Tropical climates - The Global Climate System: Patterns, Processes, and Teleconnections - page 89

the Amazon Basin consistently is a large net carbon sink in the undisturbed
rainforest; nitrogen emitted by forest soils is subject to chemical cycling
within the canopy space resulting in vegetation re-sequestration of much of
the soil-derived NO x ; forest vegetation is both a sink and a source of VOC;
and concentrations of aerosol and CCN are seasonal, with a pronounced
maximum in the dry (burning) season (Andreae et al. 2002 ).
Results obtained from the atmospheric mesoscale wet season (WETAMC)
campaign indicate that deforestation may be of secondary importance during
the wet season compared to the convection system in affecting wind regime
and cloud formation. This experiment also indicates that mesoscale convec-
tive systems are initiated over high terrain where deforestation may be
enhancing rainfall (Silva Dias et al. 2002 ). Changes in canopy structural
properties exert substantial control on rates of photosynthesis and forest
respiration (Vourlitis et al. 2002 ). Forest conversion to pasture and selective
logging can alter canopy structure, microclimate, soil water availability, and
disturbance (e.g. fire) regimes (Fearnaside 2000 ). Partition of precipitation
water in deforested areas increases runoff and decreases soil storage and
evapotranspiration thereby increasing the magnitude and/or duration of
drought during dry and transition periods that further limits transitional forest
net ecosystem CO 2 exchange (NEE) (Vourlitis et al. 2002 ).
Amazonia is a large global store of carbon and both forest clearing and
agricultural development are often cited as large net sources of CO 2 to the
atmosphere. Studies of NEE indicate that intact tropical rainforest and
savanna can accumulate 1 to 6 t C ha 1 y 1 (Malhi et al. 1999 ), waterlogged
valley forested areas can accumulate up to 8 t C ha 1 y 1 (Araujo et al. 2002 ),
and even 36 t C ha 1 y 1 was estimated on an eastern Amazonia site
(Carswell et al. 2002 ). Nepstad et al.( 2002 ) conducted an experiment on
the impact of partial precipitation throughfall on carbon cycling and found
that the net accumulation of carbon in mature Amazon forests may be very
sensitive to small reductions in rainfall. They also found that soil water
reductions were also sufficient to increase soil emissions of N 2 O and soil
consumption of CH 4 .
Global Circulation Models (GCM) are structured for global scale analysis.
Research has been conducted with the intent of developing circulation
models adapted to microscale and mesoscale simulations that could depict
land-atmosphere interactions at the level of detail needed for Amazonia,
where convective clouds and precipitation play an important role. Fine-scale
process-based multilayer canopy models (soil-plant-atmosphere) have been
used to predict hourly gross photosynthesis and respiration that can be tested
against collected tower eddy covariance flux measurements. Aggregation
of these models with coarser scale daily whole-canopy carbon fixation
and transpiration models and even a coarser ecosystem biogeochemistry