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Lecture 19

Lecture 19 - Production

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Department
Biology
Course
Biology 2483A
Professor
Hugh Henry
Semester
Fall

Description
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 – chemical energy generated by autotrophs during photosynthesis and chemosynthesis o 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 autotrophs o GPP depends on photosynthetic rate  Photosynthetic rate is influenced by climate and leaf area index (LAI) – leaf area per unit of ground area  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 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): NPP = GPP – RESPIRATION o NPP represents biomass gained by the plant o NPP is the energy left over for plant growth, and for consumption by detritivores and herbivores o NPP represents input of carbon in ecosystems  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: o NPP is the ultimate source of energy for all organisms in an ecosystem o Variation in NPP is an indication of ecosystem health o 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: o Fine roots turn over more quickly than shoots – they die and are replaced quickly o Roots my exude carbon into the soil, or transfer it to mycorrhizal or bacterial symbionts  Harvest 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  Vegetation has a high NDVI value; water and soil have low NDVI values  NDVI is measured over large spatial scales and can estimate CO 2ptake and NPP, deforestation, desertification, and other phenomena  NPP can also be estimated from GPP and respiration measurements. Change in CO conc2ntration is measured in a closed chamber  Sometimes whole stands of plants are enclosed in a chamber or tent to study CO exchange 2  The net change in CO 2s GPP minus total respiration – net ecosystem production or exchange (NEE)  Heterotrophic respiration must be subtracted from NEE to obtain NPP  Thus, NEE is a more refined estimate of ecosystem carbon storage than NPP  Eddy correlation or eddy covariance: NEE is estimated by measuring CO at2various heights in a plant canopy  Instruments are mounted on towers to take continuous CO mea2urements  A network of eddy covariance sites has been established in North America to increase our understanding of carbon and climate  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  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 NPP  Remote sensing of chlorophyll concentrations in the ocean using satellites provides estimates of marine NPP. Indices are developed to indicate how much light is absorbed by chlorophyll, which is then related to NPP 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  But ecosystem carbon storage (NEE) does not necessarily increase. Warmer temperature also increase respiration rates and loss of carbon  Climate influence on NPP can also be indirect, mediated by factors such as nutrient availability  In grassland sites across the central United States, it was observed that NPP variation with precipitation was greater over the range of sites, than year to year at one site  The different responses were attributed to variation in species composition at the sites  Different grass species have different growth
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