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Lecture 19

Biology 2483A Lecture 19: Lecture 19

Course Code
BIOL 2483A
Hugh Henry

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Lecture 19: Production
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 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.
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 (GPP)total amount of carbon fixed by
GPP depends on photosynthetic rate.
Photosynthetic rate is influenced by climate and leaf area index (LAI)leaf
area per unit of ground area.
o LAI varies among biomes:
o Less than 0.1 in Arctic tundra (less than 10% of the ground surface
has leaf cover).
o 12 in boreal and tropical forests (12 layers of leaves between the
canopy and the ground, on average).
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
All plant tissues lose carbon via respiration, but not all tissues are
photosynthetic (e.g., tree trunks). Trees tend to have higher respiratory
Respiration rate increases with temperature, so tropical forests have higher
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.
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Plants can respond to environmental conditions by allocating carbon to the
growth of different tissues.
o Example: Grassland plants allocate more NPP to roots because soil
nutrients and water are scarce.
Allocation of NPP to storage products (e.g., starch) provides insurance
against loss of tissues to herbivores, disturbances such as fire, and climatic
events such as frost.
Substantial amounts of NPP (up to 20%) may be allocated to defensive
secondary compounds.
It is important to be able to measure NPP:
NPP is the ultimate source of energy for all organisms in an ecosystem.
Variation in NPP is an indication of ecosystem health.
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.
Harvest techniques: 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:
Fine roots turn over more quickly than shootsthey die and are replaced
Roots may exude carbon into the soil, or transfer it to mycorrhizal or
bacterial symbionts.
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.
Chlorophyll concentrations can be a proxy for GPP
and NPP. They 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.
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