Nov 19, 2013
Lindeman studied energy relationships among the organisms and non living components in a
lake ecosystem. Rather than grouping its plants, animals, and bacteria according to their
taxonomic categories, they were grouped based on how they obtain their energy.
The term ecosystem was first used by Tansley (1935) to refer to all the components of an
ecological system, biotic and abiotic, that influence the flow of energy and elements. The
elements are primarily nutrients, but may also include pollutants.
The ecosystem concept integrates ecology with other disciplines such as geochemistry,
hydrology, and atmospheric science.
Primary production is the chemical energy generated by autotrophs during photosynthesis and
chemosynthesis. (chemosynthesis generates a very small amount of energy)
Primary production represents an important energy transition (conversion of light energy from
the sun into chemical energy that can be used by autotrophs and consumed by heterotrophs)
Primary productivity is the rate of primary production.
Energy assimilated by autotrophs is stored as carbon compounds in plant tissues, thus carbon is
the currency used to measure primary production.
Gross Primary Production
Gross primary production (GPP) is the total amount of carbon fixed by autotrophs in an
ecosystem. GPP is equivalent to the total photosynthetic rate, thus it depends on the
GPP is controlled by the rate of photosynthesis
(influenced by climate) and leaf area index (LAI)—
leaf area per unit of ground area (how much of the
ground surface has leaf cover). The LAI is a unitless
LAI varies among biomes
Less than 0.1 in Arctic tundra (less than 10%
of the ground surface has leaf cover).
12 in boreal and tropical forests (12 layers
of leaves between the canopy and the
ground, on average). Shading increases with the addition of each leaf layer. Because of shading, the incremental gain
in photosynthesis for each added leaf layer decreases. Eventually, the respiratory costs
associated with adding leaf layers outweigh the photosynthetic benefits.
Plants use about half of the carbon fixed in photosynthesis for cellular respiration to support
biosynthesis and cellular maintenance. All plant tissues lose carbon via respiration, but not all
tissues are photosynthetic (e.g., tree trunks). Therefore, trees tend to have higher respiratory
losses than herbaceous plants. The respiration rate increases with temperature, so tropical
forests have higher respiratory losses.
Net Primary Production
Net primary production is the energy remaining after respiratory losses.
Net primary production (NPP): NPP = GPP – Respiration
NPP represents biomass gained by the plant. NPP is the energy left over for plant growth, and
for consumption by detritivores and herbivores. NPP represents input of carbon in ecosystems
(what remains in ecosystem, not just what it lost).
Carbon not used in cellular respiration can be allocated to growth, reproduction, storage, and
defense against herbivory. Plants can respond to environmental conditions by allocating carbon
to the growth of different tissues. The allocation of carbon is an investment in potential future
NPP, but the demands for other resources determine whether it pays off. Plants tend to
allocate NPP to tissues that acquire the resources that limit their growth.
Example: Grassland plants allocate more NPP to roots because soil nutrients and water
are scarce. In contrast, plants growing in dense communities with neighbors that may
shade them may allocate NPP preferentially to stems and leaves in order to capture
more sunlight for photosynthesis.
Allocation of NPP to storage products (e.g., starch and carbs) provides insurance against loss of
tissues to herbivores, disturbances such as fire, and climatic events such as frost. This is stored
in stems and roots.
Substantial amounts of NPP (up to 20%) may be allocated to defensive secondary compounds
when herbivory is high.
It is important to be able to measure NPP. NPP is the ultimate source of energy for all organisms
in an ecosystem (determines amount of energy available to support that ecosystem). Variation in NPP is an indication of ecosystem health which varies tremendously over space and time.
Year to year changes may be indicative of stresses to that ecosystem (drought, acid rain).
Additionally, NPP is associated with the global carbon cycle. As a result, it is associated with
global climate change.
In terrestrial ecosystems, NPP is estimated by measuring increase in plant biomass in
experimental plots, and scaling up to the whole ecosystem (several methods).
The traditional technique is using harvesting methods.
To do this, you harvest aboveground biomass before and after growing season (at its
maximum). The difference in plant biomass between the two is used as an estimate for
NPP. This is a reasonable estimate of aboveground NPP if corrections are made for
herbivory and mortality.
Measuring belowground NPP is more difficult. The proportion of carbon in roots
exceeds that in aboveground tissues in many ecosystems (as seen before)Fine roots turn
over more quickly than shoots meaning that they die and are replaced quickly. In
addition, roots may exude
carbon into the soil, or transfer
it to mycorrhizal or bacterial
symbionts. Therefore, harvests
must be more frequent, and
additional correction factors are
Minirhizotrons are underground
viewing tubes with video
cameras. They allow direct
observation of root growth and
death, and have advanced the understanding of belowground production processes.
