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Lecture 19

Biology 2483A Lecture Notes - Lecture 19: Leaf Area Index, Normalized Difference Vegetation Index, Spectral Signature


Department
Biology
Course Code
BIOL 2483A
Professor
Hugh Henry
Lecture
19

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Integrates ecology with other disciplines such as geochemistry, hydrology, and atmospheric science.
Ecosystem: refers to all the compounds of an ecological system, biotic and abiotic, that influence the flow of
energy and elements.
Primary Productivity: 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.
Primary Production: the chemical energy generated by autotrophs during photosynthesis and
chemosynthesis.
GPP depends on photosynthetic rate.
Photosynthetic rate if influenced by climate and leaf area index (LAI) - leaf area per unit of ground
area.
Gross Primary Production (GPP):total amount of carbon fixed by autotrophs.
Less than 0.1 in Arctic tundra (less than 10% of the ground surface has leaf cover.
12 in boreal and tropic forests (12 layers of leaves between the canopy and the ground, on
average)
LAI varies amount biomes:
Because of shading, the incremental gain in photosynthesis for each added leaf layer decreases.
Note: What starts to limit the leaf area is when the bottom layers are not contributing to the net
gain of photosynthesis.
Bottom leaf has almost a matching respiration and photosynthesis rate.
When costs outweigh benefits you won't get any more leaves being produced.
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.
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 respiratory losses.
Leaf Area Index
Net Primary Production
Net Primary Production = Gross Primary Production - Respiration
Lecture 19: Production
Thursday, December 3, 2015
2:51 PM
Lecture Slides Page 1

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NPP represents biomass gained by the plant.
NPP is the energy left over for the plant growth, and for consumption by detritivores and herbivores.
NPP represents input of carbon in ecosystems.
Plants can respond to environmental conditions by allocating carbon to the growth of different tissues.
Grassland plants allocate more NPP to roots because soil nutrients and water are scarce.
Example:
Plants have well adapted ways in which they try to maximize their growth
Everything of the left side has a high competition for light so they don't allocate their time
to root growth.
Right side doesn't have much light competition so they allocate most of their growth to the
area were resources are most limited (roots)
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.
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 as reasonable
estimate of aboveground NPP if corrections are made for herbivory and mortality.
Important to be able to measure NPP:
How to Measure NPP
Fine roots turn over more quickly than shoots - they die and are replaced quickly.
Roots may exude carbon into the soil, or transfer it to mycorrhizal or bacterial symbiosis.
Harvests must be more frequent, and additional correction factors are needed.
Measuring belowground NPP is more difficult:
Lecture Slides Page 2

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Minihizotrons are underground viewing tubes with video cameras. They allow direct observation
of root growth and death, and have advanced the understand of below ground production
processes.
Harvest techniques are impractical for large of 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.
NDVI (normalized difference vegetation index):
NIR = Near-infrared wavelengths (700-100 nm)
Red= red wavelengths (600-700nm)
Indices for estimated NPP from reflection of several different wavelengths have been developed:
This number will give you an estimate of how green the landscape is.
Chlorophyll absorbs blue and red wavelengths and has a characteristic spectral signature.
Vegetation has a high NDVI value; water and soil have low NDVI values
NDVI is measured over large spatial scales and can estimated CO2uptake and NPP, deforestation,
desertification, and other phenomena.
Lecture Slides Page 3
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