Lecture 18: Production Flashcards
(32 cards)
Ecosystem
- refers to all the components of an ecological system, biotic and abiotic, that influence the flow of energy and elements
- 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
- 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
- depends on photosynthetic rate
Photosynthetic rate
- is influenced by climate and leaf area index (LAI)—leaf area per unit of ground area
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 (NPP)
GPP - Respiration
- GPP: all carbon that’s been fixed
- NPP: is the energy left over for plant growth, and for consumption by detritivores and herbivores
- Where water is plentiful (tropics), less NPP to roots, more to shoots to get more light (lots of competition)
Allocation of NPP to storage products (starch)
- provides insurance against loss of tissues to herbivores, disturbances such as fire, and climatic events such as frost
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.
- Anything put into NPP is pulled out of the atmosphere
Measuring below ground NPP is more difficult
- 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 symbionts.
- Harvests must be more frequent, and additional correction factors are needed
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.
NDVI (normalized difference vegetation index)
- is measured over large spatial scales and can estimate CO2 uptake and NPP, deforestation, desertification, and other phenomena.
NDVI = (NIR – red)/ (NIR + red)
NIR – near-infrared wavelengths (700-1000nm)
Red = red wavelengths (600-700nm)
High and low NDVI values
- Vegetation has a high NDVI value
- water and soil have low NDVI values.
NEE (Net ecosystem production or exchange)
- More carbon coming out (source) or is carbon going in (carbon sink)
- Wants ecosystems to be sinks rather than sources
- NEE = GPP - (AR + HR)
Eddy correlation or eddy covariance
- NEE is estimated by measuring CO2 at various heights in a plant canopy
- Instruments are mounted on towers to take continuous CO2 measurements
- At night, level near plants, soil respiring, lot of CO2
- Daytime, plants sucking up CO2, see drop in CO2
Phytoplankton
- do most of the photosynthesis in aquatic habitats
- Phytoplankton have short life spans, so biomass at any given time is low compared with NPP; harvest techniques are not used
Environmental controls on NPP
- NPP varies substantially over space and time. Much of the variation is correlated with climate.
- NPP increases as precipitation increases, up to a point. At very high precipitation levels, there is usually heavy cloud cover, so less light, and wet soils can become hypoxic.
- NPP increases with increasing average annual temperature
- ecosystem carbon storage (NEE) does not necessarily increase. Warmer temperature also increase respiration rates and loss of carbon.
What nutrient controls NPP in terrestrial ecosystems?
- Nitrogen
- Phosphorus
- More nitrogen/phosphorus leads to greater plant growth
Experiment: Dry and wet meadows - In the dry meadow, nitrogen limited NPP.
- In the wet meadow, both nitrogen and phosphorus limited NPP.
Co-limitation
two nutrients limiting growth
resource-poor vs resource-rich communities
- Plants from resource-poor communities tend to have low growth rates, and require less nutrients, and thus respond less to fertilization, than plants from resource-rich communities.
- When nutrient-poor communities are fertilized, there is often a change in species composition.
NPP in rivers and streams
- NPP is often low. Most of the energy is derived from terrestrial organic matter.
- Water flow limits phytoplankton growth: most NPP is from macrophytes and attached algae as opposed to free-flowing algae
NPP in ocean
- In the open ocean, NPP is mostly limited by nitrogen.
Global Patterns of NPP
reflect climatic constraints and biome types
- Average rate of NPP for the land surface is higher than for oceans: 54% of carbon is taken up by terrestrial ecosystems, 46% by the oceans.
NPP in terrestrial biomes
variation is associated mostly with leaf area index and length of growing season.
