Carbon Sequestration as a Co-benefit of Riparian Restoration

Summer 2014 Ecesis, Volume 24, Issue 2

This gigantic cottonwood, whose diameter exceeds 196 cm, represents about 27.5 tons of carbon dioxide removed from the atmosphere, and a value of more than $300 in carbon credits.

This gigantic cottonwood, whose diameter exceeds 196 cm, represents about 27.5 tons of carbon dioxide removed from the atmosphere, and a value of more than $300 in carbon credits.

When riparian restoration along the middle reach of the Sacramento River began more than 25 years ago, the primary intent of the project was to provide habitat for threatened and endangered species, such as the least Bell’s vireo and the Valley elderberry longhorn beetle. Because the restoration consisted of planting native vegetation, and did not attempt to restore altered ecosystem processes like the flood regime, it made sense for the focus to be squarely on biodiversity goals.

However, the thousands of acres of restored forests have nonetheless provided multiple benefits. For instance, much of the restored area has been opened to public hunting. Another unintended but not unexpected benefit is carbon sequestration — the storage of atmospheric carbon dioxide in plants and soils. 

Recently, the state of California has adopted mechanisms for paying forest landowners for the sequestration they capture by reforesting or by changing forest management and harvesting. These tradeable carbon credits are part of California’s cap-and-trade system for reducing greenhouse gas emissions. The price of a ton of carbon dioxide equivalent, or approximately the amount of carbon locked up in a good-sized, 20-year-old Fremont cottonwood (Populus fremontii), is currently $11.43.

With colleagues Erik Nelson and Cedric Puleston, I have been seeking to understand if the existence of the carbon credits will help encourage riparian restoration, either by helping public agencies recoup costs for grant-funded projects, or by inducing private landowners to take marginal floodplain acreage out of cultivation and put it into habitat. We have analyzed the carbon credits earnable over 20 years in the Sacramento River restoration by examining a “chronosequence” of restoration sites. By chronosequence, we mean that we are looking at sites of different ages that are otherwise very similar, for example, in soil type, vegetation community, climate, etc. We assume that taking measurements in sites that differ only in age is equivalent to taking measurements in the same site over a long period. 

We used the state-approved forest protocol to measure tree biomass and soil organic matter and to count carbon credits in sites ranging from 3 to 21 years old. We then compared this income to the costs associated with planting the forests and registering the project to earn credits.

Our first finding was disappointing — the credits earned were calculated at ZERO! Of course the trees stored carbon… but the protocol imposes deductions from the carbon credits based on the state’s risk that the credits might be wrongly estimated, or lost in the future. For instance, about 4% of the credits must be sacrificed to insure the state against the loss of the forests to wildfire.

In the case of our analysis, the credits were lost because of a “confidence deduction,” which reduces earnable credits if the estimate of carbon storage has too much uncertainty associated with it. In the early years of reforestation, when trees are still relatively small and the variability within plots is high, it can be difficult to avoid the confidence deduction. This should be factored in to any calculations about when the up-front costs of restoration could be offset by income from carbon credits.

To get a more useful answer than “zero,” we re-analyzed the data under two additional scenarios, based on different ideas about how much sampling would be necessary to reduce the uncertainty. We found that net income after 20 years of growth ranged from $3,573 to $7,271 per acre. Sampling costs associated with the two scenarios ranged from $4,971 to $5,610 per acre. In terms of the proportion of planting/verification costs that could be paid by carbon credits, it ranged from a low of 72% of costs to a high of 128% of costs. 

Probably any public agency or nonprofit would be happy with either of those figures. However, we determined that carbon credits are unlikely to provide much of an inducement for agricultural landowners to switch from growing crops to growing habitat. The principal uses of the Sacramento River floodplain currently are almonds, walnuts, prunes, and pasture. Almonds are California’s most lucrative crop, and can bring in up to $8,900 in net income per acre annually. So, carbon credits don’t compete well with crop income in this system. Some of our future work will explore what would need to change — about the policy, or about agricultural viability — for this calculus to be any different. — by Virginia Matzek, Department of Environmental Studies and Sciences, Santa Clara University