W. A. Fitz Maurice and Edward F. Anderson

Succulent plant flora

The Republic of Mexico has a great diversity of habitats and flora. Succulents are found throughout the country, from the tropical forests in the south and south-east to the arid regions along the international border of mainland and Baja California with the United States to the north. Two main mountain chains run down through the mainland of the Republic: the Sierra Madre Occidental in the west through the states of Sonora, Chihuahua, Durango, Sinaloa and Nayarit; and the Sierra Madre Oriental to the east through the states of Puebla, Hidalgo,

Veracruz, San Luis Potosi, Nuevo León, and Tamaulipas. Below this is the Trans-Mexican volcanic belt, and in the region between these three ranges the high plateau of north-central Mexico. All of these broad geographical zones have a rich succulent flora, as do the arid and semiarid zones of Puebla and Oaxaca, and the Baja California peninsula. Although less diverse, there is also a unique succulent plant component in the vegetation of the low, hot, humid coastal regions and the southern and southeastern tropical forests.

The extensive desert zones of Mexico consist of the Chihuahuan Desert, including parts of the states of Chihuahua, Coahuila, Durango, Zacatecas, San Luis Potosi, Nuevo León, Tamaulipas, Guanajuato, Querétaro, and Hidalgo; the Sonoran Desert of Sonora and northern Baja California; and the Vizcaino Desert, including parts of the states of Baja California and Baja California Sur.

The main vegetation zones in which succulent groups are found are the Chihuahuan Desert, Sonoran Desert, Vizcaino-Magdalena Desert, Desert Shrubland, Tropical Shrubland, Tropical Forests, and the Oak-Pine Forest. However of the 32 types of vegetation in Mexico (Rzedowski 1986) all but the aquatic, semiaquatic, and páramo types contain succulents. The Matorral Xerófilo and its subdivisions probably have the greatest diversity of

Box 3.5 Estimated number of succulent genera and species per family in Mexico

Cactaceae: There is some disagreement in the number of taxa of Mexico in this family. Bravo-Hollis and Sánchez-Mejorada (1978, 1991) recognise 1080 taxa (854 species and 226 varieties and forms), whereas Hunt (1992) lists 925 provisional and fully accepted species. According to Hernández and Godinéz (1994), following Hunt's taxonomic framework, there are 563 well recognised species. Cactus nomenclature here follows Hunt, with the addition of new discoveries and published varieties.

Crassulaceae: Following Walther (1972), together with recent discoveries, there are over 150 taxa of Mexican Echeveria. Jacobsen (1960) lists 29 Mexican taxa of Dudleya, more than 30 of Villadia and 89 of Sedum. Additionally, using Jacobsen and including recent discoveries, there are 13 species of Pachyphytum, 14 of Graptopetalum, 4 of Lenophyllum,3 of Thompsonella, 1 of Tacitus and 3 of Kalanchoe. Thus, there are of the order of 350 Mexican taxa of Crassulaceae.

Agavaceae: Gentry (1982) recognises over 140 taxa of Agave. In addition, there are approximately 21 species of Manfreda (Piña-Luján 1978) together with 11 of Furcraea,3 of Hesperaloe, and 23 of Yucca (Standley 1920-1926 and Jacobsen 1960). This gives a total on the order of 200 taxa for the family in Mexico.

Fouquieraceae Henrickson (1969) lists 12 species of the single genus in this family, Fouquieria, all of which are found in Mexico.

Other families: Other families with succulent representatives in Mexico, so-called caudiciform succulents, or species considered 'succulent' by collectors, include:

Aizoaceae: Carpobrotus( 1) and Mesembryanthemum (1) . Anacardiaceae: Pachycormus (1) . Apocynaceae: Plumería (5). Asclepiadaceae: Gonolobium (several), Asteraceae: Senecio (at least 2). Begoniaceae: Begonia (several). Bombacaceae: Ceiba (4) and Bombax (2). Bromeliaceae: Hechtia (several). Burseraceae: Bursera (2 or 3) and Beiselia (1). Commelinaceae: Tradescantia (several), Convolvulaceae: Ipomoea (at least 3). Cucurbitaceae: Ibervillea (at least 2). Dioscoreaceae: Testudinaria (1). Euphorbiaceae: Euphorbia (at least 4),Jatropha (several) and Pedilanthus (several). Fabaceae: Erythrina (several). Lentibulariaceae: Pinguicula (several). Li I iaceae: Aloe (1) . Moraceae: Dorstenia (several) and Ficus (at least 3). Nolinaceae: Beaucamea (6), Dasylirion (15), Noiina and Calibanus (18). Oxalidaceae: Oxalis (several). Piperaceae: Peperomia (at least 1). Portulacaceae: Portulaca (several) and Talinum (several). Urticaceae: P/'/ea (1) . Vitaceae: Cissus (at least I).

Totals: Overall, aside from the cacti and excluding orchids, this gives a minimum of 750 succulent taxa, and, including plants as yet unnamed, there are possibly as many as 1000. Thus, there is probably a total of between 1600 and 2000 Mexican cacti and other succulent plants. The total number varies according to the definition of 'succulent' followed.

Table 3.14 Main vegetation types and cactus diversity (Source: Ariasi993)

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