Forest Ecology Notes

Carbon and Productivity

• Carbon is equivalent to energy.
• Productivity is the rate of carbon fixed, typically expressed in grams of carbon fixed per square meter per year ($g C fixed/m^2/yr$).
• Atmospheric $CO_2$ is the source of all carbon in organisms.
  • It is fixed via photosynthesis.
  • Carbon flows through the food chain.
  • It is released back into the atmosphere via respiration, decomposition, and combustion.

Carbon Cycling Inputs and Outputs

• Inputs:
  • Photosynthesis
  • Diffusion/transport (aquatic)
• Outputs:
  • Respiration
  • Decomposition
  • Combustion (terrestrial)
  • Diffusion (aquatic)

Energy Capture

• The amount of energy captured by green plants through photosynthesis and fixed into carbon, measured at the ecosystem scale.

Daily Variations in Carbon Cycling

• Carbon dioxide concentration fluctuates throughout the day.
  • From Dawn to Dusk: XXX
  • From Dusk to Dawn: XXX

Seasonal Variations in Carbon Cycling

• The production and use of carbon dioxide fluctuate with the seasons.
• The fluctuations are greater in the Northern Hemisphere.

Gross Primary Productivity (GPP)

• $GPP$ – plant respiration ($Respplant$).
  • $GPP$ is the total energy fixed and does not account for respiration.
• Estimated as the rate of biomass accumulation over time.
• Highest where conditions are optimal for plant growth.
• Foundation for all processes in an ecosystem.
  • The physiological mechanism that produces merchantable yield.
  • Base of the food web.
• A key part in determining ecosystem structure and function.

Components of Net Primary Productivity (NPP)

• New plant biomass: 40-70% of $NPP$
• Leaves and reproductive parts (fine litterfall): 10-30% of $NPP$
• Apical stem growth: 0-10% of $NPP$
• Secondary stem growth: 0-30% of $NPP$
• New roots: 30-40% of $NPP$
• Root secretions: 20-40% of $NPP$
• Root exudates: 10-30% of $NPP$
• Root transfers to mycorrhizae: 10-30% of $NPP$
• Losses to herbivores and mortality: 1-40% of $NPP$
• Volatile emissions: 0-5% of $NPP$
• It is rare for all components to have been measured in a single study.

Global Patterns of Net Primary Productivity

• Data taken from: IBIS Simulation (Kucharik, et al. 2000) (Foley, et al. 1996)
• Net Primary Productivity is measured in $kg-C/m^2/year$
• Data from Atlas of the Biosphere, Center for Sustainability and the Global Environment, University of Wisconsin-Madison

NPP and Environmental Factors

• $NPP$ is shown in $g C m^{-2} yr^{-1}$.
• Radiation-limited conditions.
• Water-limited conditions.
  • Gridded data from Global Primary Productivity Data Initiative (GPPDI).

Correlation of NPP with Temperature and Precipitation

• $NPP$ (in units of biomass) is greatest in warm, moist environments such as tropical forests, and lowest in cold or dry ecosystems such as tundra and deserts.
• In tropical forests, $NPP$ declines at extremely high precipitation (> 3m yr^{-1}), due to indirect effects of excess moisture, such as low soil oxygen and loss of nutrients through leaching.

