Geoscience Reference
In-Depth Information
Mountain building and fluctuating sea levels significantly influenced the flora
and fauna of the forests. Lower sea levels exposed land bridges from North to
South America and through the Pacific Islands of the Sunda Shelf, allowing for the
interchange of species and changing the composition of plants and animals within
these areas. The return to higher sea levels left islands and continents temporarily
isolated, providing the opportunity for species endemism.
Although the distribution of many taxa found in the tropical seasonal forests
can be related to their past, the current structure and appearance of today's forest is
probably very different. Long-term human occupancy in these seasonal forests, the
use of fire, and the conversion of land for shifting agriculture have significantly
changed the nature and extent of the world's Tropical Seasonal Forest Biome.
Climate
Temperatures in the Tropical Seasonal Forest Biome remain warm throughout the
year. Average monthly temperature varies from 75
C), depending
on the location of the forest. Temperatures show some seasonal variation, ranging
from 68
-81
F(24
-27
C). Tropical seasonal forests remain frost free. The unifying
climatic variable of all tropical seasonal forests is the strong seasonality of rainfall.
Annual precipitation can be high, ranging from 40-80 in (1,000-2,000 mm), and
occurs mostly in the summer months (see Figure 4.3). The Tropical Seasonal Forest
Biome is characterized by a four- to seven-month dry season, sufficient to cause
many trees, vines, and other plants to shed their leaves. The abundant seasonal rain-
fall, distinct dry season, and warm temperatures make this type of tropical climate a
tropical wet and dry climate (Aw in the Koeppen classification system) or tropical
monsoon climate (Am in the Koeppen classification), depending on the location.
Tropical seasonal forests vary considerably in the amount of rainfall and dura-
tion of dry periods. The amount and timing of rainfall is largely controlled by the
seasonal shifts of the Intertropical Convergence Zone (ITCZ) and the occurrence
of tropical monsoons. The ITCZ is a zone of low pressure, clouds, and rainfall that
migrates north and south with the sun, creating a strong wet-dry seasonality in the
tropical seasonal forest regions. Rainfall is concentrated in the periods when the
ITCZ is present, with the dry season occurring when the ITCZ moves away. In the
Northern Hemisphere, the dry period occurs from about December to March. For-
est structure, canopy height, and total biomass are influenced by rainfall amounts
and average length of dry season. See Chapter 1 for a full explanation of global cir-
culation patterns and energy budgets in the tropics.
-86
F(20
-30
Monsoons
Several regions of the Tropical Seasonal Forest Biome, particularly Asia, are
greatly influenced by monsoons. A monsoon is a significant shift of prevailing
