Geoscience Reference
In-Depth Information
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Coping with Drought
Crassulacean acid metabolism (CAM) photo-
synthesis is an adaptation plants in arid regions
have evolved to limit water loss while still carry-
ing out photosynthesis. During the day, their
stomata (plant pores) are closed to prevent the
loss of water. In the evening, the stomata open
and the plant absorbs carbon dioxide and
stores it as malic acid. The final stage of photo-
synthesis is carried out during the day using
the energy of the sun and converting the malic
acid to carbohydrate. This process is common
in desert plants and is named after the plant
family Crassulaceae in which the process was
first discovered.
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water and nutrients needed to flower during the dry
season when photosynthesis is limited or nonoccur-
ring. The development of a large rootmass allows
for the storage of moisture and energy needed to
carry out these functions. Some trees develop
extensive horizontal roots, while others have deep
roots. Many roots have an abundance of fine hairs.
In a short dry forest, roots can make up as much as
50 percent of the total biomass of the forest. Many
trees, lianas, and vines exhibit this adaptation.
Some tropical seasonal forests can appear
almost lifeless during the dry season. Reddish-
brown bark and gray branches dominate before
the arrival of the rains. Leaves begin to appear,
usually within 10 days following the first intense
rain. Over the next few weeks, the leaves darken
and turn the bare trees into a lush, dense forest
with an abundance of plant and animal life.
Tropical Thorn Scrub or Thorn Forests
Tropical thorn scrub is found in all three regions in areas where precipitation is
extremely low and seasonal. Annual rainfall can be as low as 20-25 in (500-640
mm), with the dry season lasting up to seven months. In Central America, this veg-
etation type is also called ''cactus scrub'' due to the abundance of cactus species
present. Thorn scrub consists of low-growing, thorny trees, shrubs, and stem succu-
lents. The sparse upper layer contains only a few species. The trees attain heights
of 20-30 ft (6-9 m). The woody species are deciduous and small leafed; many have
thorns or spines. The understory is poorly developed and not continuous, consist-
ing of spiny and xerophytic shrubs and mosses, with a high proportion of dry land
exposed at the surface. Acacias and other trees in the legume family are widespread
throughout the thorn scrub forests of the world, cacti occur in the Neotropics,
and members of the euphorb or spurge family are more common in Africa and Asia
(see Figure 4.8). Stem succulents, xerophytic palms, epiphytic mosses and lichens,
and—in the Neotropics—terrestrial bromeliads are common.
Tropical Woodlands
Tropical woodlands or savanna woodlands are areas of densely or loosely scattered
trees with an understory of grass. Trees can be either evergreen with sclerophyllous
leaves or deciduous. Tropical woodlands are susceptible to fire. Many of these
woodlands have been replaced by savannas due to frequent fires, tree clearing for
fuelwood, or grazing animals.
Specific forests or ecoregions within this biome include the caatinga of north-
east Brazil; the Chaco of Paraguay, southern Bolivia, and northern Argentina; the
