Carbon Cycling

What is forest carbon cycling?

The continuous movement and exchange of carbon between forest ecosystems and the atmosphere.

Why is forest carbon cycling important?

Carbon is critical for biological activity, energy transfer, and the structure of organisms.

What does forest carbon cycling represent?

Represents a dynamic element, moving between different pools over time scales of week to decades.

Three important components of forest carbon cycling?

1. Carbon pool (sinks)

2. Carbon flux

3. Carbon sequestration

What is a carbon pool (sink)?

A reservoir having the capacity to release or store carbon (e.g. soils).

What is carbon flux?

The transfer of carbon from one carbon pool to another.

What is carbon sequestration?

The removal and storage of atmospheric carbon in carbon pools (such as oceans, forests, or soils). The rate at which carbon is being sequestered (like how much tree growth is happening in a year.

What is the largest terrestrial carbon sink?

Forests represent the largest terrestrial carbon sink.

• Offset 10-20% of US fossil fuel emissions per year.

Are young or old forests better for the carbon cycle?

It depends:

• Young forests are growing more quickly, and therefore sequester carbon at a rapid rate. Better for carbon sequestration.

• Old forests do not grow as quickly, but the trees themselves are larger and thus store more carbon. Better for carbon storage.

Why do tree species have “maximum heights” even under ideal growing conditions?

Hydraulic Limitation Hypothesis: With increasing tree height, it becomes progressively more difficult to transport water up the trunk.

How do trees transport water up the stem?

Capillary action (adhesive forces between water and the tubes within the tree) outweighs gravity to allow it to travel to the stomata for transpiration. Water molecules also have H-bonding to one another, forming a sort of chain through adhesion. Furthermore, as water evaporates from the stoma via transpiration, negative pressure is created that tugs the water molecules below the evaporating molecules, pulling more water into the stoma. Root pressure also plays a role. Difference in pressure in the air vs difference in the pressure of its tissue is used to pull water up the stem.

What are some ways that management effects carbon cycling?

• Harvesting removes carbon stored in living biomass.

• Increased soil temperatures leads to higher decomposition rates (greater respiration = release of CO2 to the atmosphere due to more light speeding up respiration rates and providing more energy).

• Pool sizes increase with forest/ecosystem age.

• Larger trees, accumulation of dead material, soil organic matter.

How did studies show how carbon storage differs between managed and unmanaged forests?

Tree carbon storage is clearly higher in unmanaged forests compared to managed forests.

How does management effect sequestration?

Forest management often involves thinning to maximize the growth of particular trees for profit. This means that sequestration rates are increased, even if you are losing carbon storage from thinning.

What is the most carbon dense forest type in the world? Why does this seem strange?

Eucalyptus regnans in Tasmania are the most carbon dense forests, despite this species not being the largest in the world. 

How are Eucalyptus regnan forests the most carbon dense in the world if they are not the largest?

The amount of aboveground carbon stored in any forest is a function of the total aboveground biomass (the mass of trees in a given area). 

• Function of tree size and wood density (often expressed as specific gravity- density of wood normalized to density of water (g/cm3)

  • Eucalyptus regnans specific gravity = .49

  • Coast redwood specific gravity = .36

How has the specific gravity of trees targeted by the timber industry changed throughout the last couple hundred years?

Less dense tree species (spruce, fit) were prioritized in the past because we used to transport trees by floating them down rivers. A lower specific gravity meant that it would easily float. We now prioritize denser woods with higher specific gravities, especially now that we can transport wood via trucking.

REDD Program

UN program pays landowners to reduce deforestation and degradation.

Carbon offset programs

Pay landowners for carbon sequestered in a project. 

• Carbon offsets need to be verifiable and permanent, and this creates a great need for forest measurements.

What are carbon offsets?

Tradable certificates or financial units given to individuals, companies, or governments to compensate for their own greenhouse emissions by funding external climate-mitigation projects.

What needs to be performed for carbon offset programs to be possible?

