Distribution of Agave (Agavaceae) in Mexico

Abisaí García-Mendoza

Summary: The genus Agave (Agavaceae) is endemic to the American continent. Its distribution extends from the southern United States to Colombia and Venezuela. Agave has approximate­ly 200 species and 247 taxa, of which 150 species and 186 taxa — or 75% of the total number, and 69% of endemic species — are found in Mexico. Mexican species can be found at altitudes from sea level up to 3400 m, and they are especially abundant in xerophytic plant communities. The region richest in the number of endemic species is the valley of Tehuacán-Cuicatlán, in the south-central area of the country, with 15 species, eight of them endemic.

Figure 1. Distribution of Agave and the number of species growing in each country. The quantities are: Species + Infraspecific categories = Total taxa (%). Agave has 200 species and 247 taxa.

Agave (Agavaceae) is endemic to the American continent, with a distribution extending from the southern United States (with two dis­junct species in Florida) to Colombia and Venezuela. This area includes all of the Caribbean islands, from the Bahamas to Aruba, Curaçao and Trinidad & Tobago, which are adjacent to the South American coast. Views on the generic delimitation of Agave and the number of its species have varied over the years. Berger (1915) recog­nized 274 species in subgenera Manfreda, Littaea and Euagave. For Breitung (1968) recognized some 110 species in Littaea and Euagave. Irish & Irish (2000) esti­mate the number of species to be 200-250, while Thiede (2001) recognizes 210 species in subgenera Littaea, Agave and Manfreda (included in the latter are the genera Polianthes and Prochnyanthes). In this paper I con­clude that Agave (subgenera Agave and Littaea) has approximately 200 species, plus 47 infraspecific cate­gories, giving a total of 247 taxa. Figure 1 shows the dis­tribution of the genus and the number of species in each country. Mexico, the United States, Cuba and Guatemala are the countries with the highest number of taxa; the other coun­tries have less than 8 species each, less than 3% of the total number. These numbers are based on available taxonomic information, on observa­tions made in habitat and on our examinations of specimens in several Mexican herbaria.

Figure 2. Agave parrasana showing detail of leaf and its armature. Sierra de Parras, Coahuila.

Among the contributions that have appeared since Gentry's (1982) work, we should mention the publication of at least six new taxa, as well as six additional ones soon to be published, eight changes in nomenclature, the recognition of some taxa cited by Gentry (1982) as syn­onyms, as well as the rediscovery in the field of several others previously known only from cul­tivation: Agave warelliana in Chiapas and Guatemala (Garcia-Mendoza & Lott, 1994), A. albomarginata from Querétaro (Hernández, in this same issue) and A. ellemeetiana, which was rediscovered in Oaxaca (Garcia-Mendoza & Munn, in prep.). These new contributions have appeared because of better taxonomic and phytogeographic information as well as herbarium specimens that support this knowledge. For example, at the MEXU herbarium, the Agave collection has increased to nearly 3000 speci­mens, making it one of the most important col­lections not only of the genus but of the entire Agavaceae (5000 specimens).

Figure 3. A giant plant of A. atrovirens var. atrovirens growing under oaks on the Sierra Mixteca, Oaxaca.

One of the ongoing taxonomic problems of the family are the limits between the genera Agave, Manfreda, Polianthes and Prochnyanthes. Molecular research by Clary & Simpson (1995), Bogler & Simpson (1966) and Eguiarte et al. (2000) supports the view that the entire lineage of Agave, Manfreda, Polianthes and Prochnyanthes should be contained in one genus. Nevertheless, the transfer by Thiede (2001) of the last three genera to Agave results in a very large genus with different morpholo­gies, ecologies and reproductive biologies. Another option would be to divide Agave into several monophyletic genera. The disadvantage of this alternative is that there are no clear characters remaining to separate the groups (Eguiarte et al., 2000). Therefore I believe — as do the aforementioned authors — that we need far more morphological, ecological, biochemi­cal and molecular research to understand the real evolution of the group and to be able to propose a more adequate classification. I there­fore consider the genus Agave to consist only of the subgenera Agave and Littaea.

Figure 4. A. americana in natural habitat on a mountain near Ciudad Victoria, Tamaulipas.

Richness of species in Mexico

The number of species of Agave in Mexico has varied depending on the taxonomic delimita­tion of the genus. Trelease (1920) recognizes 170 species, Gentry (1982), in his systematic analysis of the genus, considers there are 125 species, while for Thiede (2001) there are 149. The total number is directly related to the taxonomic criteria used and it is also influenced by the taxonomic knowledge of each taxon, its dis­tribution and its habitats.

