The Evergreen Forest of Mexico: An Overview

The evergreen rainforest or tropical evergreen forest is characterized by the predominance of trees over 25 meters (16 feet) high that retain their foliage all year round, which is why they are called evergreen trees (from the Latin perenne, permanent, and folia leaf). However, not all components of the rainforest are strictly evergreen, as some lose leaves during a short season in the dry part of the year, which often coincides with the flowering season of the tree. Despite this, and mainly due to the lack of coincidence of the period of leaf fall between the different species that make it, the forest never loses its greenness completely.

In Mexico, the evergreen forest develops in regions with humid tropical climates; commonly at altitudes between 0 and 1,000 meters above sea level, although in some parts of Chiapas it ascends to 1,500 meters and towards the boreal end of its distribution area, the upper altitudinal limit is approximately 600 meters. The jungle can be named in different ways depending on the author. Some synonyms of the evergreen forest are the following: tropical evergreen forest, evergreen ombrophilous forest, high evergreen forest, Pluvirsilvae, and tropical rain forest.

Distribution

The evergreen forest occupies a wide and almost continuous extension in the east and southeast of the country, starting in the region of Tamazunchale and Ozuluama located southeast of San Luis Potosi and north of Veracruz, being found in some border regions of the state of Veracruz, with the states of Hidalgo, Puebla, and Oaxaca; in some portions of Tabasco and the north and northeast of Chiapas Abarca, also, most of the territory of Campeche and Quintana Roo. It is also found on a long and narrow strip on the Pacific slope of the Sierra Madre de Chiapas, which is isolated on the west side of the Isthmus of Tehuantepec but continues towards Central America. The areas where this formation is still best to present correspond to some portions of the Yucatan Peninsula, the Selva Lacandona in northeastern Chiapas, the Selva del Ocote in the border region of Chiapas, Oaxaca, and Veracruz, and the Selva de los Chimalapas in Oaxaca.

Weather

The mean annual temperature is no lower than 20°C and rarely exceeds 26°C, although it is not very high in any case; the 0°C extreme minimum temperature isotherm functions as the thermal limit of the biome. The difference between the mean temperatures of the coldest and hottest months of the year does not exceed 11°C and is often less than 6°C. Diurnal temperature oscillations average 8 to 12°C (46 to 54°F). Within the rainforest, the temperature is homogeneous because the canopies of the trees in the canopy are sometimes so close to each other that they act as a screen that intercepts both sunlight and rain so that not all layers of the forest participate in the same climate but have different microclimates. For example, in the upper portion of the canopy, the sunlight reaches directly, and there are violent winds and daily variations in humidity and temperature. Whereas above the protected floor the air is still, the humidity is always high and the temperature varies comparatively little. The average annual rainfall is consistently 1,500 to 3,000 mm and in some areas is over 4,000 mm. The number of dry months is generally less than three per year.

Vegetation strata

The division of forest vegetation into strata is useful for describing its structure, but it is not always easy to define precisely where one stratum ends and another begins. In reality what is seen is a continuous greenness that extends from the highest point of the canopy to the ground, with sparser foliage at the bottom. Likewise, the division into strata is made difficult by the population dynamics of the vegetation, because in the forest there are always some plants that are growing while others are decaying or dying so the forests can have different strata according to their growth cycle. An example of this is the pioneer patches, which are those that colonize space or clear between vegetation and tend to have only one stratum of the same age. Mature patches, on the other hand, have a different vertical structure and often consist of more than one level. Thus, for this simple descriptive purpose, three vegetation strata have been generally considered for this biome: the emergent, the canopy, and the understory.

In the jungle, we can find from gigantic trees with thick trunks to tiny, barely perceptible plants.

The emergent stratum is the tallest; in it, it is possible to find trees up to 30 meters high, and frequently there are individuals of more than 45 meters high that protrude above the rest of the trees, their trunks go through the canopy and their enormous crowns open well above the rest of the trees. Although the vegetation of this stratum enjoys sunlight first, it must undoubtedly withstand high temperatures, low humidity, and strong winds, although it has already begun its development in the different environmental conditions of the lower stratum. It is common for the emerging stratum not to form a closed canopy but to consist of more or less isolated individuals. Below is the canopy or main stratum, composed of trees with flat or rounded crowns, which together with the emergents constitute the complete canopy. These trees fill the clearings between the emergents and form with them a continuous layer that, as already indicated, is the cause of the diverse microclimates.

