Functional Variation Along Climatic Gradients
Functional characteristics of a tropical forest can be implied, to a certain extent, from their climatic classifications:
• Energy flow along environmental gradients. Energy potentially available to tropical ecosystems increases along a gradient from moist to dry forest because of the decreasing cloud cover. However, energy that can be used by plants decreases because moisture scarcity in dry forests limits photo-synthetic activity. Therefore, primary productivity decreases along this gradient. Decomposition also decreases because moisture scarcity limits bacterial activity. Energy available to tropical ecosystems, and thus primary production, decreases along an altitudinal gradient from low to high elevations because of increasing cloud cover near the top of mountains. Another factor that limits primary production at high elevations is temperature that is below optimum levels for photosynthetic activity.
• Nutrient cycling along environmental gradients. The tightness of nutrient cycles (the efficiency with which nutrients are recycled) will generally decrease along a gradient from wet to dry. In dry forest ecosystems, there are lower rates of decomposition and less potential leaching. Nutrient conservation is less essential for survival of the forest, so fewer nutrient-conserving mechanisms (Chap. 2) are present. Along an elevational gradient, the tightness may increase. Many cloud forests have characteristics such as sclerophyllous leaves (Grubb 1977) and aboveground root concentration (Weaver 1995), characteristics that are similar to forests on nutrient-poor soils (Table 3.1). In cloud forests, nutrient scarcity may occur because of slow decomposition of the litter and humus on top of the soils.
• Species diversity along environmental gradients. Species diversity is generally higher in moist forests than in dry forests. It is low, however, where soils are saturated such as in swamp forests or mangroves. Diversity also generally decreases with increasing elevation on mountainsides. For purposes of species conservation, diversity alone is an inadequate index. In high elevation environments, diversity may be low, but there may exist endemic species that are rare or endangered, and whose presence would not be indicated by a simple diversity index.
Although some aspects of forest function are reflected in climatic classifications, others such as tightness of nutrient cycling are not. Because of the prevalence of poor soils in many regions of the lowland tropics and the consequent frequent occurrence of forests adapted to nutrient conservation, climatic classifications alone are often an inadequate system on which to base forest management plans.
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