Figure 5. A. rhodacantha near the Rio Atoyac, between Izúcar and Acallan, Puebla.

Based on the work of Gentry (1982), García-Mendoza and Lot (1994), García-Mendoza (1995), García-Mendoza and Galván (1995), as well as on regional work by several authors and on extensive field work, I consider that Mexico has 150 species, plus 36 infra-specific taxa, giv­ing a total of 186 taxa. This represents 75% of the total number of species known worldwide.

The number of 150 taxa is similar to the number recognized by Thiede, except that I exclude the species of Manfreda, Polianthes and Prochnyanthes.

Figure 6. A. chiapensis near Villa Alta, Oaxaca.

Ecological distribution

The species of Agave in Mexico can be found from sea level to 3400 meters, although they are most commonly found between 1000-2000 m. The number of species exclusive to each altitudinal range is higher nearer the sea and diminishes as the altitude increases. Table 1 shows the altitudinal range of distribution in the genus.

Figure 7. A. atrovirens var. mirabilis. A large specimen near the top of the Sierra Madre Oriental, in the border zone between Puebla and Veracruz.

Between sea level and 500 m we have 28 species, most of them found on the peninsula of Baja California and in coastal areas. As the alti­tude increases towards the central region of the country, the number of species also increases, the highest number being found between 1000-1500 m and 1500-2000 m, where they can be seen on the mountain sides and the interior plateau; at these altitude it is common to see them along river ravines or in steep places exposed to sunlight. Only three species (Agave atrovirens, A. filifera and A. montana) grow above 3000 m. A. atrovirens can be found on the slopes of the volcano Pico de Orizaba and on the Cofre de Perote, as well as on the highest mountains of the Sierra Madre of Oaxaca. A. filifera is a species growing mainly between 2300-2800 m; nevertheless, in the Cerro El Zamorano, near the border between the states of Querétaro and Guanajuato, it can be found at 3250 m, where it is abundant on the mountaintop in damp places covered by fog during the rainy season. A. montana grows exclusively between 3200-3400 m in the mountainous region between Nuevo León and Coahuila (Villarreal, 1996) in areas where the temperature drops below 0°C during the cold season.

Table 1. Altitudinal distribution of Agave in Mexico.

In Mexico agaves can be found in many different environments. Table 2 groups these plants according to the vegetation types in which they are most frequently found. They are scarce in tropical evergreen forests as well as in tropical subdeciduous forests of southern Mexico; some species that arc exclusive to these forests are Agave colimana, A. grijalvensis, A. pendula and A. wendtii.

However, agaves are also abundant in tropical deciduous forests, thorn forests, grasslands, deserts and chaparral. In the first and last of these environments we find not only the highest num­ber of species but also the highest number of indi­viduals. In some areas their abundance is so high that the vegetation typo is named after these plants. Such is the ease with the "matorral rosetó-filo", where Agave and other rosette-forming gen­era such as Dasylirion and Yucca sometimes dominate. In these four warm-dry communities we have a little more than 100 species—approxi­mately 68% of all the species found in the country. Some taxa, like A. asperrima, A. bracteosa, A. dif-formis and A. victoriae-reginae, are adapted to very dry, hot conditions. They are also important in temperate Pinus and Quercus forests, where we find 44 species, or 30% of all the Mexican species. Only four (A. atrovirens, A. attenuata, A. ellemeetiana, and A. warelliana) grow in cloud forests, a community characterized by high humidity and fog during most of the year.

Figure 8. A. marmorata near Miahuatlán, Oaxaca; rosette with massive leaves and large teeth.

Figure 9. A. attenuata on limestone cliffs near Temascaltepec, state of México.

The types of soils favored by the plants are also diverse; they do well on igneous acid rocks as well as on basic limestone soils of marine origin. It is common to see them in temperate forests or in dry scrub on substrates derived from volcanic rocks like those found along the Eje Neovolcanico (Trans-Volcanic Belt) that crosses the country from west to east, between the states of Jalisco and Veracruz, extending through Mexico City. The species found on these soils include A dasylirioides, A. hookeri, A. horrida, A inae-quidens, A. obscura and A vilmoriniana,

Table 2. Ecological distribution of Agave in Mexico

Agaves are also abundant in dry forests on limestone soils of marine origin. Many species can be found there, especially those that grow in the arid zones of Mexico, such as A. lechuguilla, one of the species with the widest distributions.

