The New Neotropical Companion

litter layer (plate 6- 7). Unlike the northern coniferous

forests, for example, which typically have a thick

spongy carpet of soft fallen needles, or the broad-

leaved temperate forests, where layer after layer of

fallen oak, hickory, and maple leaves accumulate, a

rain forest floor is often sparsely covered by fallen

leaves. This is even more noteworthy when you keep

in mind that more and heavier leaves occur in rain

forest. The explanation for this seeming paradox is

that the rates of decomposition and recycling occur

with much greater speed in rain forests than in

temperate forests. Just as productivity is continuous,

uninterrupted by the frozen soils of a northern

winter, biogeochemical cycling continues unabated

throughout the year. In tropical wet forests, litter

may be totally decomposed in less than one year, and

minerals efficiently conserved. There is, of course,

variability, and leaf decomposition rates are higher

on richer compared with poorer soils.

The most important variable in the rate of litter

decomposition is climate; a warm and very wet climate

hastens the decomposition rate. But beyond climate, the

quality of the litter strongly affects the decomposition

rate. Litter rich in phosphorus decomposes more

rapidly than litter with less phosphorus (plate 6- 8).

Tropical moist forests typically cycle minerals

“tightly.” The resident time of an atom in the nonliving

component of the ecosystem is usually brief. This

means that litter does not tend to accumulate. Estimates

are that perhaps up to 80% of fallen leaf biomass is

decomposed and recycled per year.

Rain Forest Soil Types and Nutrient

Cycling

In some regions, such as the eastern and central Amazon

Basin, soils are old and mineral poor (oligotrophic),

while in other regions, such as volcanic areas of Costa

Rica or much of the Andes, soils are young and mineral

rich (eutrophic). Soil characteristics, referred to as

edaphic characters, vary regionally in the Neotropics

because soil is the product of several variables: climate,

vegetation, topographic position, parent material, and

soil age. Even within a relatively limited region there

may be variability among soil types. For instance, a

single day’s ride in southern Belize will take you from

orange- red iron- rich soil to clayey gray- brown soil. The

gray- brown soil is calcium- rich, having weathered from

limestone, and common throughout much of Belize.

Most of the soils throughout the humid tropics

fall into one of three classifications: ultisol, oxisol, or

alfisol. Ultisols are generally well- weathered, meaning

that minerals have been washed (leached) from the

upper parts of the soils. Oxisols, also called ferralsols or

latosols, are deeply weathered, old, acidic, and found on

well- drained soils of humid regions; typically, these soils

occur on old geologic formations, such as the ancient

Guiana Shield in northeastern South America. These

soils have high iron content and are reddish (plate 6-

9). Common throughout the global tropics, oxisols

are heavily leached of minerals and are strongly acidic.

Alfisols are common in the subhumid and semiarid

Plate 6- 7. Leaf litter is abundant in tropical moist forests, but

decomposition is sufficiently rapid that in many areas the litter

layer is relatively thin. Photo by John Kricher.

Plate 6- 8. The species of leaf and the overall quality of the leaf

litter, including the quantity of such components as available

phosphorus, are important in determining how rapidly it will

be decomposed. Photo by John Kricher.

chapter 6 essential dirt: soils and cycling 85