The vegetation of the central ridges of the isthmus is, for the most part, transitional between the tall rainforest of the Gulf Coastal Plain and the low xerophytic scrub forest of the semi-arid Pacific Coastal Plain. On the northern slopes of the ridges the rainforest is more poorly developed than on the plains to the north. Many of the same species of trees are present, including Ceiba pentandra, Cedrela mexicana, Swietenia macrophylla, and Ficus sp.; nevertheless, these seldom are as large as members of the same species in the forest on the plains. Other species present on the forested slopes include Tabebuia Donnell-Smithi, Zanthoxylum melanostictum, Pithecolobium arboreum, and a species of Pterocarpus. The structure of this forest differs from that on the Gulf Coastal Plain in that there is no continuous upper canopy and there is a dense undergrowth ([Pl. 3, fig.1]). This type of forest extends from Mogoñe southward to about Matías Romero.
In the vicinity of Matías Romero open pine-oak forest (Pinus caribaea and Quercus sp.) is found on some ridges as low as 250 meters above sea level.
On the Plains of Chivela in the southern part of the central region the vegetation takes on a semi-arid appearance, especially in a savanna on the plains. Clumps of small trees and bushes, consisting of Croton nivea, Cordia cana, Jacquinia aurantiaca, Calycophyllum candidissimum, and Cassia emarginata, are scattered on a grassy plain, from which rise widely-spaced palms of an unknown species ([Pl. 3, fig. 2]).
Pacific Coastal Plain
The vegetation of the Pacific lowlands definitely is semi-arid in character. Most of the trees are deciduous, thorny, and short. During the dry season the landscape presents a barren appearance, but shortly after the first summer rains dense green foliage appears ([Pl. 4, figs. 1] and [2]). Between Juchitán and La Ventosa few trees are more than two meters high ([Pl. 5, fig. 1]). In many areas the trees and bushes form an almost impenetrable tangle, whereas on especially rocky soils or on slopes those plants are more widely spaced. Abundant and widespread species of trees on the Plains of Tehuantepec include Acacia cymbispina, Prosopis chilensis, Caesalpinia coriaria, Caesalpinia eriostachys, Celtis iguanaea, Cordia brevispicata, Jatropha aconitifolia, and Crescentia alata.
Montane Vegetation
In order to illustrate the interruption of subtropical and temperate types of vegetation by the lowlands of the Isthmus of Tehuantepec, it is necessary to digress for a moment from the isthmus and consider the types of vegetation present on the adjacent highlands. On the higher peaks, such as Cerro de Zempoaltepetl, above about 2500 meters is fir forest (Abies religiosa); lower on the slopes are extensive pine forests, which on some slopes are mixed with oak or replaced entirely by oaks. Subtropical cloud forest, characterized by relatively cool temperatures and high humidity, is found at elevations usually between 1000 and 1800 meters on the windward slopes of the Sierra Madre Oriental in Veracruz and northern Oaxaca and on the northern and southern slopes of the Chiapan-Guatemalan Highlands. None of these forest types is continuous across the Isthmus of Tehuantepec.
The Sierra de los Tuxtlas
Although actually located in the region of the Isthmus of Tehuantepec, the Sierra de los Tuxtlas, because of its isolated position, need not be considered in great detail in analyzing the distribution of animals inhabiting the lowlands of the isthmus. Nevertheless because some species living in the highlands adjacent to the isthmus also live in the Tuxtlas, this range is briefly described here. The Sierra de los Tuxtlas is a range of volcanos lying near the Gulf Coast in southern Veracruz between the mouths of the Río Papaloapan and the Río Coatzacoalcos. Volcán San Martín, the highest peak, rises above 1800 meters. This range of volcanos is surrounded by lowlands, which immediately to the south and west are covered with savanna and in places by scrub forest. The luxuriant nature of the vegetation on these volcanos indicates that this range receives much more rainfall than the surrounding lowlands. Especially on the northern slopes, tropical rainforest is well developed; this is replaced at about 1200 meters by cloud forest. The southern and western slopes are drier, for the lower slopes are covered with a scrubby, but evergreen, forest.
Detailed comments on the herpetofauna of the Tuxtlas have been omitted purposefully, for the reptiles and amphibians of the region currently are being studied by Douglas Robinson.