Richness centers in Mexico

In order to define the species-richest areas in Mexico we used a map divided into 1° squares, on which the numbers of taxa were placed. The information was obtained from the specimens at the MEXU herbarium and from the available liter­ature. The map, published by Garcia-Mendoza (1995), is shown here (Fig. 2). It shows that the area richest in species is in the phytogeographic province of Valle de Tehuacán-Cuioatlan, where one of the squares has 15 species and the one next to it has nine. This province is located in the central-southern region of the country, in the area near the borders of Puebla and Oaxaca, and has a semiarid climate, a mean average annual rainfall of 400 mm and a mosaic of plant communities dominated by Tropical Deciduous Forest, Thorn Forest and Chaparral, all communities rich in succulent, spiny and rosette plants.

Figure 10. A. grijalvensis along Río Grijalva, N of Tuxtla Gutiérrez, Chiapas.

The second richest species-density in Agave corresponds to three squares found in the Sierra Madre Occidental, one with nine species and two with eight. This zone is in the north-west of the country, in the border regions of Sonora, Chihuahua and Sinaloa, where there are several plant communities of arid and temperate zones. However, the dominant ones are those with sever­al types of conifer and oak forests. Some of the agaves found here are Agave bovicornuta, A. jai-boli, A. multifilifera, A. polianthiflora, A. shrevei and A. vilmoriniana.

Figure 11. Richness centers of the genus Agave in Mexico.

The third most important area corresponds to three squares found in the Chihuahuan Desert, two with nine species and one with six. This area is in the north-east, bordering San Luis Potosí, Nuevo León and Tamaulipas, an arid zone domi­nated by several types of desert and chaparral floras that thrive on alluvial plains separated by mountains. The climate is dry, with high tem­peratures in summer and a mean average annu­al rainfall of less than 400 mm. Species that prosper under these conditions are Agave asper-rima, A. funkiana, A. lechuguilla, A. lophantha and A. salmiana. Other areas also rich in species are found in Baja California Sur, Coahuila, Michoacán, Chiapas and the state of México.

Figure 12. A. stricta on arid hills near Tehuacán, Puebla.

Centers of Endemism

There is an important endemic component in the distribution of Agave in Mexico. Table 3 shows that of the 186 taxa reported for Mexico, 129—or 69%—are endemic. This percentage can be even higher if one considers that the natural regions of Mexico extend further than the country's political boundaries.

According to Rzedowski (1991), the Sonoran, Chihuahuan and Tamaulipan arid northern provinces extend northward into the United States, a bio-geographical territory known as Megamexico 1, and the southern Mexican provinces extend to the north of Nicaragua to form Megamexico 2. The territory that would include both these areas is called Megamexico 3. On this basis, the biogeographical area that runs from the southern United States to Nicaragua has 192 taxa of Agave, 78% of the total number. Of this number, 186 (97%) are taxa that are exclusive to this territory.

Figure 13. Flowers of A. cupreata, Tixtla, Guerrero.

Of the total number of endemic species reported for Mexico, a high percentage are micro-endemic taxa because their distribution is con­fined to less than 30,000 km2 and include only 1-3 squares (1° x 1°), or less than 3% of the area. In this category are 88 taxa, 68% of the 129 endemic taxa of Mexico (Table 3). The distribu­tion of the micro-endemic taxa is not uniform; they are mostly found in dry mountainous areas and are absent from the warm-humid areas of Yucatán and the coast of the Gulf of Mexico (Fig. 3). Thirty-two of the taxa are found only in one square, though they can be locally abundant: Agave guiengola, A. macroacantha, A. par-rasana, A. titanota, A. triangularis and A. stric-ta; or, on the contrary, they can have populations with few individuals: Agave gypsophila, A. lurida, A. nayaritensis, A. nizandensis, A. peacockii and A. warelliana.

The three species-rich areas described above also have micro-endemic taxa. One of these areas is the Valle de Tehuacán-Cuicatlán, with eight species: Agave karwinskii, A. lurida, A. macroacantha, A. peacockii, A. scaposa, A. stricta, A. titanota and A. triangularis, besides other micro-endemic Agavaceae, such as Beschorneria calcicola, Polianthes bicolor and Yucca mixtecana. From this data we can see that this phytogeographical province is not only the most important for the Agavaceae in Mexico but also for a great many other plant communi­ties and species. The total number of species is thought to be some 2700, 30% of them endemic, which makes this province one of the world's centers of bio-diversity (Dávila & Herrera-MacBryde, 1997).

Table 3. Endemic species of Agave in Mexico.

Figure 14. Distribution of micro-endemic taxa of Agave in Mexico.

Figure 15. Flowers of A. schidigera.

Figure 16. Inflorescence of A. marmorata.

