Ecosystems

Learning goals:
* Be able to describe the components that make up an ecosystem and why it is different from communities
* Understand what primary production is and why it is important
* Learn the variety of ways primary production is measured
* Understand the factors that control rates of primary production in different ecosystems

Focus of ecosystem studies is on the movement of energy and elements between organisms and the physical components of the ecosystem: soil, water, organic matter
The ecosystem concept provides an approach for integrating ecology with other disciplines such as geochemistry, hydrology, and atmospheric science
Ecosystem studies have traditionally ignored the identity of the organisms involved in processes, emphasizing instead the functional groups involved: autotrophs, detritivores/ decomposers, herbivores, carnivores
Microbes- bacteria, cyanobacteria, and fungi, play important roles in ecosystem function; identity rarely known

Primary Production
Primary production is the chemical energy generated by autotrophs, derived from fixation of CO2 in photosynthesis and chemosynthesis.
Energy assimilated by autotrophs is stored as carbon compounds in plant tissues; carbon is the currency used for the measurement of primary production
Gross primary production (GPP)—total amount of carbon fixed by autotrophs in an ecosystem.

GPP depends on the influence of climate on photosynthetic rate and the leaf area index (LAI)—leaf area per unit of ground area.
LAI varies among biomes:
Less than 0.1 in Arctic tundra (less than 10% of the ground surface has leaf cover), to 12 in boreal and tropical forests (on average, there are 12 layers of leaves between the canopy and the ground).
Because of shading, the incremental gain in photosynthesis for each added leaf layer decreases, while the respiratory costs per leaf stay the same. Eventually, the respiratory costs associated with adding leaf layers outweigh the photosynthetic benefits.

Correlation between photosynthesis (GPP) and growth is influenced by allocation of carbohydrates and respiration rate
On average, plants use about half of the carbon fixed in photosynthesis for cellular respiration to support biosynthesis and cellular maintenance- varies according to allocation to photosynthetic versus non-photosynthetic tissues

Net primary production (NPP) is the energy fixed by plants available for growth, defense, and reproduction, and the energy available for consumption by herbivores and eventually detritivores (organisms that consume dead organic matter):  NPP = GPP – plant respiration
NPP is the ultimate source of energy for all organisms in an ecosystem, variation in NPP is an indication of ecosystem health—changes in primary productivity can be symptomatic of stress, and NPP is associated with the global carbon cycle.

How NPP is allocated to different plant tissues varies with the environment: Where soil resources (water, nutrients) are in short supply, plants allocate more to roots over stems and leaves.  Where aboveground competition for light occurs, plants allocate growth to stems and leaves.

NPP changes in ecosystems as they develop following a disturbance (i.e. during succession).  Change in NPP is associated with the type of vegetation, influence of age on photosynthesis rates, LAI, and balance of photosynthetic vs. non-photosynthetic tissues   Peak NPP is usually reached in intermediate stages of succession.

Measurement of NPP
Most estimates of terrestrial NPP are based on aboveground estimates including: biomass harvests, forest tree algorithms (correlations between tree size and growth), chamber gas flux measurements (incorporates belowground respiration)- Net ecosystem CO2 exchange or NEE
Measuring belowground NPP is more difficult. Roots turn over more quickly than shoots; that is, more roots are “born” and die during the growing season.  Roots may exude a significant amount of carbon into the soil, or transfer carbon to mycorrhizal or bacterial symbionts.
Estimate of belowground NPP include: root cores- standing crop, sequential harvests, root ingrowth cores,  root windows-minirhizotrons

Larger spatial scales of NPP measurement:
Eddy covariance modeling- Intensive measurements of CO2 and microclimate in and through plant canopies; measures net exchange of CO2 with the atmosphere (NEE), including heterotrophic respiration
Remote sensing- reflectance of specific solar wavelengths from aircraft or  satellite

Aquatic ecosystems
Phytoplankton do most of the  photosynthesis in aquatic & marine habitats. Phytoplankton turn over much more rapidly than terrestrial plants, so biomass at any given time is low compared with NPP; harvest techniques are not used.  In situ  method: photosynthesis and respiration are measured in water samples collected and incubated at the site with light (for photosynthesis) and without light (for respiration).  The difference in the rates is equal to NPP.  Remote sensing is used at large spatial scales.

Environmental controls on NPP
Both physical and biotic factors control spatial and temporal variation in NPP
NPP varies substantially over space and time- there is as much as a 50-fold difference among biomes.
NPP of terrestrial ecosystems is correlated with climate (temperature and precipitation) on a global scale.
Correlation between climatic factors suggests some potential causal links:
1) Photosynthetic rates are related to water availability- stomatal effect
2) Temperature is influencing photosynthesis rates through its effect on enzyme activity
3) Climate influences LAI and photosynthetic biomass among biomes
Other factors may also be influencing the correlations: climate influences nutrient availability
plant species may exert some influence on the observed variation
Experimental determination of the causal factors involved approached using manipulations of potential factors controlling the amount of NPP (i.e. limiting factors)
Such experiments have demonstrated some general trends:
Many terrestrial ecosystems exhibit nutrient limitation of NPP
Most temperate and polar ecosystems are limited by N availablity, some by P
In deserts water is usually most limiting, but N may also be important
Lowland tropical forests tend to be limited by P and sometimes calcium and potassium, while montane tropical forests are more limited by N