Facts about carbon storage in grasses do not support assumptions of native plant advocates

We have received many comments from native plant advocates regarding carbon storage.  These comments defend projects in the Bay Area to destroy non-native forests and “restore” native plants by claiming that native plants will actually sequester more carbon than the forest that they propose to destroy.  As always, we are grateful for comments that give us the opportunity to research the issues and report what we have learned about this complex and important subject.

Carbon cycling in a terrestrial plant-soil system

The storage of carbon in plants and soil occurs as plants and soil exchange carbon dioxide (CO₂) with the atmosphere as a part of natural processes, as shown in the following diagram (1):

Green Arrow:  CO₂ uptake by plants through photosynthesis

Orange Arrows:  Incorporation of Carbon into biomass and Carbon inputs into soil from death of plant parts

Yellow Arrows:  Carbon returns to the atmosphere through plant respiration and decomposition of litter and soil Carbon.  Carbon in plant tissues ultimately returns to atmosphere during combustion or eventual decomposition.

Rates of carbon uptake and emissions are influenced by many factors, but most factors are related to temperature and precipitation:

• Higher temperatures are associated with faster plant growth, which accelerates photosynthesis and carbon uptake.
• Higher temperatures also accelerate decomposition of plant materials, thereby accelerating the return of stored carbon into the atmosphere.
• The effect of moisture in the soil on decomposition can be graphed as a “hump.”  In extremely dry soils, decomposition is slow because the organisms that decompose vegetation are under desiccation stress.  Conditions for decomposition improve as moisture in the soil increases until the soil is very wet when lack of oxygen in the soil impedes decomposition.

Although temperature and precipitation are important factors in carbon storage, they don’t change appreciably when one type of vegetation is replaced with another.  Therefore, these factors aren’t helpful in addressing the fundamental question we are considering in this post, which is “Does native vegetation store more carbon than the forests that presently occupy the land in question?”

Where is carbon stored?

Much of the carbon stored in the forest is in the soil.  It is therefore important to our analysis to determine if carbon stored in the soil in native vegetation is greater than that stored in non-native forests.  The answer to that question is definitely NO!  The carbon stored in the soil of native vegetation in Oakland, California is a fraction (5.7 kilograms of carbon per square meter of soil) of the carbon stored in residential soil (14.4 kilograms in per square meter of soil). (9)  Residential soil is defined by this study as “residential grass, park use and grass, and clean fill.”  This study (9) reports that the amount of carbon stored in the soil in Oakland is greater after urbanization than prior to urbanization because Oakland’s “wildland cover” is associated with “low SOC [soil organic carbon] densities characteristic of native soils in the region.”

Native plant advocates have also argued that the carbon stored in the soil of perennial native grasslands is greater than non-native trees because their roots are deeper.  In fact, studies consistently inform us that most carbon is found in the top 10 centimeters of soil and almost none is found beyond a meter (100 centimeters) deep. (1, 4) In any case, we do not assume that the roots of perennial grasses are longer than the roots of a large tree.

Another argument that native plant advocates use to support their claim that native perennial grasslands store more carbon in the soil than non-native trees is that native grasses are long-lived and continue to add carbon to the soil throughout their lives.  In fact, carbon stored in the soil reaches a steady state, i.e., it is not capable of storing additional carbon once it has reached its maximum capacity. (1)

It is pointless to theorize about why grassland soils should store more carbon than forest soils.  The fact is they don’t.  In all regions of the United States forest soils store more carbon than either grassland or shrubland soils.  (9, Table 5)

We should also describe Oakland’s native vegetation before moving on:  “Vegetation before urbanization in Oakland was dominated by grass, shrub, and marshlands that occupied approximately 98% of the area.  Trees in riparian woodlands covered approximately 1.1% of Oakland’s preurbanized lands…”  (5)  In other words, native vegetation in Oakland is composed of shrub and grassland.  When non-native forests are destroyed, they will not be replaced by native trees, especially in view of the fact that replanting is not planned for any of the “restoration” projects in the East Bay.

The total amount of carbon stored within the plant or tree is proportional to its biomass, both above ground (trunk, foliage, leaf litter, etc.) and below ground (roots).  Since the grass and shrubs that are native to the Bay Area are a small fraction of the size of any tree, the carbon stored within native plants will not be as great as that stored in the trees that are being destroyed.

Whether we consider the carbon stored in soil or within the plant, the non-native forest contains more carbon than the shrub and grassland that is native to the Bay Area.

