Geoscience Reference
In-Depth Information
of open water that occur in these tropical zones. These oceans moderate any sud-
den changes in temperature that may occur.
Rainfall Requirements of Tropical Rainforests
Seasonal cycles of rainfall are less evident near the Equator, and more pronounced
toward the tropical margins, where maximum rainfall occurs after the sun reaches
the equinoxes.
It is both the amount and continuity of rainfall that allows tropical rainforests
to remain evergreen. Evergreen plants in the tropics can only exist where annual
rainfall amounts, in combination with water storage capacity, are sufficient to
maintain a positive water balance throughout the year. This means that the forest
encounters minimal water stress. An annual rainfall of at least 100 in (2,500 mm)
has been proposed as the minimum amount needed to maintain a tropical rainfor-
est. However, this rainfall must be distributed evenly throughout the year. A nega-
tive water balance for more than a month has been shown to have a devastating
effect on the tropical rainforest.
Like temperature, there is a diurnal cycle to rainfall within the Tropical Rain-
forest Biome. Rain typically occurs in the late afternoon when convection has
reached its maximum. Rainfall can be very heavy at times, and in some areas can
last until nightfall or well into the night. Inland regions and mountain slopes expe-
rience rain earlier in the afternoon compared with coastal areas, where rainfall typi-
cally occurs in the evenings.
Precipitation is cycled through small water cycles within forests, where precipi-
tation evaporates through photosynthetic processes then condenses and is recov-
ered as precipitation. These small water cycles have a transfer time of one to two
days and account for nearly two-thirds of the water budget. The other way water is
transferred is through the larger cycles that can encompass vast oceanic areas, such
as rainfall transported through trade winds, monsoons, and cyclones. For example,
the larger movements of water around the Atlantic Ocean feeds the African rainfor-
est, and to a smaller extent the Amazon Basin.
These large and small water cycles contribute to the creation and maintenance
of the Tropical Rainforest Biome. Any imbalance or change in these climate
regimes over short or longer time spans can have a significant effect on the tropical
rainforest, as well as climate conditions around the world.
Climate Change
Natural changes in climate have affected the Tropical Rainforest Biome through-
out time. Recent studies have challenged earlier thoughts that the rainforest
has been only weakly affected by climate changes since its origin in the Tertiary
(65 mya). New studies document that small decreases in mean temperature between
1.8 million and 12,000 years ago caused significant changes in forest stands.
When cooler temperatures prevailed, water vapor in the atmosphere decreased,
and the climate became drier than today (see the sidebar on p. 24). Alternatively,
