The peninsula is composed almost wholly of a bed of coraline and porous limestone covered with a layer of thin, dry soil formed from the slow weathering of the bedrock. The humus is rarely more than a few centimeters deep, and consequently cornfields (milpas) must be moved every two or three years. With each move a new area of the land is cleared and burned. This agricultural practice has been routine with the Mayan people for centuries and apparently has had a profound effect on the vegetation, severely limiting the extent of virgin forest.
In many places, especially in the northern part of the peninsula, the rocky surface of the terrain is perforated by natural wells or sink-holes, which are called cenotes locally. Many cenotes support small "islands" of unique vegetation, the composition of which somewhat resembles the rain forest of the southern part of the peninsula. Temporary water-holes and shallow lakes (aguadas) are sparsely scattered throughout the southern half of the peninsula. There are no rivers of importance; most that are present are in the southernmost region.
The climate of northern Yucatán is hot and dry, and the absence of high mountainous ridges to intercept the moisture-bearing clouds from the Atlantic accounts for less rain than would be expected. The southernmost part of the peninsula receives a maximum of 200 centimeters of rain annually. The amount decreases toward the north to an average of about 45 centimeters. Almost all of the rain falls from May to October. Daily temperatures range from 78° to 98° F. in the shade; cool sea winds prevail day and night throughout most of the year. The hottest months are May and June. In winter, fierce northers, temporales or nortes, occasionally sweep across this open region, bringing rain and cooler temperatures, lasting several days.
The areas adjacent to British Honduras and Guatemala receive sufficient rainfall to support forests containing mahogany (Swietenia); sapodilly (Achras); several valuable cabinet woods; logwood (Haematozylon) and other dyewoods. As one proceeds northward the flora and fauna change gradually from that of the wet tropics to that of the dry tropics.
Paynter's (1955a) system of vegetational zones on the peninsula is useful to ornithologists. He recognized three zones: Scrub, Deciduous Forest, and Rain Forest. The Scrub zone occupies Isla Holbox and Isla Mujeres and the arid northern coast of Yucatán. The zone extends only a few miles inland and consists of low-tropical thorn forest of mesquite (Prosopsis), Mimosa, Yucca, Agave, and mangrove (Rhizophora) wherever standing water occurs. Sisal in Yucatán and Isla Mujeres of Quintana Roo are in this Scrub zone.
The Deciduous Forest zone is a vast area of low, semiarid, secondary forest occupying all of the state of Yucatán not in the Scrub zone, northern Campeche, and a thin strip a few kilometers wide in Quintana Roo along the Yucatán-Quintana Roo border and a similar narrow band along the eastern coast. The island of Cozumel is also included in this zone. Collections were made in the Deciduous Forest at Champotón, Campeche, Isla Cozumel and Puerto Juárez, Quintana Roo, and Pisté, Yucatán.
The Rain Forest zone occupies the remainder of the peninsula and includes most of the territory of Quintana Roo, and the southern half of Campeche. Collections were made in this zone at Escárcega, Campeche, and at Felipe Carrillo Puerto and Pueblo Nuevo X-can, Quintana Roo. Isla del Carmen cannot be accurately assigned to any of these three zones because most of the island has been planted in coconut palms. It is an area of high rainfall and may once have supported rainforest.
In the present report the term "second growth" refers to areas that have been recently cut over. In these areas, the forest is characteristically low and in a stage of transition. In the Rain Forest zone "second growth" areas usually contain species of deciduous trees.
Paynter (1955a) has given an excellent descriptive summary of the topography, climate, and phytogeography of the Yucatán Peninsula and there is little need to go into further detail here. During summer, the rainy season, the Deciduous Forest is much more luxuriant in appearance than in winter when most of the trees have lost their leaves. Also, the numerous cenotes in this zone support a limited growth of evergreen trees. For example, the large Cenote Seco about two kilometers east of Chichén-Itzá contains elephant ear (Caladium), mamey (Mammea), figs (Ficus), and other large trees the trunks of which are some 10 meters below the surface of the surrounding tableland. These plants project five to 10 meters above the surrounding low forest. The floor of this cenote is near the watertable but not below it.
GAZETTEER