Forest Classification Based Upon Soil Nutrient Status
A forest classification that considers nutrient status could be made according to soil fertility. For example, in the humid Amazon lowlands, Spodosols, Oxisols, Ultisols, and Alfisols represent a gradient of increasing soil fertility. However, to classify a forest as "forest growing on an Oxisol" emphasizes characteristics of the soil instead of the forest. To characterize tropical forests according to their degree of adaptation to low soil nutrients, Jordan (1985) classified forests along a continuum from those adapted to extremely low nutrient supplies (oligotrophic forests) to those on relatively fertile soils (eu-trophic forests). The characteristics used in this classification are listed as parameters in Table 3.1.
Net primary productivity increases along the gradient from oligotrophic to eutrophic forests. In addition, species diversity may increase along the gradient. For example, the high caatinga forest on Podsol sands of the Amazon is a highly oligotrophic forest. Compared to the eutrophic forest in Panama, it has a high root biomass, a slightly lower above-ground biomass, a high root:shoot ratio, a high concentration of roots above the soil surface, a low specific leaf area (the leaves are thick and tough), a low leaf area index, an average leaf biomass (the thickness of the leaves compensate for the lower number of leaves), a slow rate of leaf decomposition, and low concentrations of nutrients in the leaves.
Nutrient use efficiency (the amount of biomass produced per unit of nutrient taken up by plants) is an indication of the degree to which a nutrient is limiting (Vitousek 1982, 1984). Because nutrient use efficiency is an indicator of the tightness of the nutrient cycle, these values are used in parameters 12 and 13 of Table 3.1. Nitrogen use efficiency is highest in the caatinga forest, but phosphorus use efficiency is highest in the Oxisol forest. Estimation of values of nutrient use efficiency for a number of sites at high elevations and in lowland forest in the tropics has led to the hypothesis that phosphorus is frequently a limiting nutrient in the lowlands, while nitrogen is frequently limiting at high elevations (Vitousek 1982, 1984). Phosphorus is limiting in the highly weathered soils of the lowlands because it is immobilized by the iron and aluminum, while nitrogen is limiting at high elevations because it is immobilized in litter that decomposes very slowly due to low temperatures (Jordan 1985).
Implications for Management
• Species diversity is greater on relatively rich soils than on poorer soils. However, species diversity does not indicate the abundance of endemic species, which may be of interest to conservationists. If a desirable species is rare, it does not matter if it occurs on nutrient-rich or nutrient-poor sites. In either case, care must be taken to ensure its reproductive success.
• Productivity and decomposition are lower on nutrient-poor soils, because the metabolism of trees and decomposers is limited by a scarcity of nu-
trient elements. Because oligotrophic forests are less productive, their economic potential is less. They might serve better as nature reserves than as sources of timber.
• Nutrient cycles will be tighter on nutrient-poor soils than on nutrient-rich soils. If management is carried out on nutrient-poor soils, care must be taken to preserve the soil organic matter and humus and litter on top of the soil. Otherwise, the productive potential of the site will quickly deteriorate.
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