experience an annual rainfall of between 50 and 200 cm
(about 20– 80 in), most of it falling in a five- to eight-
month wet season. Rainfall varies among years, and
periodic droughts occur, some of which may be severe.
Though annual precipitation on a savanna may be
substantial, for at least part of the year there is drought
stress, which ultimately favors grasses and dry- adapted
trees. In most areas, rainfall is the most critical variable
in determining whether an area is essentially grassland
(low rain), savanna, or dry woodland (moderate rain).
Though savannas throughout Venezuela, Colombia,
Bolivia, Suriname, Brazil, and Cuba all experience
a significant dry season exceeding three months,
savannas in Central America (Nicaragua, Honduras,
Belize) as well as in coastal areas of Brazil and the island
of Trinidad do not have protracted dry seasons. For only
three months at the most is rainfall below 10 cm (4 in)
per month. Additional factors, particularly soil quality,
contribute to savanna formation in these areas.
Fire
Fire frequency is an important variable upon which
many savannas depend. Savannas typically experience
frequent mild fires, but there may be major burns every
few years or so. Many savanna and dry- forest plant
species are pyrophytes, plants that are adapted in various
ways to withstand occasional burning. Grass is an
outstanding example. Grasses have dense underground
root systems that are protected from surface fire and
allow rapid aboveground growth following fires. A fire
burns the plants’ aboveground vegetation, releasing
minerals, fertilizing the upper soil layer, and enhancing
regrowth. Trees are less adaptable to fire, though
ancient charcoal remains from Amazon forest soils
dated prior to human invasion suggest that even moist
forests also occasionally burn.
Experiments in which fire is suppressed suggest
that if fire did not occur in savannas, plant species
composition would significantly change. When
burning occurs, it prevents competition among plant
species from progressing to the point where some
species exclude others. Frequent fire generally favors
grasses and selects against woody vegetation.Soil Characteristics
Many savanna soils, like many rain forest soils, are
typically oxisols and ultisols (chapter 6), with a low pH
(4– 4.8) and notably low concentrations of phosphorus,
calcium, magnesium, and potassium. Aluminum levels
are high. Some savannas occur on waterlogged soils,
others on dry, sandy, well- drained soils.
Waterlogged soils occur in areas of flat topography
or poor drainage. Because these soils usually contain
large amounts of clay they become water- saturated. Air
cannot penetrate between the soil particles, making
the soil oxygen poor. In extreme cases, hardened pans
form, as in the case of lateritic arecife soil and caliche
soil, which is hardened with calcium carbonate.
By contrast, dry soils are sandy and porous, their coarse
texture permitting water to drain rapidly. Sandy soils are
prone to the leaching of nutrients and minerals and so
tend to be nutritionally poor. Though most savannas are
found on sites with poor soils (either because of moisture
conditions or nutrient levels or both), poor soils can and
do support lush rain forest. The white, sandy soils of the
upper Amazon (chapters 6 and 12) support such forests,
unless the forest is cut and burned.
This wide range of soil characteristics may seem
unusual, but it really means that extreme soil conditions,
either too wet or too dry for forests, are satisfactory
for savannas. More moderate soil conditions support
moist forests. Indeed, soil degradation is blamed
for promoting savanna formation in sites that once
supported moist forest.Plate 14- 5. The Campo Flicker (Colaptes campestris) ranges
throughout southeastern South America. It frequents open
areas, cerrado, dry woodland, and savannas, where it is often
on the ground, searching for ants. Photo by Nancy Norman.
252 chapter 14 don’t miss the savannas and dry forests