The spreading branches, the upper portions of the trunk, and even the small twigs provide a suitable environment for the orchids, bromeliads, ferns, and lichens that live above them, and it is common for the canopy trees to be interwoven and united by an abundance of climbing and woody plants such as lianas or lianas. Below the main canopy, we find the understory, a stratum formed by lower trees that often have pyramidal or vertically elongated crowns, some of which are young trees that will eventually reach the canopy, while others are slow-growing trees that have reached their full development. This stratum contains young canopy trees, smaller adult trees, mature woody plants, shrubs, young plants, and herbs. In this stratum, sunlight hardly penetrates, so few plants can adapt to this penumbra.

Biodiversity

The rainforest is one of the biomes with the greatest volume of species, containing more than half of the world's terrestrial flora and fauna, estimated at ten million species. It is believed that approximately 80% of all insect species are confined to the tropics. Similarly, it is not unusual to find more than 100 different kinds of trees of all sizes in one hectare. The total number of plants and animals found in one square meter is enormous, almost always greater than that found in any other square meter of the earth's surface. Finding two trees belonging to the same species sometimes takes a long time. Precipitation seems to be one of the factors determining this diversity. In general, the wettest forests are the most diverse. Another important factor is the fact that different kinds of trees provide different types of bark on which several varieties of epiphytic species can grow.

A characteristic of several species of jungle plants is that their leaves have a drip-shaped end which allows the water to slide quickly and not accumulate on the surface.

The number of species in the forest is so large that sometimes identification problems arise; not only are there many species in an area, but even in some well-studied areas it is common to find species that have not yet been classified. A major problem for the classification of species in the case of vegetation is the inaccessibility of structures such as leaves, flowers, and fruits that are necessary for their identification. In some cases, trees can be recognized by observing the bark or by making a small cut with a machete.

Life forms

In the rainforest, we can find gigantic trees with thick trunks, often striated or with buttresses at the base, to tiny, barely noticeable plants. The trees and shrubs of the jungle are structures on which other plants can live, they provide innumerable habitats, all of them different from each other and where many animals find an ideal place to live. For example, mosses, ferns, and fungi live in the dark and humid spaces of the roots. The trunk of the trees, which is a difficult place to colonize, is used by various orchids and climbing plants with large leaves, which are supported by their aerial roots. The larger branches are horizontal and on them grow epiphytic orchids and bromeliads; the bromeliads accumulate diverse residues to form their soil and on this soil worms, snakes and many insects develop. The canopy of the trees also serves as a support for vines and large creepers that lean on the thick trunks, a very characteristic feature of the jungle.

The leaves of most rainforest tree species are remarkably similar in shape and size, with pointed tips and no pubescence or hairs on the surface. In the understory area, it is common to find single-stemmed saplings of no more than 3 meters and often also proliferate small palms and palmettos. Other representative plants of this biome are the soft-stemmed ground grasses, many of which are very similar, with large leaves with elongated tips and reddish underneath and green above. As for the fauna, the rainforest is home to an unsuspected variety of insects of very different forms, in addition to the decomposing organisms of matter. Both high and low in the jungle we find hundreds of butterflies flying in groups or individually.

In the rainforest, we also find "clearings" or open spaces between the vegetation, which are colonized by pioneer plants.

Surrounding trunks or branches, it is easy to find structures made of wood fragments or mud; these are termite nests, where thousands of organisms that have found a communal way of living in the rainforest live. Among the main organisms that present this type of life called social are ants, bees, and wasps, in which there is a form of organization where the different functions of the community are perfectly distributed. Termites play a crucial role in the processing of deadwood and layers of fallen leaves so that they can be used again by plants.

In the sparse layer of dry leaves, there is also a great variety of invertebrate animals such as mites, mice, centipedes, snails, and slugs that also play a fundamental role in the life of the ecosystem. Many other insects such as cockroaches, beetles, and crickets live on the plants and also on the jungle floor. Among the major vertebrates, we find animals with large-toed limbs or powerful claws that inhabit the high strata of the jungle, as well as dozens of monkeys with long tails that jump from one tree to another. Armadillos, anteaters and tapirs are also common.

Big and small cats live in the undergrowth, hiding and stalking their prey, although they can also be seen climbing the thicker branches of trees. Since each feline species has its feeding tastes and does not compete for food, several of them can be found in a given area. These mammals are predators, feeding on meat and sometimes carrion, but they almost always prefer to hunt for their food themselves.

The rainforest is home to an unsuspected variety of insects.

The rivers and streams that pass through the jungle are home to many reptiles such as some turtles and crocodiles that feed on fish, plants, insects, and some small vertebrates. Among the green leaves of the trees or in the trunks, frogs, perfectly camouflaged chameleons, or immobile snakes stalking their prey can go unnoticed. No place in the world hosts a greater variety of brightly colored birds than the jungle, where parakeets, macaws, toucans, and quetzals, among many others, illuminate the treetops with their brightly colored feathers and beaks. The upper strata of the trees are the habitat of eagles that nest at heights of more than 50 meters, as well as hawks and owls.

