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Primary Production

In tropical regions where temperatures are continuously warm and rainfall is abundant throughout the year, the growing season is continuous (Jordan 1971a). At higher latitudes, the growing season is shorter. Even though there is impinging solar radiation at higher latitudes or higher elevations, plants cannot use it when daily temperatures drop below freezing. In the dry or seasonally dry tropics, the growing season is also shorter. Lack of moisture inhibits photosynthesis and so the impinging radiation cannot be used by plants.

Because of the year-round availability of moisture and solar radiation, the wet tropics have often been thought of as an environment where forest production would be a maximum. For example, Wallace (1878, p. 65) believed that tropical forests had great productive potential when he wrote, "The primeval forests of the equatorial zone are grand and overwhelming by their vastness and by the display of a force of development and vigour of growth rarely or never witnessed in temperate climates." Ecological studies in the 20th century have confirmed this impression. Net primary production (wood increment plus leaf litterfall) is higher in wet tropical ecosystems than in any other terrestrial ecosystem (Table 2.1). These estimates, however, may be somewhat low. Nemani et al. (2003) found that global climatic changes between 1982 and 1999 resulted in an increase in net primary production in ecosystems throughout the world by 6%. The largest increase was in tropical ecosystems. Amazon rain forests accounted for 42% of the global increase, mainly owing to decreased cloud cover and the resulting increase in solar radiation. In tropical plantations, wood production can be higher than that of native forests (Wadsworth 1997; Dabas and Bhatia 1996). Plantations, however, receive anthropogenic subsidies that can increase their productive potential over naturally occurring forests.

The high rate of net primary production in native tropical forests (Table 2.1) has important implications for nutrient dynamics in the soil, which in turn affects the ability of the soil to sustain production when the forest is logged or converted to plantations or agriculture. High rates of energy transfer to the soil due to leaf litter input and tree fall are accompanied by high rates of decomposition, which in turn results in high rates of carbonic acid formation (see Sect. 2.6.3). This causes relatively high rates of mineral weathering and nutrient leaching, which can decrease the productive capacity of the soil.

The distribution of energy between wood and leaves may be important with regard to energy input to the soil. Leaf litter decomposes more rapidly than wood biomass, due to its greater ratio of surface area to volume and

Table 2.1. Net primary productivity in ecosystems of the world. (Reprinted from Whittaker and Likens 1975)

Ecosystem

Net primary productivity (dry

matter)

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