Native Habitats Contain Clues to Cultural Needs for Orchids
THE MAJOR PARTS OF SEVERAL continents that lie between the tropics of Cancer and Capricorn make up the tropics, an important portion of the earth’s land mass. Perhaps the most significant fact about these lands is their diversity. Topography in the tropics varies from coastal plains to rugged mountains. Tropical climates range from wet rainforests to arid deserts. Orchids are found growing in many of these environments.
The biologically complex rainforest is perhaps the most publicized yet least understood tropical habitat. Much drama and myth about it was created through the tales of early European explorers, whose descriptions of the sights they saw in rainforests provided fuel for vivid imaginations. Certainly the examples of flora and fauna that were returned to Europe from these regions did little to dispel this image, as they were often frighteningly dissimilar to the plants and animals with which the residents of Europe were familiar.
For centuries, the rainforest was the stuff of fable and adventure, and more recently, its swift demise the cause for global alarm. “Save the rainforest” has become such a popular phrase in the environmental lexicon that many of us have forgotten or failed to recognize that there are many different types of tropical forests.
Tropical forests are not necessarily the impenetrable tangle of vegetation that was promoted in old adventure novels and early Hollywood films. Certainly there are locations, particularly along rivers, that can be difficult to penetrate, but in many tropical forests it is comparatively easy to move about. They are not always as dark and steamy as most of us envision. Breezes across the branches of treetop canopies create an ever-changing mosaic of shifting light patterns through the layers of vegetation below them.
Tropical forests are not the dank and dismal haunts of myriad poisonous insects and reptiles, as well as ferocious beasts, lurking in the darkest corners in search of human prey. Yet, without a doubt, such forests harbor an unfathomable diversity of as yet undiscovered plant and animal species that makes the urgency of the quest to discover new life forms on other planets a point to ponder. Still, the tropical rainforest does contain aspects of its popular legend and can provide a basis for comparison with other tropical climates.
It is important to note that there is no universally accepted classification system to apply regarding ecological systems. Most models are based on plant communities because they are considered to be more stable than those of animals (which frequently range through several biomes). Average annual temperature and precipitation are the main factors considered, but differences in soil types and seasonal climatic patterns are but two of several additional criteria that may come into play.
ORIGIN The term tropical rain-forest is just a bit more than a century old; first applied by the 19th-century German naturalist A.F.W. Schimper. His 1898 treatise on plant geography described four types of tropical forests based on progressively drier, more seasonal climates: rainforest, monsoon forest, savanna forest and thorn forest. Two drier tropical climates unable
to support woody vegetation he called tropical grassland and desert. This system provided the basis for most of the classification schemes that followed, although most subsequent systems have further divided Schimper’s six basic groups. Even under the best of these schemes, classifications tend to blur and are transitional at their boundaries.
Rainforests in the tropics are found primarily at lower altitudes, from sea level up to perhaps elevations of 3,000 feet (915 m) and this habitat is favored by many species of orchids. The largest of these occur in the Amazon and Orinoco river basins of South America. Other important rainforest locations include Central America, the Zaire Basin of Africa and the Indo-Malayan region.
Constant high temperatures typify the tropics and, by definition, any climate with a mean temperature of 65 F (18 C) or higher during the coldest month may qualify as tropical. This eliminates some mountain areas that fall within the tropics that have somewhat cooler average temperatures. Still, many cooler growing cloud forest orchid species come from these higher elevations.
Temperatures in the tropical rain-forest are consistently high throughout the year. The temperature variation is so small that, in fact, there is a greater difference in the temperature extremes from day to night than there are from the warmest season to the coldest.
Moisture, however, is the most dramatic factor in a rainforest. At least 100 inches (250 cm) must fall throughout an entire year for a rainforest to exist. Periods without rain are brief and infrequent, lasting only a few days or weeks.
THE CANOPY Trees in tropical rainforests are lofty and evergreen. The tree canopy towers from 100 to more than 160 feet (30 to 48 m) above the ground. It is composed of a mixture of trees whose differences in crown shape and color are easy to discern from the air. So many kinds of plants grow in the tropical rainforest that it is said there are more species in just a few acres of rainforest than in the entirety of Europe. The trees are shallow rooted and many have buttressed roots. Sporadically, tall trees, called emergents, protrude above the canopy. Some of these reach heights of well more than 200 feet (60 m) and their presence gives the canopy, when viewed from above, an uneven appearance.
Although tropical rainforest plants are evergreen, most do not grow continuously. New growth frequently occurs in flushes and the new foliage is often colorful. For most species, flowering and fruiting occur annually. Leaf fall varies by species and is often heaviest after a new flush of growth.
Woody vines, known as lianas, proliferate, scrambling from tree to tree in search of light. Epiphytes, including bromeliads, ferns and orchids, abound. Although many exotic blossoms and colorful foliage plants originate in such forests, it is a mistake to envision the rainforest as a gaudy floral display. Still, it is written that the sheer profusion of vegetation in tropical rainforests is staggering. The canopy zone has the greatest diversity and teems with plant and animal life in this biome as nowhere else on earth.
