112 Ecosystems of South America herbs, ferns, and shrubs Lowland forest trees are generally shallow rooted and many of them develop buttresses at the base of their trunks, an adaptation thought to provide mechanical support The epiphytes and vines are conspicuous in tropical forests, where the epiphytic Orchidaceae and Bromeliaceae are usually prominent The barks of lowland tropical rain forest trees are usually thin and smooth, and this morphological characteristic makes the anchorage of epiphytic forms difficult In the Andean cloud forests, the heights of trees decrease at increased altitudes and their shapes become irregular (e.g., elfin forest) In cloud forests, the bark of trees and the forest floor are covered with mosses, lichens, and ferns, and the diversity of fungi, algae, bryophytes, and seedless vascular plants is remarkable Functional Aspects Tropical Rain Forests A mean monthly temperature of 426 1C and mean annual rainfall usually exceeding 1800 mm characterizes the Tropical Rain Forest (Af) climate that coincides with the geographic distribution of tropical rain forest (Figures and 3) Some areas of tropical rain forests are the result of orographic precipitation Isolated mountains (e.g., Sierra Nevada de Santa Marta in Colombia) or coastal escarpments (e.g., the Atlantic Brazilian Shield and the lower slopes of the Andes in the Pacific coast of Colombia and Ecuador) act as barriers to airflow, forcing moist air to ascend upslopes where adiabatic cooling generates clouds and precipitation (Terborgh, 1992) These geophysical conditions conduce to forested landscapes with a physiognomy similar to the Amazon wet evergreen forest In areas of lowland tropical forest (e.g., Amazonia), minimum changes in temperature between summer and winter and continuous heating coupled with the high evapotranspiration rates of vegetation create a predictable pattern of daily convectional rain that falls during the middle afternoon It is estimated that up to 80% of the precipitation falling in Amazonia is retained within the hydrological cycle of the basin The geological material underlying the hydrographic network also influences the development of forest ecosystems In the Amazon lowlands, large areas of forest are subject to inundation (varzea) and these ecosystems differ from those that occupy high terrain (terra firme) Rivers running from the Andes carry large amounts of silt and have a light brown color (whitewater) These large loads of silt are deposited in areas of low relief and form broad floodplains of high fertility During the flooding season, these areas remain under several meters of water In contrast, rivers running through the old Precambrian Shield are poor in nutrients and minerals and their waters are clear The Shields have been exposed to millions of years of leaching and in order to conserve nutrients, plants protect their foliage with tannins The tannins leached into the rivers give a tainted coloration to the water (blackwater) The Rio Negro and other blackwater rivers also have floodplains, but they are different because of the lack of sediments and because plant communities are adapted to acidic conditions (igapo) Because of the high temperatures and abundant rainfall of tropical forest ecosystems, there is intense chemical action on bedrock, and soils and all soluble components continuously leach, producing characteristic Ultisol and Oxisol soils (Terborgh, 1992) These soils are rich in iron, manganese, and aluminum, which stay behind after all other soil soluble constituents have been leached Large amounts of these minerals arrange in stratified layers forming laterites Field studies have shown that tropical forests exploit a larger volume of soil than previously thought Soil depth is therefore quite important in water balance variability because it allows evergreen forest to keep evapotranspiring during dry periods by absorbing water from soil depths of more than m Although the most rapid changes in water availability take place at the soil surface, where it is depleted by plants, the process of hydraulic lift can take water from deep soil layers and discharge it into the dryer upper layers During the day, the water potential (c) gradient causes water to move from the ground into the roots, shoots, and through stomata to the atmosphere During the night when stomata are closed, c in the plant equals c in the deep soil and a water potential gradient can move water from the deeper moist soil and plant interior into the dryer upper soil layers Decomposition and nutrient dynamics proceed rapidly under the warm, moist conditions of tropical forest ecosystems High temperatures also favor an intense bacterial activity on upper soil layers and therefore there is no accumulation of humus Many nutrients are stored in the forest biomass where they are kept from leaching Nutrients are partitioned in different compartments of the forest and, on a weight basis, leaves contain the highest concentrations Most of the phosphorous is stored in the leafy biomass and potassium and calcium in stem tissues Nutrient losses in intact forests are generally low because of the high concentration of roots in the upper soil layers The presence of mycorrhizal associations in roots enhances their nutrient uptake Epiphytes also reduce the loss of nutrients washed away from standing biomass Over long successional periods, the forest biomass is progressively stored in the woody components and nutrient accumulation in leaves decreases Invertebrates can account for up to 60% of the animal biomass of tropical forest ecosystems About 19% of the woody fraction of the biomass is consumed by termites, beetles, and the larvae of insects Litter is the main source of food for the animal community and leaves, flowers, and fruits are consumed mainly by insects, birds, bats, and small mammals Chemical defenses are common among tropical forest plants and many species have developed mechanical defenses by incorporating silica, lignin, and fibers in their tissues Tropical Semideciduous and Dry Forests Along the gradients from tropical rain forest to dry forest, the structural and functional attributes of the ecosystems change Tropical deciduous forests correlate with climates of two welldetermined seasons: a rainy period followed by a dry one More than 50% of the tree species shed their foliage during the dry period The predominant characteristic of the habitat of these forests is the prolonged seasonal drought, causing desiccation of the topsoil and lowering atmospheric humidity The length of the dry season determines the degree of divergence in physiognomy and structure of the seasonal forest (Gentry, 1995) From wet evergreen to dry deciduous forests, prolonged drought correlates with a reduction in