The idea that forest structure reflects productive potential led the 19th century naturalists and explorers to believe that tropical forest regions have great potential for wood and crop production. They were impressed with the huge structure and impenetrable vegetative growth of rain forests (Jordan 1982). In their homelands, such structure and growth indicated a high productive potential. However, in many tropical forests, structure does not reflect function. For example, in the Luquillo Mountains of Puerto Rico, the Colorado forest (Cyrilla racemiflora), which dominates between 600 and 800 m above sea level, has a tree density and basal area greater than the Tabonuco forest (Dacryodes excelsa) which grows below 600 m elevation (Lugo and Scatena 1995). However, the net primary productivity of the Tabonuco forest averaged 4.86 Mg ha-1 year-1 (Jordan 1971 b), while that of the Colorado forest averaged only 0.59 Mg ha-1 year-1 (Weaver 1995). In the Amazon region, the structure of forests on nutrient-poor soils also does not reflect the productive potential of the site. In spite of growing on poor soils, the forest has biomass (above ground plus roots) in the range of 320 to 400 Mg ha-1 (Jordan 1985), values that are comparable with other humid tropical forests growing on a variety of conditions (324 Mg ha-1, Achard et al. 2002). Nevertheless, above-ground wood production on the nutrient-poor sites was 3.9 to 4.9 Mg ha-1 year-1, compared with a value of about 7 Mg ha-1 year-1, the average for all mesic tropical forests (Fig. 2.6).
Management for wood production based upon structure is satisfactory when structure reflects function. Structure reflects function when moisture, for example, is a critical factor. Figure 3.1 shows that forests in arid climates differ greatly in wood biomass, leaf biomass, height, and basal area from forests in moist climates. In the case of nutrient status, structure does not always reflect function. Nutrient-conserving mechanisms can compensate for low nutrient availability, with the result that the biomass of a forest on nutrient-poor soils differs only slightly from that of a forest on rich soils (Table 3.1), despite the fact that wood production on the two sites differs greatly.
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