Forest carbon monitoring: We must be able to measure forest carbon to know how much these people are owed.

Measuring Forest Carbon

• Basic accounting of size and stocks (pools) and fluxes between pools.

• “Additionality”

What is additionality?

Carbon storage that is above and beyond what would have happened in a “business as usual” scenario.

What is typically measured to quantify forest carbon?

Allometric equations are built in different forests and environments by going out, cutting down trees, and weighing them to determine biomass. 

• This creates a basic relationship to scale tree size to biomass. 

As of 2019, what % carbon still needed to be reduced to fall below the 1.5C target?

9%.

What is the % budget imbalance of sinks vs. sources of carbon?

4%

What is exacerbating climate change?

Forest degradation and deforestation.

• Increased prevalence of wildfires is contributing to carbon emissions.

How is the forest carbon sink changing?

The forest carbon sink is declining. This is due to deforestation, but beyond that, even uncut forests are capable of storing less and less carbon.

• Some of the places with the greatest amount of forests are now acting as carbon sources rather than carbon sinks.

Nature-Based Solutions: Carbon Dioxide Removal (CDR)

1. Remove CO2 from the atmosphere.

2. Store it somewhere else for millenia.

What are the major CDR types.

1. Land-use or ocean management to protect or enhance sinks. Afforestation is the simplest and cheapest option.

2. Biomass for energy use and carbon capture and storage (BECCS).

3. Enhanced natural weathering (distributing silicate minerals).

4. Industrial CO2 removal - primary issues are chemical wastes.

Carbon Removal Technologies

• Reducing Emissions from Deforestation and Degradation (REDD)

• Deforestation and forest degradation contribute ~1Pg C/yr to the atmosphere, one-tenth of fossil fuel emissions (van der Werf et al. 2009 Nature Geosciences).

• Co-benefits: In addition to carbon outrage, benefits include biodiscovery, water recycling, and, likely additional biophysical cooling.

What is the currency of carbon markets?

Most basic singular unit is 1 ton of CO2 eq

What is the social cost of carbon?

The social cost of carbon is the cost of the damages created by one extra ton of CO2 emissions.

• In the US Obama administration the price was $43, for Biden $51, for Trump $3-5.

• NASEM data suggests $181

• Putting a price on carbon drives economic incentives for clean energy, and forces producers and consumers to internalize the costs. Can also be used to return collected fees to people and thus drive pollution down.

Mandatory Compliance Markets

Governed by national, regional, or provincial law and compel emission sources to meet GHG emission reduction targets. Because compliance program offset credits are generated and traded for regulatory compliance, they typically act like other commodity pricing.

Voluntary Carbon Markets

Enables carbon emitters to offset their unavoidable emissions by acquiring carbon credits generated by initiatives aimed at removing or decreasing GHG emissions from the environment. Companies can engage in the voluntary carbon market on their own or as a part of an industry wide-program.

What is a potential drawback of companies using their PR budgets to buy from voluntary Carbon Markets?

Buying cheap carbon offsets can be done mainly for PR, but this has caused backlash toward companies (Microsoft) when flaws of offsets were investigated.

What is Cap and Trade?

A system by which company emissions are capped, but they can offset some emissions through the purchase of credits and trading with companies that had left over allotments.

• Some systems gradually reduce the caps/allotments over time to spur reduced emissions.

• First cap and trade program from the successful US Clear Air Act of 1990 (smokestack emissions).

• The EU program for Carbon started in 2005, California in 2013.

What are the cheapest nature-based solutions?

Natural forest management and reforestation.

What are the two major categories of nature-based climate solutions?

1. Increase carbon sinks.

2. Protect Carbon reserves.

What falls under “Increase carbon sinks?”

• Reforestation

• Afforestation

• Restoration

• Natural regrowth of secondary forests

What falls under “Protect carbon reserves?”

• Reduce deforestation and degradation

• Avoid land use change to non-forested areas

• Mature and old-growth forests are a priority.