The New Neotropical Companion

to be a primary limiting factor in forest net primary

productivity, wet forests that experience high rates of

leaching could conceivably experience high nitrogen

loss from leaching and denitrification (a chemical

reduction process by which certain bacteria liberate

gaseous nitrogen into the atmosphere).

The complexity of nitrogen flux and the patterns by

which basic elements are differently concentrated in

tropical soil and vegetation is essential to an understanding

of Neotropical ecology. Generalities may be of limited

value. Variability is apparent throughout the tropics.

Is It All about Phosphorus?

In answer to the question posed in this heading: well,

yes, it is. As stated above (see “Rapid Recycling”),

phosphorous is a key nutrient for plants and the most

difficult to procure. There is widespread agreement

that phosphorus (P) limits primary productivity on

many sites throughout the global tropics, particularly

on soils of poor quality. Researchers have identified

different “strategies” associated with plants on P- rich

soils compared with those on P- poor soils. On rich

soils with little P limitation, trees grow rapidly and

retain P for short periods. But on P- poor soils, growth

is slower and plants retain P for longer periods. In both

cases, the forests appear to thrive. It is not possible just

from observation to readily see the difference between

the extremes (rich and poor) in the structure of forests

even though the efficiencies of nutrient cycling of

phosphorus vary dramatically between them.

Blackwaters and Whitewaters: The

Poor and the Rich

White, sandy soils are usually drained by rivers called

blackwaters, best seen at areas such as the Río Negro

near Manaus, Brazil, or Canaima Falls in southeastern

Venezuela. Water appears tea- like, dark and clear, colored

by tannins, other phenols, and related compounds and

by the humic matter. Blackwaters occur throughout

the tropics and the temperate zone, including North

America, especially such habitats as boreal peatlands

and coniferous forests with mineral- poor, sandy soils.

Part of the humic matter in blackwaters consists of

defense compounds (chapter 11) leached from fallen

leaves. It has been hypothesized that leaves are costly

to grow on poor soils because raw materials to replace

a fallen or injured leaf are in limited supply. Therefore,

leaves on plants growing on white, sandy soils tend to

concentrate defense compounds that help discourage

herbivory. Carl Jordan has shown that leaf production

in these conditions may be less than half that in forests

on richer soils, and leaf decomposition time can be in

excess of two years. When the old leaf finally does drop,

rainfall and microbial activity eventually leach it of

tannins and phenols, making the water dark, a kind of

tropical tannin- rich “tea,” called blackwater (plate 6- 11).

In South America, blackwater tributaries drain into the

Río Negro (Black River). This dark water is clear because

there is little unbound sediment to drain into streams

and rivers. Gallery forests bordering blackwater rivers

(called igapo forests), are subject to seasonal flooding,

and their ecology is intimately tied to the flooding cycle

(chapter 12).

Plate 6- 12. Thick deposits of Andean sediment, which is rich in

nutrients, characterize this section of the Río Napo in Ecuador.

Colored by the sediment load it carries, it is known as a

whitewater river. Photo by John Kricher.

Plate 6- 13. This photo shows sediment deposit on an island

along the Amazon River approaching Manaus, Brazil. The river

deposits sediment but also sweeps it away, depending on the

stage of the flood cycle. This creates dynamic islands within

the river. Photo by John Kricher.

chapter 6 essential dirt: soils and cycling 89