Evergreen broad-leaved forests form dense, closed canopies in humid tropical regions of the Amazon, central Africa, and southeast Asia, as well as the temperate maritime climates of Japan and New Zealand. Evergreen, tropical forests occupy about 15 percent of the global land area. Broad-leaved evergreen trees also exist in a sparser woodlands in seasonally dry climates, such as the Mediterranean-type climates in Australia, California, Chile, Europe, and South Africa. Evergreen woodlands consist of a mix of herbaceous and woody plants, with mixtures ranging from 20/80 to 80/20 (Scholes and Archer 1997). Globally, these woodlands constitute more than 20 percent of the terrestrial biosphere (Melillo et al. 1993).
The aseasonal behavior of the tropical forests contrasts with the strongly seasonal behavior of subtropical and Mediterranean-type forests.
Months after July 1999
Figure 15.5. Seasonal variation of net ecosystem CO2 exchange of a tropical forest growing in the Amazon near Manaus, Brazil. Adapted from Araujo et al. (2002).
Published data on the seasonal patterns of NEE from evergreen tropical forests are relatively scarce (Grace et al. 1995; Malhi et al. 1998; Araujo et al. 2002). The emerging pattern is that the seasonality of daily-integrated NEE is weak, because of the small range in day length and temperature (Figure 15.5); day length is close to 12 hours long, year-round, and the amplitude of monthly mean air temperatures is less than 4°C. Furthermore, the nearly equal day and night length and warm temperatures experienced by evergreen tropical forests cause them to respire at greater rates and for a longer time than forests, during the growing season, at northern latitudes. As a consequence, daily net rates of carbon exchange are lower in evergreen tropical forests than in temperate broad-leaved forests during the peak of their growing season (Malhi et al. 1998).
Day-to-day variations in GPP are mainly caused by changes in clouds and their modulation of sunlight. Ironically, the seasonal occurrence of drought, which reduces both GPP and R , is a major factor modulating NEE of wet tropical forests.
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