Amazon Tropical Forest

In the temperate maritime climate of New Zealand, broad-leaved, evergreen Nothofagus forests experience rates of carbon exchange rates that resemble those of the tropical forests but with lower nocturnal respiration rates (Hollinger et al. 1994). The carbon exchange rates of these forests are also more seasonal than those of tropical forests, operating more like deciduous forests but without the dormant phase. Light-use efficiency of these forests is enhanced by diffuse radiation (Hollinger et al. 1994).

Subtropical and Mediterranean Climates

In the subtropical regions of Africa (tiger bush), Australia (Eucalypt bush), and Brazil (cerrado), the timing of the wet and dry season is the major control on photosynthesis and respiration (Miranda et al. 1997; Eamus et al. 2001; Vourlitis et al. 2001). These woodlands are carbon sinks during the wet season, are carbon neutral during the dry season, and are net sources during the transition period from dry to wet. Changes in vapor pressure deficit and soil moisture are the major modifiers of the maximum value and initial slope of the relationship between sunlight and canopy photosynthesis (Eamus et al. 2001; Vourlitis et al. 2001).

Carbon dioxide exchange of Mediterranean-type ecosystems is out of phase with the seasonal pattern of carbon exchange in temperate forests (Figure 15.6). The prolonged summer drought limits gas exchange (photosynthesis and respiration) (Reichstein et al. 2002a), but rainfall is plentiful during the winter, so the plants are physiologically active when temperatures are cooler, days are shorter, and less sunlight is available than in summer. Consequently, Mediterranean-type ecosystems have less potential to acquire carbon than temperate systems that are active during the warmer and brighter summer.

Another prominent feature, relatively unique to savanna and Mediterranean-type ecosystems, is the effect of episodic rain on soil respiration during the summer (Reichstein et al. 2002b; Rey et al. 2002; Xu and Baldocchi 2003). Two mechanisms may produce enhanced respiration rates after rainfall. One is a physical displacement of soil air and CO2 by the downward-moving front of water in the soil. But this effect is shortlived, and the volume of air in the soil profile is relatively small. The other mechanism is rapid activation of heterotropic respiration (Birch 1958).

0 0

Post a comment