Geoscience Reference
In-Depth Information
canopy, maintaining constant temperatures. The transfer of heat energy on the
ground is minimal.
Very little of the solar radiation entering the forest is reradiated or returned to
the atmosphere. The high water vapor content and high carbon dioxide levels, pro-
duced by plant respiration and decomposition within the forest, serve to absorb the
outgoing heat energy, returning it to the tropical forest system. This leads to an
almost uniform daily cycle in the radiation balance measured in the tropics. Insola-
tion steadily increases from sunrise until noon, and gradually decreases until night
fall. Patterns of annual (yearly) radiation (light and heat energy input) show high
values during the equinoxes, with slightly decreased values during the rainiest sea-
son. In tropical areas farther from the Equator, a distinct seasonal change is noted.
These fluctuations play a role in the distribution and physiology of some tropical
plants at the tropical margins.
Heat Budget
Understanding the exchange of heat from the atmosphere to the canopy and forest
floor is crucial in understanding tropical rainforests and their climate. These
changes are more apparent daily than annually. During the day, insolation is pri-
marily received by the tree canopy. Within the forest, energy is absorbed and
released through evaporation and transpiration by plants. This evapotranspiration
cycle causes variation in water vapor content within the forest. Despite these
changes in energy and water vapor, the air temperature within the forest remains
relatively constant at 79
C). Besides evapotranspiration, energy is
released during the night through the condensation of water vapor. This release of
energy is absorbed by the atmosphere, as well as the tree bases at ground level.
These conditions regulate temperature and moisture and minimize drastic changes.
In fact, the largest temperature variation and energy exchange occurs at the canopy
level. These fluctuations in heat and energy transfer are more evident toward the
subtropics, where pronounced wet and dry seasons occur.
Daily temperature changes influence tropical rainforest vegetation. Average
daily temperatures show the greatest variation in open, treeless areas within the
equatorial rainforest. Daily temperature variation is greater near the margins of the
tropics (around 8
-81
F (26
-27
C]). These small diurnal temperature effects are also
more pronounced in the central or interior parts of the rainforest, as coastal areas
show minimal change. The minimum daily temperature occurs shortly before sun-
rise and temperature rises steadily reaching as much as 5
F [4.5
C) higher
around noon, then decreases until nightfall and through the night. Cloud cover can
add to or moderate these temperature changes.
Although the fluctuation of heat into and out of the forest can vary during the
day, the annual cycle remains steady. Due to the steady input of light and heat, an-
nual temperature cycles show little variation. Temperatures near the Equator
change seasonally by as little as 2
-9
F(3
-5
C). The consistency in average monthly
temperature is influenced by the consistent day length, as well as the large expanses
F(1