Finally, it should be said that the richness of endemic species of Agave in Mexico is largely due to the heterogeneous habitats of the country— with their differences in climate, geology, soils, topography, altitude, etc.—as well as to the intrinsic properties of each taxon, such as their genetic plasticity, ecological tolerance, disper­sion capacity, seed germination and their diverse biotic interactions with other organisms, includ­ing pollinators. Other factors that should not to be forgotten are the historical events that took place in North America and that influenced the present-day distribution of the species. All of these factors have made "Megamexico 3" the richest area in species today, the area with the highest diversity of form, where the known fossils of the family have been discovered, and where— with certainty—the genus and the whole family originated.

Figure 17. A. americana on a plain of the Chihuahuan desert N of Saltillo, Coahuila.

Nevertheless, it is important to underline that we still need basic studies on this genus in rela­tion to its taxonomy, biogeography, morphology, ecology, evolution, etc. Fortunately, Mexico has a team of scientists in different fields who have undertaken the difficult (but also fascinating) task of trying to understand several biological aspects of these marvelous plants.

Figure 18. A. horrida on the lava field near Cuernavaca, Morelos.

Figure 19. A. lechuguilla near Cardonal, Hidalgo.

Acknowledgments

I wish to thank Felipe Villegas for the excellent maps and Jennifer Bain for her translation.

References

Berger, A. 1915. Die Agaven. Verlag, Jena.

Bogler, J.D., & B.B. Simpson. 1996. Phylogeny of Agavaceae based on ITS rDNA sequence variation. Amer. J. Bot. 83: 1225-1235.

Breitung, J.A. 1968. The Agaves.

G. Glass & R. Foster (Eds.). The Cactus and Succulent Journal 1968 Yearbook. Abbey Garden Press, Reseda, California.

Clary, K.H., & B.B. Simpson. 1995. Systematics and character evolution of the genus Yucca L. (Agavaceae): Evidence from morphology and molecular analyses. Bol. Soc. Bot. México 56: 77-88.

Dávila, A.P., & O. Herrera-Macbryde. 1997. Tehuacán-Cuicatlán Region Mexico. In S.D. Davis, et al. (Eds.). Centres of Plant Diversity. A Guide and Strategy for their Conservation. WWF-IUCN.

Eguiarte, L.E., V. Souza & A. Silva-Montellano. 2000. Evolución de la familia Agavaceae: filogenia, biología reproductiva y genética de poblaciones. Bol. Soc. Bot. México 66: 131-150.

García-Mendoza, A. 1995. Riqueza y endemismos de la familia Agavaceae en México. In E. Linares-Mazari, et al. (Eds.). Conservación de plantas en peligro de extinción: Diferentes enfoques. Universidad Nacional Autónoma de México, Instituto de Biología, México, D.F.

García-Mendoza, A., & E.I. Lott. 1994. Agave L. G. Davidse, et al. (Eds.). Flora Mesoamericana, Vol. 6. Universidad Nacional Autónoma de Mexico, Instituto do Biología, México, D.F. garcía-mendoza, A., & R. galván. 1995. Riqueza de las familias Agavaceae y Nolinaceae en México. Bol.

Soc. Bol. México 56: 7-24.

Gentry, H.S. 1982. Agaves of Continental North America. University of Arizona Press, Tucson, Arizona.

Irish, M. & G. 2000 Agaves, Yuccas and Related Plants. A Gardener's Guide. Timber Press. Portland, Oregon.

Thiede, J. 2001. Agave. U. Eggli (Ed.). Illustrated Handbook of Succulent Plants: Monocotyledons. Springer-Verlag, Berlin & Heidelberg.

Trelease, W. 1920. Agave. P.G. Standley (Ed.). Trees and Shrubs of Mexico, Contr. U.S. Natl. Herb. 23(1): 107-142.

Villareal, J.A. 1996. Una nueva especie de Agave subgénero Agave (Agavaceae) de México. Sida 17: 191-195.

Figure 20. A. congesta. Flowering plant on the Sierra de San Cristóbal de las Casas, Chiapas.

Figure 21. Cultivated variety of A. angustifolia near Totolapan, Oaxaca. This species is the source of the distilled "mezcal de Oaxaca".

Figure 22. A. attenuata in natural habitat near Tlatlaya, state of México. All photos are by the author.

Abisaí García-Mendoza

Jardín Botánico, Instituto de Biología UNAM, A.P 70-614 Del. Coyoacán 04510, D.F., México. abisai@mail.ibiologia.unam.mx

© Cactus & Succulent Journal of America, 2002