Ecology Lecture No. 19: Production
Tuesday November 13 , 2012
-The term ecosystem is used to refer to all the components of an ecological system, biotic and abiotic,
that influence the flow of energy and elements.
-Primary production is the chemical energy generated by autotrophs during photosynthesis and
chemosynthesis. Primary productivity is the rate of primary production. Carbon is the currency used to
measure primary production since that is how autotrophs store assimilated energy. Gross primary
production (GPP) refers to the total amount of carbon fixed by autotrophs. GPP depends on
-Photosynthetic rate is influenced by climate and leaf area index (LAI), which is the leaf area per unit of
ground area. The LAI varies among biomes with a value of 0.1 in the Arctic tundra (< 10% of the ground
has leaf cover) and a value of 12 in boreal and tropical forests (12 layers of leaves between canopy and
ground). Shading decreases the incremental gain in photosynthesis for each added leaf layer and
eventually, the respiratory costs associated with adding leaf layers outweigh the photosynthetic
-Plants use about half of the carbon fixed in photosynthesis for cellular respiration. All plant tissues lose
carbon via respiration, but not all tissues are photosynthetic (e.g., tree trunks). Trees tend to have
higher respiratory losses. Respiration rate increases with temperature, so tropical forests have higher
Net Primary Production (NPP):
-NPP represents the biomass gained by the plant, is the energy left over for plant growth (and for
consumption by detritivores and herbivores) and represents input of carbon in ecosystems. It is
modelled by the equation: NPP = GPP – Respiration. Plants can respond to environmental conditions by
allocating carbon to the growth of different tissues. Allocation of NPP to storage products (like starch)
provides insurance against loss of tissues to herbivores, disturbances such as fire, and climatic events
such as frost.
Measuring The NPP:
-It is important to measure the NPP since: It is the ultimate source of energy for all organisms in an
ecosystem, variation in NPP is an indication of ecosystem health and NPP is associated with the global
carbon cycle. In terrestrial ecosystems, NPP is estimated by measuring increase in plant biomass in
experimental plots, and scaling up to the whole ecosystem. These harvest techniques are used to measure biomass before and after growing season. This is a reasonable estimate of aboveground NPP if
corrections are made for herbivory and mortality.
-Measuring belowground NPP is more difficult since: Fine roots turn over more quickly than shoots (they
die and are replaced quickly) and roots may exude carbon into the soil (or transfer it to mycorrhizal or
bacterial symbionts). Thus, harvests must be more frequent, and additional correction factors are
needed. Minirhizotrons are underground viewing tubes with video cameras that are used to allow direct
observation of root growth and death, and have advanced the understanding of belowground
Estimating NPP In Biologically Diverse Ecosystems:
-Harvest techniques are impractical for large or biologically diverse ecosystems. Chlorophyll
concentrations can be a proxy for GPP and NPP and can be estimated using remote sensing methods
that rely on reflection of solar radiation. Chlorophyll absorbs blue and red wavelengths and has a
characteristic spectral signature.
-Indices for estimating NPP from reflection of several different wavelengths have been developed by
using the NDVI (Normalized Difference Vegetation Index) = (NIR –red) / (NIR + red) where NIR is the near-
infrared wavelengths and red refers to red wavelengths.
-NDVI is measured over large spatial scales and can estimate CO up2ake and NPP, deforestation,
desertification, and other phenomena. Vegetation has a high NDVI value, while water and soil have low
The Net Ecosystem Exchange (NDEE):
-NPP can also be estimated from GPP and respiration measurements. The net change in CO (GPP mi2us
total respiration) is known as the Net Ecosystem Exchange (NEE). As heterotrophic respiration must be
subtracted from NEE to obtain NPP, NEE is a more refined estimate of ecosystem carbon storage than
NPP. In Eddy Correlation/Covariance, the NEE is estimated by measuring CO at various heights in a plant
canopy. Networks of eddy covariance sites increase our understanding of carbon and climate.
Photosynthesis By Phytoplankton:
-Phytoplankton do most of the photosynthesis in aquatic habitats. Phytoplankton have short life spans,
so biomass at any given time is low compared with the 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 the NPP. Environmental Controls:
-NPP varies substantially over space and time. Much of the variation is correlated with climate. NPP
increases as preci