Chapter 14 Don’t Miss the Savannas and Dry Forests

cecropias, and others, depending upon location. Local

plant species composition varies widely. In much of

Central America and the Caribbean, the most abundant

savanna tree species is Caribbean Pine (Pinus caribaea;

plate 14- 4), which is often adorned with bromeliads and

orchids. Several species of oaks are common in Central

American savannas, though no oaks are found in South

American savannas. Fire- resistant tree species such as

Byrsonima crassifolia, Casearia sylvestris, and Curatella

americana are abundant on South American savannas,

as are large stands of Moriche Palm (Mauritia flexuosa),

typically found along wet areas. Grasses and (in wetter

areas) sedges form much of the ground vegetation. Soil

ranges widely, from sandy to claylike, but is typically

described as poor.

What Causes Savanna Formation?

There is no single environmental factor that determines

that a given site will be savanna. Savannas are found on

a wide variety of soil types and experience extremes

of tropical climate. Rainfall, while usually strongly

seasonal, may in some cases be relatively nonseasonal.

Water drainage may be rapid or slow. Fire is an

important influence, and savannas tolerate fire well,

rebounding quickly after burning. Grazing by large

animals may exert strong effects on some savannas,

particularly in Africa, but grazing is not generally a

factor in the Neotropics, except where there is extensive

and prolonged cattle ranching.

Though climate has a major influence on savanna

formation, it cannot be the only influence, because tracts

of savanna frequently occur in the midst of otherwise wet

forest areas, where rainfall is evenly distributed throughout

the year. For this reason, local soil type (edaphic factors)

as well as other factors must also influence savanna

formation. Soil and climate strongly interact. In the

central Llanos in northeastern South America, heavy

rains result in soil forming a hardened crust of lateritic

ferric hydroxide, usually at some depth in the soil but

occasionally on the surface. This crust, termed arecife,

is sufficiently hard to impede the growth of tree roots,

except where the roots encounter channels through the

crust. In essence, the area becomes permanent savanna.

Lightning- set fire is a normal component of

savanna climate and is of major importance in

savanna formation and propagation. Natural fires

may be common during dry season when brief but

heavy thunderstorms are typical. Some savannas have

formed on sites where rain forest has been repeatedly

cut and burned, suggesting that human activity can

alter conditions on the site such that savanna takes

over when the site is abandoned.

Climate

Savannas and moist forests share relatively few species,

and moist forests are typically richer in species.

However, savannas of various sorts, such as cerrado, host

numerous endemic plant species and all savannas offer

opportunities to see many animal species (plate 14- 5),

as discussed later in the chapter. Savannas typically

Plate 14- 4. Savanna ecosystem on Abaco Island in the

Bahamas. The foreground is mostly grass and palmettos; the

background trees are Caribbean Pine. Photo by John Kricher.

Plate 14- 3. Pinewoods characterize savannas in the Bahamas

and much of the savanna habitat in Central America. Photo by

John Kricher.

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