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Biology 2483A Study Guide - Final Guide: Normalized Difference Vegetation Index, Spectral Signature, Chlorophyll

Course Code
BIOL 2483A
Hugh Henry
Study Guide

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Unit 6: Ecosystems
Chapter 20: 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
Primary production is the chemical energy generated by autotrophs during photosynthesis and chemosynthesis.
oPrimary 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 autotrophs.
oGPP depends on photosynthetic rate.
Photosynthetic rate is influenced by
oClimate and
oLeaf area index (LAI): leaf area per unit of ground area.
LAI varies among biomes:
oLess than 0.1 in Arctic tundra (less than 10% of the ground surface has leaf cover).
o12 in boreal and tropical forests (12 layers of leaves between the canopy and the ground, on average).
oBecause of shading, the incremental gain in photosynthesis for each added leaf layer decreases.
oEventually, the respiratory costs associated with adding leaf layers outweigh the photosynthetic
Layer 1 is the top of the canopy and layer 15 is the bottom
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Ecosystem photosynthesis increases as LAI increases
Incremental carbon gain decreases with each additional leaf layer
This may because leaves in the bottom layer become shaded
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.
Net primary production
Net primary production (NPP)  NPP = GPP – Cellular Respiration
oNPP represents biomass gained by the plant.
oNPP is the energy left over for plant growth, and for consumption by detritivores and herbivores.
oNPP represents input of carbon in ecosystems.
Plants can respond to environmental conditions by allocating carbon to the growth of different tissues.
Example: allocation of NPP to roots will depend on the nature of the biome:
oBiomes where competition for light is important  less NPP is allocated to roots
oIn nutrient poor biomes  more than half of NPP is allocated to roots
E.g. tundra and grasslands
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.
Measuring NPP
It is important to be able to measure NPP:
oNPP is the ultimate source of energy for all organisms in an ecosystem.
oVariation in NPP is an indication of ecosystem health.
oNPP is associated with the global carbon cycle.
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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:
oFine roots turn over more quickly than shoots—they die and are replaced quickly.
oRoots may exude carbon into the soil, or transfer it to mycorrhizal or bacterial symbionts.
oHarvests must be more frequent, and additional correction factors are needed.
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.
oChlorophyll absorbs red and blue light resulting in dips in reflectance curves for vegetation
oVegetation reflects more near IR light than bare soil and water do
Indices for estimating NPP from reflection of several different wavelengths have been developed:
NDVI (normalized difference vegetation index):
oNIR = Near-infrared wavelengths (700-100 nm)
ored = red wavelengths (600 – 700 nm)
Vegetation has a high NDVI value; water and soil have low NDVI values.
NDVI is measured over large spatial scales and can estimate CO2 uptake and NPP, deforestation, desertification,
and other phenomena.
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