respiration, and it is common to all ecosystems, including
tropical forests. Net primary productivity is easier to
calculate than gross primary productivity, because NPP
can be measured by weighing biomass change over a
period of time (keeping in mind that some biomass will
likely be consumed by herbivores). To calculate GPP we
would need to assess respiration, which would require
detailed physiological measurements of carbon dioxide
output, making it considerably harder to determine.
Tropical forests currently cover about 7– 10% of the
global land area (this amount was considerably greater
before human deforestation), and because tropical moist
forests exhibit high net productivities, they store about
40– 50% of carbon present within terrestrial ecosystems.
That is essentially what you are looking at when you
behold the massive trees and dense green of a rain
forest: carbon stored in the form of leaves, branches,
bark, roots, wood. As noted above, tropical humid
forests, particularly rain forests, are the most productive
of Earth’s natural terrestrial ecosystems. And keep in
mind that tropical humid forests store that carbon (in
the form of wood) for long time periods. This is why
tropical rain forests are sometimes referred to as carbon
sinks. They make wood and keep it, and what is wood
but large carbon- based compounds. But it is also true
that rain forests have high rates of respiration, so they
have the capacity to emit much carbon dioxide. Rain
forests expend as much as 50– 60% of gross primary
productivity in respiratory metabolic needs. What this
means, of course, is that gross primary productivity, the
total rate of photosynthesis (net productivity plus energy
used for respiration), is very high in rain forests, but the
cost of photosynthesis, as measured in respiration rates,
is also high (plate 5- 3).
Many physical factors, such as water availability, light
intensity, and distribution of mineral nutrients, affect
the outcome of GPP, which is why productivity varies
significantly among global ecosystems. In the tropics,
solar radiation, the most fundamental ingredient, varies
mostly because of seasonal variation in cloud cover. The
component molecules necessary for photosynthesis,
carbon dioxide and water, are essential to support
primary productivity. Water is limited in many
ecosystems, including some tropical forests during
dry season, thus water availability has a significant
influence on global patterns of productivity. In
addition, GPP is dependent on atoms such as nitrogen,
phosphorus, calcium, and potassium (an incomplete
list). In tropical forest ecosystems, as you will learn in
chapter 6, phosphorus tends to be a common limiting
factor of NPP. Atmospheric carbon dioxide has been
increasing since the onset of the Industrial Revolution
and continues to rise, potentially contributing to global
GPP as well as to current climate change. If water or
other factors are not limiting, the addition of carbon
availability in the atmosphere, as is now occurring,
could act to stimulate additional primary productivity,
one of the reasons for the focus of research on tropical
forests as potential carbon sinks.Rain Forest Net Productivity in
Global ContextIn the course of one year, a square meter (10.75 ft^2 )
of rain forest captures nearly 30,000 kilocalories
of sunlight (GPP). Of this total, the plants devote
about 35% to new growth and reproduction (NPP),
using the remaining 65% for metabolic energy. These
percentages are crude, and forests vary with regard to
NPP in relation to GPP. Tropical forest is estimated to
account for one- third of global potential terrestrial net
primary productivity (fig. 5- 1).
The high productivity typical of tropical rain forests
is facilitated by a growing season considerably longer
than any in the temperate zone. At the height of the
summer growing season, the daily NPP per hectare of
temperate forest, such as is found in the Great Smoky
Mountains, will be similar to that of a tropical humid
forest. However, growth in a tropical humid forest is
essentially continuous throughout the year, neverPlate 5- 3. Tropical rain forests have high rates of NPP but also
high rates of respiration. The forest breathes. Photo by John
Kricher.74 chapter 5 sun plus rain equals rain forest