2
Methods of Estimating Aboveground Net
Primary Productivity
Osvaldo E. Sala and Amy T. Austin
Introduction
Estimating net primary productivity (NPP) has
been a central goal of basic and applied ecologists.
Very important questions rely on good estimates of
NPP: the global carbon balance, the location of the
missing carbon sink, and predictions of global climate change (see Chapter 3). Primary productivity
represents the major input of carbon and energy
into ecosystems and McNaughton (1989) proposed
NPP as an integrative variable of the functioning of
the whole ecosystem because of its relationships
with animal biomass, secondary productivity, and
nutrient cycling. From an applied perspective, primary production of grasslands determines forage
availability and constrains animal carrying capacity
whereas primary production of forests is directly
related to wood yield.
The concept of primary productivity is related to
the ideas of energy flow in ecosystems (Odum
1971). A portion of the photosynthetically active
radiation (PAR, radiation in the 400- to 700-nm
wave band) received by an ecosystem is absorbed
by green plants (Fig. 2.1). The absorbed energy is
either re-radiated, lost as latent heat, or stored by
the activity of photosynthesis in organic substances.
The last flow is known as gross primary productivity. Plants use the stored energy in these organic
compounds fixed in photosynthesis for their own
respiration (autotrophic respiration). The balance
between carbon fixation in photosynthesis and carbon loss in plant respiration is net primary productivity, or as Odum (1971) defined it: "net primary
productivity is the rate of storage of organic matter
in plant tissues in excess of the respiratory utilization by plants." Along the trophic web, herbivores
eat part of the organic matter stored in plants (herbivore consumption) but a portion is not consumed
and goes directly to decomposers. This flow of matter and energy that represents the fraction of NPP
not consumed by heterotrophs is named net community productivity. Only a portion of the consumed substances is assimilated since part is lost in
animal feces and urine. The assimilated products
are then used in supporting heterotrophic respiration or accumulation of herbivore biomass. The rate
of accumulation of organic substances in herbivores is named secondary productivity. This pattern
of partitioning of energy into the categories of assimilated, nonassimilated, and nonutilized is repeated along the food web. Finally, net ecosystem
productivity is the rate of storage or loss of organic
matter in the ecosystem in excess of the respiration
by all its organisms in all the different trophic levels
from autotrophs to decomposers.
All types of productivity are flow rates of matter
and energy through different portions of the ecosystem; they are represented by arrows in the
scheme of figure 2.1, and they have units of mass
or energy per unit area per unit time (e.g.,
g m -2 yr- 1 or kJ m -2 yr- 1 ). The terms production
and productivity are used as synonyms here. In contrast, biomasses of plants and animals are quantities
or state variables where mass and energy accumulate; they are represented by boxes in Figure 2.1,
and they have units of mass or energy per unit area
(e.g., g m- 2 or kJ m- 2 ). These two concepts, biomass and productivity, although very different, are
often confounded. In part, this confusion arises because biomass is used to estimate productivity in
31
Methods of Estimating Aboveground Net
Primary Productivity
Osvaldo E. Sala and Amy T. Austin
Introduction
Estimating net primary productivity (NPP) has
been a central goal of basic and applied ecologists.
Very important questions rely on good estimates of
NPP: the global carbon balance, the location of the
missing carbon sink, and predictions of global climate change (see Chapter 3). Primary productivity
represents the major input of carbon and energy
into ecosystems and McNaughton (1989) proposed
NPP as an integrative variable of the functioning of
the whole ecosystem because of its relationships
with animal biomass, secondary productivity, and
nutrient cycling. From an applied perspective, primary production of grasslands determines forage
availability and constrains animal carrying capacity
whereas primary production of forests is directly
related to wood yield.
The concept of primary productivity is related to
the ideas of energy flow in ecosystems (Odum
1971). A portion of the photosynthetically active
radiation (PAR, radiation in the 400- to 700-nm
wave band) received by an ecosystem is absorbed
by green plants (Fig. 2.1). The absorbed energy is
either re-radiated, lost as latent heat, or stored by
the activity of photosynthesis in organic substances.
The last flow is known as gross primary productivity. Plants use the stored energy in these organic
compounds fixed in photosynthesis for their own
respiration (autotrophic respiration). The balance
between carbon fixation in photosynthesis and carbon loss in plant respiration is net primary productivity, or as Odum (1971) defined it: "net primary
productivity is the rate of storage of organic matter
in plant tissues in excess of the respiratory utilization by plants." Along the trophic web, herbivores
eat part of the organic matter stored in plants (herbivore consumption) but a portion is not consumed
and goes directly to decomposers. This flow of matter and energy that represents the fraction of NPP
not consumed by heterotrophs is named net community productivity. Only a portion of the consumed substances is assimilated since part is lost in
animal feces and urine. The assimilated products
are then used in supporting heterotrophic respiration or accumulation of herbivore biomass. The rate
of accumulation of organic substances in herbivores is named secondary productivity. This pattern
of partitioning of energy into the categories of assimilated, nonassimilated, and nonutilized is repeated along the food web. Finally, net ecosystem
productivity is the rate of storage or loss of organic
matter in the ecosystem in excess of the respiration
by all its organisms in all the different trophic levels
from autotrophs to decomposers.
All types of productivity are flow rates of matter
and energy through different portions of the ecosystem; they are represented by arrows in the
scheme of figure 2.1, and they have units of mass
or energy per unit area per unit time (e.g.,
g m -2 yr- 1 or kJ m -2 yr- 1 ). The terms production
and productivity are used as synonyms here. In contrast, biomasses of plants and animals are quantities
or state variables where mass and energy accumulate; they are represented by boxes in Figure 2.1,
and they have units of mass or energy per unit area
(e.g., g m- 2 or kJ m- 2 ). These two concepts, biomass and productivity, although very different, are
often confounded. In part, this confusion arises because biomass is used to estimate productivity in
31