Tropical Dry Forest Ecosystems
Tropical and subtropical dry forest ecosystems occur in frost-free climates from lowlands to lower montane regions where potential evapotranspiration exceeds precipitation on an annual basis (Holdridge 1967). Generally, they occur in environments with mean annual rainfall ranging from 1000 to 2000mm and mean annual biotemperatures of 12°C and above. Forest stature, primary productivity, and tree species richness increase with increasing rainfall (Murphy and Lugo 1986a; 1995). Dry forests are seasonal forests, experiencing wet and dry periods. Air temperatures are usually high and relative humidities low. The result is that dry forest plants have multiple adaptations to dry conditions, including drought avoidance and resistance through a variety of morphological and behavioral characteristics (Lugo et al. 1978; Medina and Cuevas 1990).
Tropical dry forests occur on substrates ranging from nutrient-rich alluvial soils to nutrient-poor rock outcrops. They can occur on volcanic, limestone, or ultramorphic rocks, with soil textures ranging from sandy to clay, rocky, or organic. Substrate type can greatly exacerbate the water limitations of dry tropical forest climates. If soils are droughty and have low water holding capacity, vegetation may acquire a greater xeromorphic aspect than expected. Conversely, in locations where soils store water well or where water is channelized (as in valleys or canyons), vegetation may acquire great stature and biomass (Murphy and Lugo 1990; 1995). Tropical dry forest vegetation is generally water rather than nutrient limited (Lugo and Murphy 1986).
West Indian dry forests are characterized by smaller stature and biomass, lower biodiversity and productivity, and more seasonal tree growth, reproductive cycles, and organic matter turnover than forests in areas of higher and less seasonal rainfall (Murphy and Lugo 1995). Leaf and litterfall is seasonal (Lugo et al. 1978; Lugo and Murphy 1986; Cintrón and Lugo 1990). Soils can have high organic matter, high pH, and low bulk density (Brown and Lugo 1990). Soil nutrient content is relatively high but with low phosphorus availability to plants (Lugo and Murphy 1986). Animal activity can be highly seasonal as well. Termites are important in the decomposition and turnover of dead wood. Ants and other soil organisms turnover inorganic soil, transport seeds for great distances, and participate in a complex biotic web that processes large quantities of organic matter.
Tropical dry forests support a large fraction of the human population in the tropics, and as a result, are under intense pressure (Murphy and Lugo 1986a). Because dry climates are preferred over very wet climates in the tropics, large population concentrations occur in dry forest life zones. The result is that tropical dry forests not only provide space for the expanding human population, but are also used intensively as a source of fuelwood and charcoal. Grazing animals are also often allowed to roam free through dry forests.
The net result of human activity in this life zone is the serious degradation or disappearance of dry forests in most tropical regions. Because succession is usually slow in these forests, chronic human use results in deforestation and modification of vegetation cover. Usually, degraded stands lose their understory to grazing animals, trees are repeatedly harvested and resprout as multiple small stems, the canopy is opened, and soils are exposed to erosion (Murphy et al. 1995). In cases of extreme use, fire is introduced. Despite these problems, the current condition of dry forests opens the opportunity for restoration and management. Tropical dry forests are resilient in terms of their ability to root and stem sprout, a characteristic that can be used to rehabilitate forests and restore biomass (Murphy and Lugo 1986b; Murphy et al. 1995; Murphy and Lugo 1995). Experience with tree plantations shows promise provided they are managed carefully (Lugo et al. 1990; Wang et al. 1991).
Table 1. Basic socioeconomic and environmental indicators for the insular Caribbean
Notes and Codes. Country-island-countries and/or territories with West Indian iguana populations; () indicates countries with a total population below 100,000 (expressed as 0.0 in corresponding column). Area -given figure is equivalent to total land area, excluding inland water bodies. GNP per capita-data from World Bank Atlas method, subtotals based on available data: (a), estimated to be upper-middle-income, $3,036 to $9,385; (b) estimated to be high income, $9,386 or more; (c), estimated to be lower-middle-income, $766 to $3,035; (*) figure for lower-income countries is $765 or less. Forest Cover -'Total forest' is the sum of natural forest and plantations. Change in Forest Cover -method for data collection based on deforestation model (adjustment function) developed for correlation of forest cover change over time, incorporating ancillary variables (population change and density, initial forest cover and ecoregion). All columns-numbers may not tally due to rounding; (n.a.) no available data; (n.s.) no significant change or data indicating a very small value.
Sources of data. Socioeconomic indicators: World Development Report 1997. The State in a Changing World: Selected World Development Indicators, The World Bank, 1997; The World Bank Atlas 1995, The World Bank, 1996; World Population Prospects, United Nations, 1995; World Urbanization Prospects, United Nations, 1995; FAO Production Yearbook, 1996. Environmental indicators: State of the World's Forests 1997, FAO 1997; World Development Report 1997. The State in a Changing World: Selected World Development Indicators The World Bank, 1997; UN World Population Prospects, 1994; Proceed. XI World Forestry Congress, FAO 1997.