Converting forests to grassland

If we were starting with bare ground, it might be relevant to compare carbon sequestration in various types of vegetation, but we’re not.  We’re talking about specific projects which will require the destruction of millions of non-native trees.  Therefore, we must consider the loss of carbon associated with destroying those trees.  It doesn’t matter what is planted after the destruction of those trees, nothing will compensate for that loss because of how the trees will be disposed of.

The fate of the wood in trees that are destroyed determines how much carbon is released into the atmosphere.  For example, if the wood is used to build houses the loss of carbon is less than if the wood is allowed to decompose on the forest floor.  And that is exactly what all the projects we are discussing propose to do:  chip the wood from the trees and distribute it on the forest floor, also known as “mulching.”  As the wood decomposes, the carbon stored in the wood is released into the atmosphere:  “Two common tree disposal/utilization scenarios were modeled:  1) mulching and 2) landfill.  Although no mulch decomposition studies could be found, studies on decomposition of tree roots and twigs reveal that 50% of the carbon is lost within the first 3 years.  The remaining carbon is estimated to be lost within 20 years of mulching.  Belowground biomass was modeled to decompose at the same rate as mulch regardless of how the aboveground biomass was disposed” (8)

Furthermore, the process of removing trees releases stored carbon into the atmosphere, regardless of the fate of the destroyed trees:  “Even in forests harvested for long-term storage wood, more than 50% of the harvested biomass is released to the atmosphere in a short period after harvest.”  (1)

Will thinning trees result in greater carbon storage?

Native plant advocates claim that thinning the non-native forest will result in improved forest health and therefore greater carbon storage.  In fact, the more open canopy of an urban forest with less tree density results in greater growth rates.  (3)  Although more rapid growth is associated with greater rates of carbon sequestration, rates of storage have little effect on the net carbon storage over the life of the tree.  (6)  Net carbon storage over the life of the tree is determined by how long the species lives and how big the tree is at maturity.  These characteristics are inherent in the species of tree and are little influenced by forest management practices such as thinning. (6)

More importantly, even if there were some small increase in carbon storage of individual trees associated with thinning, this increase would be swamped by the fact that over 90% of the urban forest will be destroyed by the proposed projects we are evaluating in the East Bay.  The projects of UC Berkeley and the City of Oakland propose to destroy all non-native trees in the project areas.  The project of the East Bay Regional Park District proposes to destroy all non-native trees in some areas and thin in other areas from 25 to 35 feet between each tree, reducing tree density per acre by at least 90%.  No amount of “forest health” will compensate for the loss of carbon of that magnitude.   

Responding to native plant advocates

• The vegetation that is native to the Bay Area does not store more carbon above or below the ground than the non-native forest.
• Chipping the trees that are destroyed and distributing the chips on the ground will not prevent the release of carbon from the trees that are destroyed.
• Thinning the trees in our public lands will not increase the capacity of the trees that remain to store carbon.

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Bibliography

1.  Anderson, J., et. al., “The Potential for Terrestrial Carbon Sequestration in Minnesota, A Report to the Department of Natural Resources from the Minnesota Terrestrial Carbon Sequestration Initiative, February 2008.
2. Birdsey, Richard, “Carbon storage and accumulation in United States Forest Ecosystems,” USDA Forest Service, General Technical Report WO-59, 1992
3. Environmental Protection Agency, “Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008,” April 15, 2010., EPA 430-R-10-006
4. Fissore, C.,  et.al., “Limited potential for terrestrial carbon sequestration to offset fossil-fuel emissions in the upper Midwestern US,” Frontiers in Ecology and the Environment, 2009, 10.1890/090059
5. Nowak, David, “Historical vegetation change in Oakland and its implication for urban forest management,” Journal of Arboriculture, 19(5): September 1993
6. Nowak, David, “Atmospheric Carbon Reduction by Urban Trees,” Journal of Environmental Management, (1993) 37, 207-217
7. Nowak, David. Crane, Daniel, “Carbon storage and sequestration by urban trees in the U.S.A.,” Environmental Pollution, 116 (2002) 381-389
8. Nowak, David, et.al., “Effects of urban tree management and species selection on atmospheric carbon dioxide,” Journal of Arboriculture 28(3) May 2002
9. Pouyat, R.V. (US Forest Service)., et.al., “Carbon Storage by Urban Soils in the United States,” Journal of Environmental Quality, 35:1566-1575 (2006)