Vegetation adaptations

The trees that colonize the clearings or areas without vegetation cover have seeds that are well adapted to grow under the bright sun, high temperatures, and relatively low humidity. These species compete with each other for light, grow very fast, and die if other trees cover them. Often called pioneers because they must colonize large spaces, many of these trees are small, short-lived, and almost always end up being replaced by trees capable of withstanding shade.

The seeds of the vast majority of rainforest plants do not grow near their parent, as competition for the use of resources such as light and water becomes difficult, and the likelihood of attack by herbivores and pathogenic organisms increases. As a result of wind, water, and some animals, seeds move to distant sites. Those that are dispersed by the wind are provided with different structures that allow them to travel long distances; some are flattened and have a wing on one of their edges; others have a kind of parachute formed by fine hairs, or are covered by a silky body that allows them to fly. Another important factor in seed dispersal is animals; in this case, the fruits are fleshy, but the seeds are hard and difficult to disintegrate; when ingested (by animals such as monkeys, birds, or bats), they pass through the digestive tract without any modification and are deposited in distant places through feces.

A great variety of snakes are also found in the rainforest.

Once the seed has been dispersed, it has to germinate and grow, which is not easy because of the intense competition for space in the forest. To overcome these adversities, plants have created two strategies; one is to produce large and fleshy fruits in small numbers, but that allows them to have a good amount of reserves to feed the young seedling. The other alternative is to produce large numbers of small seeds, some of which will find a suitable place to develop. To carry out pollination of understory plants, some species have flowers with large, brightly colored stamens that give off strong odors and attract pollinators. Still, others produce their flowers on stems, trunks, and low branches and are thus more visible to pollinators, allowing them to develop large fruits that the thin branches could not support and that have a greater capacity to survive.

Wildlife adaptations

The animals that inhabit the jungle present diverse adaptations, mainly of locomotion. For example, some mammals and reptiles have membranes that allow them to move in the air from one place to another. In reality, they do not fly but move by gliding from high places to lower ones. This technique is used both to flee from predators and to obtain food. Another common adaptation in many primates and other animals that hang from trees is their prehensile tails; these function as a fifth limb that allows them to move from one tree to another, or to stop on a branch while feeding.

Several species have prehensile tails, but the ones with the best-developed tails are the monkeys of America, of which there is only one species in Mexico: the spider monkey. The tail of this animal measures one and a half times the length of its body, it is longer and stronger than its legs and it uses it, as well as its limbs, to move along the branches of the trees in the jungle. Aardvarks, which specialize in feeding on ants or termites, lack teeth and have evolved a long, stick-like tongue that allows them to slurp in ant hills; they also have a prehensile tail to help them hang from trees and sharp claws with which they can pierce. Most tree frogs have suction cup toes that secrete a sticky liquid that allows them to cling to branches without difficulty and even to the underside of leaves.

Many animal species resort to camouflage in such a way that, to the untrained eye, they are easily confused with the ground, fallen tree leaves, trunks, or green leaves. This strategy, in addition to protecting them from predators, makes them invisible to their prey. Some animals are better adapted for camouflage, lightening,g or darkening their skin from one moment to the next, this ability to change color is vital for their role as hunter and prey. The change in coloration is accomplished through a neuroendocrine process in which the nervous system intervenes and there is a release of hormones that induce the pigment cells to expand or contract and the skin to darken or lighten, respectively. Birds have also developed various adaptations to protect themselves from predators and feed. Thus, for example, toucans, parrots, and macaws have very large and strong beaks that they use to break hard shells. Hummingbirds and other small birds have long beaks with a delicate shape that allows them to reach the nectar of tubular flowers.

Agriculture

Intensive agriculture is mainly concentrated in deep and humid soils. It occupies important areas, especially in the state of Veracruz; the main plants cultivated are sugar cane, corn, citrus, bananas, mangoes, and some other fruit trees. In the first decades of the century, bananas were grown and exported on a very large scale, but due to fungal diseases, intensive planting had to be almost completely suspended. Coffee cultivation is also of some importance, mainly on rough or uneven terrain. In recent years the demand for some other fruit trees has increased greatly.

Deterioration by agriculture

The impact of human activities on the forest has been intense since pre-Hispanic times in some parts of the country and has been accentuated by the population explosion, the opening of roads, and other factors. Given the favorable climatic characteristics for agriculture that can be carried out without interruption and the need for irrigation throughout the year, the areas occupied by this type of vegetation are a strong attraction to be subjected to cultivation. In the east and southeast of the country, semi-nomadic agriculture is practiced, consisting of a sequence of clearing, burning, and planting corn for a few successive seasons and abandonment for many years, after which the same process is repeated. As a result of this practice, enormous extensions of land are affected, of which the forest, climax, disappears, becoming secondary plant communities of herbaceous, shrub, and tree type, although some believe that this practice is the least destructive form of agricultural exploitation.