FAUNA The rainforest is rich with animal life, but most visitors to one would not likely encounter very many of them. Many animals that live in the rainforest have adapted to survive in a particular zone or niche there. Many more non-flying animals are arboreal in habit in the rainforest than in the temperate forest. Probably half of the rainforest mammal species live in the forest canopy. Many of the animals that dwell in the rainforest are nocturnal. A large number of mutually dependent and unique plant and animal relationships have developed the in rainforest environment.
How many kinds of animals live in the rainforest? Among larger animals, mammals and birds, there are probably twice as many tropical species as temperate ones. Insects and invertebrates have been less studied in the tropics, but if the same ratio holds true, the tropics in their entirety may hold between two and three million animal species, many of which are as yet undescribed insect species.
Trees in the rainforest sometimes fall due to weather-related disturbances or may die and topple from old age, thus creating gaps in the rainforest canopy. These gaps allow streams of light to invade the shady understory that normally receives a mere 2 percent of the light from above. The dimension of such gaps varies depending on the size of the tree that falls and how many other trees it takes down with it. Gaps initiate a new succession of plants that rapidly flourish and grow that would otherwise be unable to compete under the shade of the canopy.
Tropical forests with ample rainfall, yet with several drier months on a regular basis (months with about 2 inches [5 cm] of rainfall or less), are called monsoon forests, or, outside of Asia, tropical seasonal forests. These forests also contain many orchid species, and these typically require a dry rest period during their growing cycle. Trees in the monsoon forest are not quite as tall as those in the tropical rainforest and the canopy tends to be more even. Numerous tree species in the monsoon forest are deciduous, if even briefly. There are fewer climbing plants and epiphytes in such forests and the number of species is somewhat reduced.
In recent decades, the term tropical moist forest has been used to include both rain and monsoon tropical forests. Although they are biologically different, they have some similar characteristics and their boundaries are often indistinct.
Within the parameters of the tropical moist forest are a number of more specifically defined kinds of forests that have received delineation and study by various scientists. Tropical semievergreen rainforest, montane rainforest, heath forest, freshwater swamp forest and peat swamp forest are among these.
Schimper’s third type of tropical forest, the savanna forest, is now more commonly known simply as savanna. It is tropical grassland with trees. The trees are scattered, their ranges controlled by soil conditions or periodic fires. Savanna climates are moderately dry, with summer rainfall that ranges from roughly 20 to 60 inches (50 to 150 cm). Most of the tree species are deciduous during the dry season. The savanna climate is extensive across Africa, Southeast Asia and South America.
Thorn forests are also widespread across the tropics, found primarily on the margins of equatorial tropical and subtropical semiarid regions. Rainfall in such areas rarely exceeds about 27 inches (68 cm) annually, again occurring primarily during the summer season. Trees with open growth habit, primarily acacias, form woodlands with an associated understory of cacti, other succulents and grasses. As the name implies, many of these trees and shrubs are armed with spines.
Of the tropical forests, it is the moist forests (rainforest and monsoon forest) and their subsidiaries, and not the dry forests (savanna forest and thorn forest) that are the primary object of global concern. This is because they are so species rich and yet face rapid destruction before researchers can even explore and catalog all of their wonders. These are also the habitats from which most of our popular tropical orchids originate. It is estimated that of the approximately 250,000 flowering plant species on earth, perhaps two thirds, or 170,000, occur in the humid tropics.
LESSONS TO LEARN For orchid hobbyists, there are several lessons to be learned from the rainforest. First, contemplating the variety of habitats included in the rainforest around the globe helps us understand why all orchids do not thrive under the same growing conditions. It is worth spending some time researching which plants in your collection are from or descended from species with uniformly wet conditions all year versus those that experience seasonally dry periods. By spending some time considering the environment from which our plants originate, we can appreciate why bright light, high humidity, air movement and a growing medium with good drainage are essential for many of our orchids.
Most of us are aware that new species of Orchidaceae continue to be discovered in tropical forests even though those forests are being destroyed at an unprecedented rate. While individually we may feel that we cannot make an impact on that problem, we can adopt a mentality of conservation. We can strive to use fewer resources, work to protect the orchid habitats in our own back yards and seek to find and assist groups and organizations that are working on the problem.
Despite the mystery that the tropical moist forest continues to hold for Western scientists, we should acknowledge that man has lived in close association with tropical forests for millennia. The essentials of life were provided in and near their borders. Industrialization and modern medicine have made life in the tropics much less dangerous than it once was and consequently life expectancies and populations in the tropics have risen dramatically. Technology has made it possible to quickly clear forest that until recently seemed indestructible. The outcome that will result from modern pressures is the real drama and peril for the remaining moist tropical forests.