Geoscience Reference
In-Depth Information
moisture-conserving response to water scarcity. In areas with a majority of months
without rain, succulents and evergreen plants with smaller drought-resistant leaves
start to dominate. Reduction in tree height is probably a result of reduced water
availability at the root level, and increased duration of the dry season. Lianas and
woody vines increase from wet to dry forests, accounting for up to 34 percent of all
species in some tropical seasonal forests. The presence of epiphytes is reduced,
likely the result of lower humidity and less dew during the dry season, creating an
unfavorable environment. During the dry season, average humidity can be
decreased to as low as 20-60 percent.
Tropical seasonal forests are high in overall biodiversity, though somewhat less
species-rich than the tropical rainforest. In East Africa, for example, the coastal
tropical seasonal forest is second to the rainforest in species richness. Researchers
estimate that a lowland seasonal forest adjacent to a lowland rainforest can contain
50-100 percent of the total plant and animal species of the nearby rainforest. Far-
ther from the rainforest, seasonal forests have, on average, 50 percent or fewer spe-
cies than a comparable area of rainforest.
Forest Structure
Tropical seasonal forests represent a number of distinct communities, each with
different structure and composition. Trees can be deciduous or evergreen with a
canopy that varies from 10-130 ft (3-40 m) in height. The tropical seasonal forests
that border the rainforest tend to have a closed or nearly closed canopy of decidu-
ous trees. In taller deciduous forests, emergents can reach above the canopy to
heights up to 145 ft (44 m). In the tropical deciduous forest, most of the canopy
trees are deciduous and dormant during the dry season, although dormancy may
not last the entire dry season. By dropping their leaves, trees are able to conserve
water. Typically, leaf fall occurs when moisture stress is high at the start of the dry
season. There may be little synchronicity in leaf fall within a given community, or
even a single tree. Within an individual tree, some branches may lose their leaves
while others sprout new ones. The leaves of deciduous canopy trees tend to be
larger and less leathery than rainforest trees. They develop quickly at the start of
the wet season. In other types of seasonal forests, evergreen trees dominate. These
trees are small in stature and their leaves are often small and leathery to resist dry-
ing out during extensive periods of drought. Evergreen trees in the seasonal forest
tend to be shorter, with an open or closed canopy. Most trees found in tropical sea-
sonal forests tend to be shorter, with a less complex forest structure than the tropi-
cal rainforest. As the duration of the dry season increases, the canopy becomes
open, with fewer layers, lower biomass, and lower net primary productivity com-
pared with rainforests (see Figure 4.5). Tree diameters are smaller than those in the
rainforest, and because of a short growing season, annual tree growth can be half
that of the rainforest.
Families of trees found in the seasonal forest are often present in the rainforest;
however, species are often quite distinct. Many trees within the legume family