Livestock

Cattle raising occupies an area of more than 37 million hectares, equivalent to 20.5% of the area, which in turn corresponds to 16% of the national livestock area, that is, excluding free-grazing livestock areas. Veracruz and Tabasco account for 68%, and these states, together with Campeche and Chiapas, account for almost all the livestock in the humid tropics (93%). In Tabasco, Veracruz, Chiapas, and Campeche, cattle raising has been displacing agriculture; these states currently dedicate 48%, 30%, 26%, and 12% of their territory, respectively, to cattle raising. There are no precise data, but it is estimated that the number of head of cattle is approximately 6.5 million.

Deterioration by cattle ranching

When grazing is not natural, the land is converted into an artificially maintained pasture. To this end, it is customary to clear the land, burn the existing vegetation and sow suitable grasses; the pasture is maintained indefinitely by setting fire to it during the driest time of the year and replanting it after burning. These pastures are used to maintain a fairly vigorous cattle ranch at the expense of the forest's natural vegetation cover.

Forest extraction

Precious woods (mahogany and red cedar), timber, and non-timber products are extracted from this biome. The wood is used as sawn logs, veneer logs, sleepers, sawlogs, sawmilling, packaging material, poles, and fuels. Quintana Roo and Campeche produce the largest amount of timber products, with Quintana Roo accounting for 57% of the precious wood and 40% of the current wood, and Campeche for 18% and 48%, respectively; Veracruz is the main producer of non-timber products with 73%.

Deterioration due to forest extraction

The disappearance of arboreal vegetation is the easiest to appreciate and evaluate as a form of deterioration of natural communities since the landscape changes radically over extensive areas. Throughout the centuries, logging has been a cause of the disappearance of the forests of the Mexican coastal plains. So far this century, the loss of forest area has been estimated at 26,000,000 hectares, this loss of area added to the loss of forests (18,000,000 hectares), means that 19% of the total Mexican territory has diminished its arboreal vegetation in less than a century.

Forests are cut down because they contain valuable timber, however, it is common to find in a forest different kinds of trees growing intermingled, so it is difficult to find individuals of the same species grouped in one area. Many of these trees produce hard and heavy wood, however, the tropical woods appreciated in the market are only a small proportion among the thousands of species that exist, a hardwood tree can require several hundred years to reach maturity, unlike the pine which only needs 50 to 100 years. Tropical rainforests, home to the world's greatest treasure trove of plant and animal life, are doomed to die if drastic conservation measures are not taken.

Other causes of deterioration

Fire is one of the elements contributing to the degradation and destruction of natural ecosystems. It can be accidental, provoked for agricultural or livestock purposes, or natural. The possibility of a fire caused by natural causes is rare, but when it is provoked and escapes the control of, for example, farmers or cattle ranchers who practice land clearing, it becomes a very important factor in the destruction of Mexico's rainforests.

Extinction of wildlife

It is alarming to note that many species have become extinct in very short periods and that others are struggling to survive. At least 200 species of mammals and birds have disappeared in the last two centuries, and several hundred more are in danger of extinction worldwide. The main factors affecting the disappearance of forest fauna are the destruction and transformation of their habitat. Any clearing of the forest for agricultural or livestock purposes disrupts the life cycles of any organism and reproduction is interrupted or becomes intermittent.

Some species of animals are used by man to satisfy his vanity so alligators and big cats are hunted for their skins, which fetch high prices in the market. The case of birds with beautiful plumage is similar; traders have penetrated the jungles and killed hundreds of them to obtain their feathers or the whole organism. Moreover, parrots, macaws, toucans, and parrots, among others, are captured alive to be sold as pets. This type of trade is illegal, but the authorities have not yet been able to combat it.

Conservation

The destruction of forests around the world is reaching impressive speeds, so their conservation is essential for the development of thousands of plants, animals, and the man himself; the gradual loss of these forests will not only cause the disappearance of many species of organisms on Earth but will possibly produce important climatic changes worldwide. It is therefore of crucial importance to talk about the role of conservation. The word conservation does not mean to avoid the exploitation of the resource, but to practice it in such a way that the damage it causes to the environment is minimal so that it allows the renewal of the forest in a short period. Some forests can only be maintained through the creation of conservation areas or less intensive logging.

In the short term, the best action to preserve endangered species is to create reserves and establish strong international cooperation to regulate the trade in endangered plants and animals. Furthermore, conservation education and a caring attitude towards the resources must be encouraged, as well as changing the socio-economic conditions in Mexico, which have so far led to the accelerated destruction of these forests.

By: Citlalli Álvarez Saulés