diguetii, Jacquemontia abutiloides, Jatropha cuneata, J. canescens, Lippia palmeri, Marina parryi, Pachycormus discolor, Porophyllum gracile, Stenocereus gummosus, and Simmondsia chinensis. Dominant island endemics include Bahiopsis subincisa, Cochemiea halei, Echinocereus barthelowanus, Gongylocarpus fruticulosus, and Opuntia pycnantha. Subordinate plants include Bebbia juncea, Bursera hindsiana, Castela peninsularis, Euphorbia lomelii, Gossypium harknessii, Ipomoea jicama, and Pachycereus pringlei.

The coastal dune vegetation (vegetación de dunas costeras) includes beaches, sand bars, and extensive dune fields that occupy about 230 km2, most on Isla Creciente and Isla Magdalena (Aguirre Muñoz et al., 2010). On the peninsula el médano amarillo is the best representation of coastal dunes. Abronia maritima, Sporobolus virginicus, and Sesuvium portulacastrum are the pioneer plants, with rhizomatous and prostrate life forms on the new or active dunes, where sand is transported mainly by wind and secondly by high tides; this result was also annotated by Johnson (1977). Stable dunes are commonly covered with Aristida californica, Astragalus magdalenae, Funastrum arenarium, F. peninsulare, Lotus bryantii, Palafoxia arenaria, P. linearis, and Psorothamnus emoryi var. arenarius.

Species such as Encelia ventorum and Isocoma menziesii are relatively common in the northern and central coast of the peninsula, but are still found in the dune fields in our area. A tropical species, Scaevola plumieri was found healthy on stable dunes on Isla Magdalena, reaching its northern geographic limit. Some shrubby species, typical in the Cape Region desert scrubland, are present here, especially Ebenopsis confinis, where it forms thick cover between dune crests. Widespread annual life forms include Amaranthus palmeri, Cenchrus palmeri, and Phaseolus filiformis which are abundant after summer rains; Boeberastrum anthemidifolium, Camissonia sceptrostigma, Perityle spp., and Sphaeralcea spp. appear after limited winter rains.

Marine vegetation consists of a few species; the surf grasses Phyllospadix scouleri and P. torreyi inhabit the breaker zone on the Pacific seashore of Isla Magdalena and Isla Margarita. These species grow in large clumps or beds exposed during low tide and submerged at high tide. They are attached to rocks in the middle to low intertidal zones to a depth of 1–6 m. Some algae may grow together and all represent an important food resource for sea turtles (Santamaría–Gallegos et al., 2003). The eel grass Zostera marina is common at shallow depths in Bahía Magdalena and Bahía Almejas. Carrera–González and de la Fuente (2003) estimate that this zone covers an area of 140 km2 (ca. 10 % of the lagoons) as intertidal beds. Spartina foliosa is also a sea grass, but is an emergent life form up to 1.3 m high that grows in stands in shallow water and is generally associated with mangroves. Its biological importance and range needs more documentation.

Geographically restricted species are relevant because there are 20 endemic taxa, only four taxa have infraspecific status. Endemism includes one genus (Gongylocarpus, with one species and two subspecies). Table 2 shows these important taxa set; in this, 1 indicates the taxa restricted to the islands, 2 to the contiguous peninsular land, and 3 to both areas.

 

Discussion

The flora of this region, with ca. 4,268 km22 of land and 140 km2 of lagoons, has 506 taxa of vascular plants. Some known regional floras in Baja California Sur state, those from Sierra de La Giganta (729 taxa in 7,369 km2; León de la Luz et al., 2008) and the Cape Region dry tropical forest (520 taxa in 2,500 km2 León de la Luz et al., 2012) are alike in surface size and number of taxa.

The vegetation landscape of the Bahía Magdalena plains contains scattered trees and some shrubs of relatively low diversity, which provide sparse canopy cover. Most of the species are herbaceous life forms and appear only after heavy rains (Table 1b). Vines and climbing species are moderately common. Hydrophytes are heterogeneous, found in marine and freshwater environments. Succulents endure not only low water availability, but alkaline and saline soil conditions. In most of the peninsula, the cardon Pachycereus pringlei is a conspicuous plant, a key species in the ecosystem because the high biomass per plant and plant density, but in this region, its density is relatively low and its size is small. It is practically absent on dunes and island and has low density in the other plant communities.

The 19 endemic taxa suggest a grade of singularity in the plant assemblages, mainly in the islands. Only two species endemic of cacti are protected by Mexican law (NOM–059; SEMARNAT, 2010): Cylindropuntia santa–maria and Stenocereus eruca. Other cacti, Ferocactus townsendianus var. santa–maria is a poorly known species since only four specimens were located in 1952, which two were sent to botanical gardens according Lindsay (1996); Bravo and Sánchez–Mejorada (1991a) suggest that this may be an invalid taxon, but additional fieldwork is necessary to locate the only population reported. The family Asteraceae contributes two taxa, of which Bahiopsis subincisa is locally abundant above 100 m on the rocky slopes of the islands. Lyrocarpa xanti is other few known taxon, some specimens from the mid–peninsula have been designated with this name, but these could belong to some of the subspecies of the widespread L. coulteri. Asclepias masonii also has been poorly collected. Two specimens, one of Bahiopsis (Asteraceae) and other of Stenotis (Rubiaceae), seem to be novelties for the local flora, and require additional work.

