Rio Mayo Project

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Cascada Basaseachic
Cascada Basaseachic, in Chihuahua, is a national park in
the upper Rio Mayo drainage in the Sierra Madre
Occidental. With a 246 meter free fall it is the highest
waterfall in Mexico (measured in 1983 by geographer R.H.
Schmidt of the University of Texas, El Paso)

In 1932 Howard Scott Gentry, just graduated from U. C. Berkeley, came to the Rio Mayo country in southern Sonora State, Mexico. He was immediately taken with the plants, the landscape, and the local people. For the next seven years, he explored the area and, in 1942, published Rio Mayo Plants: A study of the flora and vegetation of the valley of the Rio Mayo, Sonora (Publ. No. 527, Carnegie Institution of Washington). Gentry's project was ambitious: the area treated covers about 16,000 km2 (an area about the size of the state of New Jersey) and ranges from Mangroves and Thornscrub along the Gulf of California, through Tropical Deciduous Forest on the foothills, to high elevation (2900 m) Conifer Forest on the ridge tops of the Sierra Madre Occidental in the state of Chihuahua. The vegetation section of the book provides an account of the vegetation types, climate, geography, land uses by humans and a gazetteer of locations. The flora section provides scientific and local names of the plant species, with notes on the range, ecology, and ethnobotanical uses of each.

Decades later, Dr. Paul Martin of the Department of Geosciences at the University of Arizona began revisiting the sites where Gentry had worked and collected. Martin noted that Gentry's locations largely seemed stable in terms of species composition, but in traveling more extensively in the region, he observed many plant species that Gentry had never encountered. Thus began The Rio Mayo up-date project: an effort spear-headed by Paul Martin and involving literally dozens of students, staff and faculty from the U. of A. and other institutions. Between about 1980 and the present, many collecting trips have been made into the area involving most forms of conveyance known to humans (from vehicle to burro to kayak to foot). We estimate that 15000 numbers have been collected; these are housed at ARIZ and duplicates have been exchanged with a number of other herbaria (the first priority is MEXU in Mexico City).

Tropical Dry Forest in bloom
As is typical of most Tropical Deciduous Forests, many trees
bloom in the dry season when they are leafless. The dry
season picture at right shows pink flowered Tabebuia
and Ipomoea arborescens with white flowers.

The area is especially interesting because of its combination of vegetation types. The tropical dry forest is near the northern extreme of the range of this vegetation type, maintained at this latitude because the Sierra Madre Occidental provides a shield from arctic blasts that occasionally beset the same latitude in eastern Mexico. Here the tropical dry forest is bounded downslope by a unique coastal vegetation, the Sinaloan thornscrub. In this area, this thornscrub vegetation begins to meld into the Sonoran Desert to the north. Upslope from the tropical dry forest, the oak and the pine-oak forests are known for a high level of endemicity.

Tropical Dry Forest after rains
With the onset of the rainy season, the vegetation explodes
into an impenetrable mass of green. Although many species
flower during the wet season, they are not as conspicuous
as the species that bloom in the dry season, in part because
the the forest is so green.

The Rio Mayo project revised Gentry's work, adding new species discovered in the region by the team (both plants new to science and those whose ranges have been extended to the region), and documenting the effect of nearly 60 years of increasing human impact. About 2750 taxa are treated in the revised Rio Mayo Plants (compared to 1200 known to Gentry). The project was blessed by Gentry's approval before his death in 1993. Its physical home centered in the herbarium of the U. of A. and it enjoyed the support of the Southwest Center (also on the U. of A. campus), as well as of a number of private donors.

Resulting from this study was the book titled: "Gentry's Río Mayo Plants. The Tropical Deciduous Forest and Environs of Northwest Mexico". Paul Martin led the editorial team of D. Yetman, M. Fishbein, P. Jenkins, T.R.Van Devender and R. Wilson. The book also includes contributions from many experts who have authored treatments of specific groups of plants. It was published in 1998 by the University of Arizona Press.

Some new species described recently from Rio Mayo collections:

Bogler, D. 1994. Taxonomy and phylogeny of Dasylirion (Nolinaceae). Doctoral dissertation, University of Texas at Austin. 1994. [Dasylirion gentryi., Holotype (TEX), Isotype (ARIZ): P. D. Jenkins 92-117]

Daniel, T. 1995. Justicia masiaca (Acanthaceae), a new species from northwestern Mexico. Brittonia 47:408-413. [Holotype (CAS) T. Daniel et al 2546]

Nesom, G. 1993. Erigeron jenkinsii (Asteraceae: Astereae), a new species from the Rio Mayo area, Sonora. Phytologia 75:118-120. [Holotype (ARIZ) P. D. Jenkins 91-119]

Steinmann, V. and T. Daniel. 1995. Euphorbia gentryi, a new species of Euphorbia subgenus Agaloma from northwestern Mexico. Madroño 42:452-454. [Holotype (CAS) Victor Steinmann et al. 93-357]

Turner, B.L. 1992. Vernonia joyaliae . A new species of Vernonia (Asteraceae) from Sonora, Mexico. Phytologia 73:16-17 [Holotype (ASU) E. Joyal 1465]

Turner, B.L. 1993. Two new species of Senecio (Asteraceae) from Sonora, Mexico. Phytologia 74:382-384. [Senecio riomayensis, Holotype (ARIZ), Paul Martin et al. 17 March 1988, s.n.; Senecio tepopanus, Holotype (ARIZ), Howard Gentry 1411]


Gentry, H. S. 1942. Rio Mayo Plants: A study of the flora and vegetation of the valley of the Rio Mayo, Sonora. Carnegie Institution of Washington, Publ. 527, 328 pp.

Jenkins, P. D., P. S. Martin, L. A. McDade, M. Fishbein, T. R. Van Devender and A. Burquez. 1995. Plant phenological patterns of a tropical dry forest with bimodal annual rainfall: Rio Mayo basin, northwestern Mexico. Association for Tropical Biology. San Diego, CA

Jenkins, P. In press. A report on the revision of Howard Scott Gentry's Rio Mayo Plants. At The Pass Conference, University of Texas, El Paso, TX, March 1993.

P. S. Martin, D. Yetman, M. Fishbein, P. Jenkins, T.R.Van Devender and R. Wilson. 1998. Gentry's Río Mayo Plants. The Tropical Deciduous Forest and Environs of Northwest Mexico. University of Arizona Press.

McDade, L. A. and P. D. Jenkins. 1993. Research on tropical dry forest in southern Sonora. Association for Tropical Biology. San Juan, Puerto Rico.

Wilder, J. (Ed.) 1995. Explorations on the Rio Mayo. Journal of the Southwest 37:1-417.

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