Global Patterns of NPP - Ecosystem Specific

• Open ocean:
  • Surface area: 65.0%
  • Average net primary production: 125 $g/m^2/yr$
  • Percentage of Earth's net primary production: 24.4%
• Continental shelf:
  • Surface area: 5.2%
  • Average net primary production: 360 $g/m^2/yr$
  • Percentage of Earth's net primary production: 5.6%
• Estuary:
  • Surface area: 0.3%
  • Average net primary production: 1,500 $g/m^2/yr$
  • Percentage of Earth's net primary production: 1.2%
• Algal beds and reefs:
  • Surface area: 0.1%
  • Average net primary production: 2,500 $g/m^2/yr$
  • Percentage of Earth's net primary production: 0.9%
• Upwelling zones:
  • Surface area: 0.1%
  • Average net primary production: 500 $g/m^2/yr$
  • Percentage of Earth's net primary production: 0.1%
• Extreme desert, rock, sand, ice:
  • Surface area: 4.7%
  • Average net primary production: 90 $g/m^2/yr$
  • Percentage of Earth's net primary production: 0.9%
• Desert and semidesert scrub:
  • Surface area: 3.0%
  • Average net primary production: 0.04 $g/m^2/yr$
  • Percentage of Earth's net primary production: 3.5%
• Tropical rain forest:
  • Surface area: 3.3%
  • Average net primary production: 2,200 $g/m^2/yr$
  • Percentage of Earth's net primary production: 22%
• Savanna:
  • Surface area: 2.9%
  • Average net primary production: 900 $g/m^2/yr$
  • Percentage of Earth's net primary production: 7.9%
• Cultivated land:
  • Surface area: 2.7%
  • Average net primary production: 600 $g/m^2/yr$
  • Percentage of Earth's net primary production: 9.1%
• Boreal forest (taiga):
  • Surface area: 2.4%
  • Average net primary production: 800 $g/m^2/yr$
  • Percentage of Earth's net primary production: 9.6%
• Temperate grassland:
  • Surface area: 1.8%
  • Average net primary production: 600 $g/m^2/yr$
  • Percentage of Earth's net primary production: 5.4%
• Woodland and shrubland:
  • Surface area: 1.7%
  • Average net primary production: 700 $g/m^2/yr$
  • Percentage of Earth's net primary production: 3.5%
• Tundra:
  • Surface area: 1.6%
  • Average net primary production: 140 $g/m^2/yr$
  • Percentage of Earth's net primary production: 0.6%
• Tropical seasonal forest:
  • Surface area: 1.5%
  • Average net primary production: 1,600 $g/m^2/yr$
  • Percentage of Earth's net primary production: 7.1%
• Temperate deciduous forest:
  • Surface area: 1.3%
  • Average net primary production: 1,200 $g/m^2/yr$
  • Percentage of Earth's net primary production: 4.9%
• Temperate evergreen forest:
  • Surface area: 1.0%
  • Average net primary production: 1,300 $g/m^2/yr$
  • Percentage of Earth's net primary production: 3.8%
• Swamp and marsh:
  • Surface area: 0.4%
  • Average net primary production: 2,000 $g/m^2/yr$
  • Percentage of Earth's net primary production: 2.3%
• Lake and stream:
  • Surface area: 0.4%
  • Average net primary production: 250 $g/m^2/yr$
  • Percentage of Earth's net primary production: 0.3%

Net Ecosystem Production (NEP)

• $NEP = GPP – Recosys ± Other transfers$
  • $Recosys$ = Respiration of plants, animals, and soil microbes, plus other C transfers from plants.
  • Other transfers include harvest, disturbance, leaching, and erosion.

Equations for NPP and NEP

• $NPP = GPP - R_{plant}$
• $NEP = GPP - (R<em>{plant} + R</em>{heterotr} + F<em>{disturb} + F</em>{leach})$

Positive, Negative, and Neutral NEP

• Positive $NEP$; GPP > Recosys ± Other transfers
  • Ecosystem is removing C from the atmosphere = C sink.
• Negative $NEP$; GPP < Recosys ± Other transfers
  • Ecosystem is releasing C to the atmosphere = C source.
• Neutral $NEP$; $GPP = Recosys ± Other transfers$
  • Neither removing nor releasing C = C neutral
• Timeframe is IMPORTANT

Ecosystems and NEP

• What types of ecosystems have…
  • Positive $NEP$ (carbon sink)?
  • $NEP = 0$ (carbon neutral)?
  • Negative $NEP$ (carbon source)?