The New Neotropical Companion

interrupted by a cold winter when plants become

essentially dormant. Temperature hardly varies, and

because the tropical year is frost- free, there is no time

at which all plants become dormant, as they are forced

to do in winter throughout much of the temperate

zone. Thus, more NPP occurs in the course of a year in

the tropics than in other regions. Water availability is

an important variable in tropical primary productivity.

A strong dry season in the tropics may mean slower

growth. In areas where dry season is acute, many trees

are deciduous, dropping leaves at the onset of dry

season and growing new leaves with the onset of rainy

season. Droughts may exert severe effects on forest

productivity (see “Drought Sensitivity of Tropical

Forests,” below), often killing large numbers of trees.

As rain forests are cut and replaced by anthropogenic

(human created and controlled) ecosystems, much

more NPP is directed specifically toward humans (in

the form of agriculture or pasturage) and some is lost

altogether (fields and pastures are less productive than

forests), making less energy available for supporting

biodiversity and reducing the potential efficacy of

tropical forests as carbon sinks. For example, when

a forest is cleared, the slash burned, and the site

converted to agriculture or pasture, there is far less

biomass available for carbon absorption (plate 5- 4).

Agricultural ecosystems (including pastures) do not

store nearly as much carbon as forests. It is estimated

that almost 40% of the world’s terrestrial NPP has

either been directed to humans or lost due to human-

directed habitat conversion. About three decades ago

tropical forests were estimated to store about 46% of

the world’s living terrestrial carbon and 11% of the

world’s soil carbon. That figure is likely less now due to

forest clearance.

Net Ecosystem Productivity

Though tropical humid forests throughout Earth have

the highest net primary productivities per hectare of

any terrestrial ecosystem, one must realize that these

forests are more than just trees. Ecologists understand

that net primary productivity and respiration of the tree

community does not, in itself, explain carbon flux (the

rate in which carbon enters and leaves an ecosystem)

at the ecosystem level. Why? Well, animals and fungi

and bacteria are why. They live there too, and they all

expire carbon dioxide. What this means is that carbon

flux in the ecosystem is dependent on the net primary

productivity of all plants minus the total respiration of all

plants, animal consumers, and decomposers. Ecologists

call this net ecosystem productivity (NEP). This simple

equation describes this process:

NEP = NPP – R (plants) – R (consumers) – R (decomposers)

The overall metabolism of a tropical forest, especially

considering its complex decomposer food web, makes

it possible for tropical forests to release more carbon (in

Plate 5- 4. This old pasture contains far less biomass and thus

stores far less carbon than the tropical forest it replaced. Photo

by John Kricher.

Open ocean

Tropical rain forest

Temperate forest

Savanna

Northern coniferous forest (taiga)

Continental shelf

Agricultural land

Temperate grassland

Woodland and shrubland

Estuaries

Swamps and marshes

Desert scrub

Lakes and streams

Tundra (arctic and alpine)

Extreme desert

Type of ecosystem

Average world net productivity (billion kcal/yr)

20 40 60 80 100 120 140 160 180 200

Figure 5– 1. This figure plots the net primary productivity

(NPP) of major ecosystem types. The open ocean and tropical

rain forest are close to equal, but their NPP rates are high for

very different reasons. The oceans are vast in area but low in

per unit area NPP. The rain forests are much more limited in

area but high in per unit area NPP. Reprinted with permission

from Kricher, John. Tropical Ecology. Princeton, NJ: Princeton

University Press, 2011.

chapter 5 sun plus rain equals rain forest 75