In 1889, T. S. Brandegee collected sterile exemplars of Mimosa sp. on Isla Margarita with the annotation, "small bush neither in flower nor fruit". In 1928, J. N. Rose made a revision of Mimosaceae for North American flora and considered the inclusion of "Mimosa (?) Margaritae Rose, sp. nov" for such exemplars; since then nobody had collected additional material, but we found some exemplars during the fieldwork.

The grass family contributes two endemic taxa. Muhlenbergia brandegeei has an unusual distribution: Isla Margarita on the Pacific side of the peninsula and Isla Espíritu Santo on the Gulf of California. Tuctoria fragilis appears sporadically and massively on the llanos de Hiray, this species and Stenocereus eruca are the only endemics in the study area that are not found on the islands. The beach plant Helianthus niveus of the northern part of the peninsula was reported by Brandegee (1889) during his trip to Bahía Magdalena, but this taxon has never been collected again in locations south of the Desierto Vizcaino in the middle of the peninsula.

We found two gymnosperms on dunes, Ephedra californica and E. aspera (Ephedraceae), paleo–endemics of North American deserts, both extend their previous known range by almost 300 km southward following Turner et al. (1995) and Villanueva–Almanza and Fonseca (2011).

The endemic shrub Gongylocarpus fruticulosus in the Onagraceae has two varieties (see Table 2 and floristic list), both taxa are difficult to differentiate based on the type of vestiture. It is a common plant in the hills of Isla Margarita and Isla Magdalena islands, characterized by year–round blooms of large blue–magenta flowers. This is a good candidate for horticulture, similarly, the small Agave margaritae has this same potential.

From a purely floristic analysis, the Bahía Magdalena region has been considered a subunit of the Sonoran Desert province (Shreve, 1951; Shreve and Wiggins, 1964; Wiggins, 1980), based on landscape features and plant composition. The presence of great stands of mangrove highlights a tropical and subtropical feature, as well as some other species.

In the sarcocaulescent scrubland, the majority of the species present also occur in the dry tropical environment of the Cape Region, but two conspicuous species give here a particular touch Larrea tridentata and Atriplex canescens that are common in northern dry deserts. The marine plants reflect the influence of cold water current such as Zostera marina, Phyllospadix spp., and Spartina foliosa. There is no record of the rhizomatous grass Jouvea pilosa (J. Presl.) Scribn., which is common along the beaches of the Gulf of California; instead, Sporobolus virginicus occupies here the same niche. Both species converge on the Gulf of California side at the southern end of the peninsula. Avicennia germinans is a mangrove that has its northern limit just in this area.

Cattle grazing have been present in this area for at least two centuries. It is difficult to determine how livestock practices have impacted plant composition and structure. According to some anecdotal information from local elder population, mortality of plants is noticeable when rainfall is absent.

The farmed land area (Valle de Santo Domingo) has not expanded since the 1970s because the subsurface aquifer has been drawn down with consequent salt–water incursion; only 40–50 % of the converted land area is currently in agricultural use (some 750 km2 in our study site polygon, considering also urban and sub–urban areas); these abandoned fields has been unable to progress in a secondary succession process, probably as a consequence of changes in the soil structure caused by mechanized farming. Along the coast, shrimp farms are a potential threat to mangrove stands. Invasion of exotic plants seem to be a minor concern, with the exception of some induced grasslands for cattle raising and buffel grass Pennisetum ciliare alongside the roads, this is an invasive plant that was introduced as pasture during 1960 decade. Also, Tamarix aphylla has been introduced in ranchos and fish camps as a shade plant, it is potentially an important invasive threat.

In summary, our study area contains several distinct landscapes. The most characteristic is the fog scrubland due its particularity in the peninsular context, but from a botanical perspective, the most important environmental units are the islands because of the many endemic species. Mexican environmental authority is evaluating the establishment of a Natural Protected Area within the surface that we surveyed. The boundaries are still undefined, although the marine zone seems to be the main objective. This includes mangroves, which are breeding area for marine animals that support several fisheries; the extensive beaches and adjacent coastal dunes, where at least five species of sea turtles lay eggs, and the relatively shallow bays and lagoons where gray whales (Eschrichtius robustus) breed in the winter and support an important ecotourism economy. If the initiative progresses authorities should include the rocky island areas where the polygon for Magdalena bay protected area will be defined, since an important number of endemic plants inhabit in these environments.

 

Acknowledgements

We thank Miguel Córdoba Matson of CIBNOR for English and editorial services. We also thank Miguel Domínguez for field assistance and fishermen of Puerto Cancun and Puerto Chale. Field trips in the Magdalena bay area were supported by Comisión Nacional para el Conocimiento y Uso de la Biodiversidad (CONABIO, grant HJ 002). Collection permits were issued by Dirección General de Vida Silvestre (SEMARNAT: SGPA/DGVS/03824/09, SGPA/DGVS/01928/10, SGPA/DGVS/07132/10, SGPA/DGVS/07004/11, and SGPA/DGVS/09748/12). We are also grateful with reviewers of Biological Sciences for their valuable comments and suggestions in the review of the early manuscript.

 

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