Harvest techniques are impractical for large or biologically diverse ecosystems (due to their
labor intensive and destructive nature)
Several non destructive techniques have been created which allow for more frequent
estimation, over much larger spatial scales. However, they are not as precise as harvest
Remote sensing is a method that relies on the reflection of solar radiation. The amount
of light reflected depends on the wavelengths of the light. Different kinds of objects
absorb or reflect some wavelengths more than others. Remote sensing takes advantage
of light reflection and absorption to estimate the density and composition of objects on
earths surface, in water, and in the atmosphere. Remote sensing can be done at large
scales using satellites which transmit their measurements to receiving stations. It can
estimate CO u2take and NPP, deforestation, desertification, and other phenomena.
Ecologists use remote sensing to estimate NPP by taking advantage of the unique
reflective pattern of chlorophyll containing plants, algae and bacteria. Chlorophyll absorbs blue and red
wavelengths and has a
signature. Ecologists measure
the reflection of specific
wavelengths and estimate NPP
using several indices that have
been developed. NDVI
vegetation index) is an index
that uses differences between visible light and near infrared reflectance to estimate the
density of chlorophyll.
NIR = Near-infrared wavelengths (700-1000 nm)
red = red wavelengths (600 – 700 nm)
Vegetation has a high NDVI value; water and soil have low NDVI values. The above
image shows that vegetation reflects more near infrared and red light (600-700nm and
700-1000nm) than bare soil or water. Also, due to the fact that vegetation absorbs red
and blue, there are dips in the reflectance curve (unlike soil and water which absorb one
Below image shows that terrestrial NPP is highest in the tropics, and it declines heading
North and South
NPP can also be estimated from GPP and respiration measurements (directly). Change in CO
concentration is measured in a closed chamber. Sources of CO2 in a closed system are by plants
and heterotrophs undergoing respiration.
Sometimes whole stands of plants are enclosed in a chamber or tent to study CO 2
exchange. The net change in CO 2s GPP minus total respiration: Net ecosystem production or
Heterotrophic respiration must be subtracted from NEE to obtain NPP. Thus, NEE is a
more refined estimate of ecosystem carbon storage than NPP (which only subtracts
We can also estimate NEE using frequent measurements of CO2 and microclimate at various
heights of the plant canopy, and into the open air above the canopy. This is known as the Eddy
correlation or eddy covariance. It takes advantage of the gradient in CO2 concentration
between the plant canopy and the atmosphere that develops because of photosynthesis and
During the day (plants are photo synthetically active), CO2 is lower in the canopy than it
is in the air above. At night, CO2 in the canopy is higher due to respiration.
Instruments are mounted on towers to take continuous CO mea2urements.
A network of eddy covariance sites has been established in North America to increase
our understanding of carbon and climate.
Phytoplankton do most of the photosynthesis in aquatic habitats (algae and bacteria).
Phytoplankton have short life spans, so biomass (plant material) at any given time is low
compared with NPP; harvest techniques are not used.
Photosynthesis and respiration are measured in water samples collected and incubated onsite
with light (for photosynthesis) and without light (for respiration).
The difference in the rates is equal to NPP (note that there are errors associated with the
artificial environment of the bottles).
Remote sensing of chlorophyll concentrations in the ocean using satellites provides estimates of
marine NPP. Indices are developed to indicate how much light is absorbed by chlorophyll, which
is then related to NPP.
Below image shows that NPP is highest in zones of upwelling in coastal areas! Environmental Controls on NPP
NPP varies substantially over space and time. Much of the variation is correlated with climate
(temperature and precipitation).
NPP in Terrestrial Ecosystems
NPP increases as precipitation increases, up to a point (2,400mm per year). Once it reaches a
certain point, NPP decreases in some ecosystems. At very high precipitation levels, there is
usually heavy cloud cover, lowering the amount of available sunlight. High precipitation also
leaches nutrients from the
soil, and high soil water
content results in hypoxic
NPP increases with
increasing average annual
temperature (does not
mean carbon storage (NEE)
is higher because
respiration/carbon loss is
also higher with warmer
NEE may actually decrease.
We know that water availability and temperature has a direct effect on NPP. However, climate
influence on NPP can also be indirect, mediated by factors such as nutrient availability, or the
particular plant species found within an ecosystem.
An experiment looked at how NPP in a grassland ecosystem responded to year to year
variation in precipitation. They also looked at the average annual NPP and precipitaton
across several grassland ecosystems at different locations in the U.S. In grassland sites
across the central United States, it was observed that NPP variati