Lecture 5: Terrestrial production: Lecture Notes

Lecture 5: Terrestrial production, Decomposition, NPP, NEP, and Carbon Storage

NPP: Net Primary Production

NPP = GPP (photosynthesis) – Ra (autotrophic respiration)
One way to think of NPP is as growth expressed as an annual average of above ground production.

NPP is considered a flux: Mass per unit area per unit time. (Units usually given as some multiple of Tons of Carbon per Hectare per Year or GtC/Ha * Y or GtC * Ha-1 * yr-1)

Overview Picture of today's lecture:
Global Terrestrial Carbon Uptake and Release
This image describes total flow of Carbon through the biome.

Heterotrophic decomposition:

All of the biomass that falls in forests is potentially decomposable. Because of the cold weather in winters of northern climates decomposition is sometimes slowed, leaving organic matter stored in humus (dark soil in soil pit photo). Over time there is a huge storage of carbon in mineral soil available for plant use.

Decomposition:

Fungi and bacteria are primarily responsible for decomposition.

The decomposing carbon is consumed to produce energy and carbon dioxide is released to the atmosphere.
All organic material is decomposable but some material is more labile (easy to decompose), such as glucose and amino acids
Other materials such as lignin are more recalcitrant (difficult to decompose). "Lignin is what makes wood woody."

Decomposition in models:

The CENTURY model presents carbon in three pools:
Metabolic, active and passive depending on the availability of that carbon for metabolic use.
Metabolic - Decays in approx. 6 months
Active - Decays in approx. 2-3 years
Passive - Rate of decay has a half-life of approx. 100 years (thus CENTURY model)
How material passes between these pools is an extremely important, unanswered question in modern ecological research.

Impediments to Decomposition

Standing water or lack of oxygen can impede or even halt decomposition.
Aerobic decomposition is a much faster, and more efficient than anaerobic decomposition.
Foresters used to allow logs to sit at the bottom of lakes to preserve them.

Primary controls on rates of Decomposition

1) Climate
Temperature and moisture
Combined temperature-moisture terms like AET (actual evapotransporation)

2) Litter quality
Amount of lignin
Ratio of lignin: N (because N is sometimes an indication of how labile the litter is)

Topography:

Topography and soil drainage can significantly influence carbon storage.
Bottomlands and marshes are excellent carbon sinks because of the slow decomposition rates (due to the anaerobic conditions).
Recently these wetlands have been drained for agriculture, freeing huge amounts of carbon for decomposition.

Excellent example are peat bogs which are rumored to be over 100 m deep with stored organic matter.

Flux Towers and their importance to carbon exchange research

Michigan has a tower at UMBS called the PROPHET tower.
These towers typically use an eddy-flux technique to measure C exchange, NEP or NEE.
In essence, as an air eddy passes through carbon dioxide is moved up or down. Every 15 seconds an instrument measures air movement and then reconstructs, based upon CO2 levels, water vapor, and oxygen levels, the total carbon exchange.

In the context of what we've been talking about, flux towers measure:

GPP = Gross Primary Production (or Photosynthesis)
NPP = Net Primary Production which equals GPP - Ra (autotrophic respiration).
NEP = Net Ecosystem Production = NPP - Rh (heterotrophic respiration)
NEE = Net Ecosystem Exchange

NEE is an "instantaneous" measurement (discrete moment in time), while NEP is the accumulation of NEE over a period of time.
In other words, you integrate NEE over time to calculate NEP for an ecosystem.

The number of flux towers is growing worldwide. Most of the towers are in North America and Europe, Africa and Australia are particularly under-represented.

Most measurements take place in "wildland" ecosystems far away from human land use, which means that scientists do not have good data for the areas disturbed by humans, making it difficult to model biome-level NEP using measurements from these towers.

A negative NEP frequently means a movement of carbon into an ecosystem, but researchers are not always consistent - be sure to check which convention is being used when reading data.

Inventory methods of calculating net ecosystem exchange (NEE)

Place tags on trees, measure the diameter and then return after several years to calculate growth.
Measure litter depths along transects.

Theoretically, changes in a terrestrial ecosystem carbon stock could be measured by repeated inventories; BUT there's a lot of noise in the data that hampers some ecology and biogeochemistry research. These inventories are useful when calculating a country's NEP, which can be used for carbon tax and credit systems. Much of the existing literature and research uses NEP.

NECB is a newer, more precise alternative to NEP.

NEP derives from ecologically significant considerations, such as photosynthesis and respiration, but can neglect some fluxes of carbon in an ecosystem.
NECB stands for the Net Ecosystem Carbon Balance and is considered more precise than NEP (precise measurements required for Kyoto reporting).

NECB includes additional carbon fluxes, both organic and inorganic, that are sometimes but not always negligible.
It also includes sideways movements of C in and out of an ecosystem, through both terrestrial and aquatic ecosystems.

Disturbance and NBP

Death or disturbance of individual trees is a common occurrence within a forest.
Even deaths of large stands of trees due to disease is a relatively common and natural event.
Fires, flooding, insects, windthrow, and ice storms are examples of large infrequent disturbances (LIDs) which significantly alter an ecosystem. For examples, 6 major hurricanes have effected the forests of New England since the 1600s. These LIDs, while considered infrequent on human timescale, are part of the regular "disturbance regime" of the forest.

More and more some of these LIDs are influenced by human presence: Invasive insect irruptions and forest fires.
Case in point: the gypsy moth, introduced in Boston and spread very quickly. They're capable of defoliating huge areas of land.

NBP
Integrates NECB over time
Includes disturbance over time
Includes scaling up across all ecosystems in the landscape

Succession

Succession is a broad principle in ecology.
"The tendency for one species or community to be replaced by another over time."
Frequently succession is predictable but not always.
Primary succession occurs on newly exposed land - lava flows or areas where glaciers have retreated.
Secondary succession occurs after disturbance of an already established ecosystem.

Early and late successional species have very characteristic life history strategies.
Early plants produce many small seeds that are easily dispersed (think wind) and can lie dormant for many years before germinating, whereas (later-successional species have larger, heavier seeds that are typically spread by animals.

Mechanisms of succession.

Often one stage of succession yields the conditions for the next stage.
For example:
Early-successional trees cannot regenerate well under shady conditions but late-successional trees regenerate well under shady conditions and eventually eclipse the early colonists.

Theoretically succession leads to a "climax state" but these climaxes are usually dynamic, cycling between several different ecosystem states. The theory of a "climax state" was an important theory in the evolution of ecology, but has been replaced in recent years with theories of "early" and "late" succession.

The "end point" of succession frequently varies substantially with environmental conditions. This has somewhat diminished the idea of a single climax landscape state. Disturbance is a constant factor, constantly changing the characteristics of an ecosystem, preventing one single climax state and resulting in dynamic system.

Carbon storage and cycling in terrestrial ecosystems

Ecosystems or landscapes can be considered a source or sink of carbon.

Moment to moment, this can be considered Net Ecosystem Exchange (NEE).

Alternately, carbon in a landscape can be thought of as linkages of pools of carbon. Questions such as "how does carbon accumulate in these pools" or "how long does carbon stay in this pool" and "how can it move between pools" can inform discussions of carbon flow through many ecosystems or across huge landscapes.

In accounting for carbon storage, where do we include the carbon stored in woody debris?
Woody debris isn't usually considered part of vegetation or soil. Usually it will be accounted as a separate carbon pool.

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