• Dr. Lee Anne Willson

Nature-based climate solutions

Nature-based solutions

How much of what we need to do for the Ames CAP could be achieved by “nature-based solutions”? This includes planting more trees, creating more prairie areas, using more deep-rooted plants in landscaping, capturing water where it falls, and encouraging and using biochar to enrich the soil. I’m not including changes in agricultural practices in this list, because I’m focusing on what we can do in the city of Ames.


The Ames Sycamore

Suppose we set a goal of 1 tree planted for every resident of Ames over the course of 40 years? I use 40 years because the estimated sequestration of carbon by a tree is 1 ton over 40 years. That is also a reasonable average lifetime for a tree in Ames: Natural tree lifetimes can be centuries long, urban trees have relatively short lifetimes of a few decades, and a suburban tree probably falls between those extremes. Using 40 makes the arithmetic easy and illustrates nicely what is involved.

There are about 60,000 residents in Ames, so that would be 1500 trees per year. Each tree removes 1/40 ton/year, so we would gradually build up to 1500 tons/year after 40 years, and then this number would stay at 1500 tons/year if we kept replacing trees, or diminish gradually if we did not. Is this an unreasonable number? Ann Arbor, with 83% larger population, has set a goal of 10,000 trees, and achieved 3000 new trees in one year, so 1500/year is not an unreasonable goal for Ames. While this includes trees planted by the city, most are planted by residents in their yards.

Or, if we chose to count all the tons associated with one tree at the time we plant the tree, that would be 1500 tons/year as long as we keep planting trees, ending when we stop planting them. The same total sequestration results in either case; the difference is when we give ourselves the full credit for planting the tree.

For comparison: Ames currently emits about 1200 ktCO2e per year, so 1500 tons/year = 1.5 ktCO2e/year is 0.1% of the total. I like to say we need 100 1% solutions, but even on this scale the contribution to be made from local tree-planting is modest at best.

What is the cost of planting one tree? That varies quite a bit. Some organizations that use local labor to plant trees in developing countries charge just a few dollars per tree – I’ve been rewarding myself for exercise with an app that plants 5 trees for every mission accomplished. Here, the Ames Foundation suggests $250 as a contribution for one tree; this is a typical cost for a 5-8 foot tree ready to plant. When I look at this as an option for our household, I compare it to my yardstick value, $50/ton (see the Cost of Carbon blog), and it is clearly more expensive than some other options.

On the other hand, if I decide to plant some trees in our yard, I can cut the initial cost substantially by planting a smaller tree – sometimes these are available at very low cost, or even free from organizations around Ames. I then have the labor of protecting and nurturing it until it is big enough to need less attention. Outside of cost, most of my motivation is probably to get shade, wind-protection, and improved visual appeal. If I plant deciduous trees on the south side of the house (but not shading solar panels), they will lower my A/C use in the summer while still allowing some direct passive solar heating in the winter.

Regardless, I still get carbon sequestration benefits. The total carbon sequestered from the time I plant the tree until 40 or so years in the future is, surprisingly, about the same whether I plant a tiny tree or a 5’ tree. This happens because the mass of the wood in the mature tree is much more than the mass in even a 5-10’ young one.

After the August Derecho people have been concerned about the possibility of damage from falling trees, and some have been cutting down healthy, mature trees out of that concern. One thing that will be needed for a tree-planting initiative is some assistance for homeowners in planning the type of tree and the location of the tree so as to minimize that risk. We also have a lot of above-ground electrical and phone wiring, and part of climate resilience planning may include burying more of these lines underground.

If we can achieve at least 0.1% of the goal with trees, but gain the other benefits of a good tree canopy, that is something to include in our planning.

Prairie preserves

Neal Smith Prairie

Prairie Preserves

According to https://tallgrassontario.org/wp-site/carbon-sequestration/, "Soils under long-established prairie grasslands can contain more than 10 tons of roots per acre with most of this bulk in the top 24 inches. The roots of some prairie plants can extend to a depth of 10 feet or more. Various studies of the potential for tallgrass prairie carbon storage have shown that the storage rates vary between .30 and 1.7 metric tons per acre per year.” The carbon is stored underground, where it does not go up in smoke of there is a wildfire, in contrast with a forest of trees.

From this, a rough and easy number is 1 ton/acre/year for carbon sequestration by prairie land. At present levels of emission from Ames, if this is all we did, we’d need 18 acres per household in Ames to offset all our emissions, or a total of almost 1700 square miles! Story County covers just 574 mi² and Ames = 27.92 mi², so obviously we can’t achieve our goal just with prairie plantings. However, as a 1% solution, this would be 17 square miles, probably too much for Ames but just 2.9% of Story County.

What would this cost? Consider two examples. First, assume that we are able to convert grass areas inside the Ames city limits, using some combination of city-owned parks and individually owned lawns. If we could get 100 acres, roughly 1% of the area of Ames, converted to prairie, we would sequester roughly 100 tons/year. The seeds to get this started would run about $250/acre (based on using a CP42 prairie seed mix and the price Story County Conservation charges for planting a prairie area). For larger areas, there are ongoing costs, including a regular prescribed burn. An average cost for a prescribed burn is about $50 per acre. Over 40 years that would give a total cost around $750-$800, or about $20/year. For yard-sized areas, maintenance costs would be minimal. If a controlled burn is not an option, small prairies can be maintained by mimicking a controlled burn with occasional mowing or weeding.

A second example is that a local philanthropist decides to buy a 100-acre farm and convert it to prairie. Farmland is running nearly $10,000/acre so the upfront investment is $1 million. Then there are all the same costs as for the previous example. If we assume that the prairie persists for 100 years, then the initial investment is $100/acre/year, and the total per-year for the century is close to $120/acre/year or $120/ton of carbon sequestered. However, there is another factor here: Land used to grow corn ends up emitting between 1 and 2 tons/acre of greenhouse gases. Thus the philanthropist might be spending $120/acre/year, but ends up with a net reduction in greenhouse gases of 2 to 3 tons/acre, and the cost is now between $40 and $50 per ton. For this example, there are also a variety of grants and incentives available to reduce the actual cost to our hypothetical philanthropist.

Converting a suburban lawn to prairie does have the drawback that some of the other functions of a lawn – as a place to play, for example – don’t work with a prairie planting. However, there are native grasses with deep roots that create a fine lawn – indeed, a lawn that requires less care than the varieties most often used. The city of Ames has a brochure on this subject: NativeTurf_Brochure.pdf


Biochar and other soil enrichment

When plant residue decomposes, it releases greenhouse gases, and in particular, it releases methane. However, if we convert the plant residue to biochar, it can be used to enrich the soil, and it is thought to be a very stable way to keep carbon out of the air. Biochar added by native Americans more than 1000 years ago can still be found in the soil.

You can buy biochar for your garden, in small quantities, for a modest sum. However, that amounts to about $1000/ton of sequestered carbon. A program at Iowa State University that recently received a $1M X-prize grant to develop large-scale biochar production (https://www.news.iastate.edu/news/2022/04/22/xprizemilestone) hopes to get the price down to between $40 and $150 per ton. They estimate that farmers will be interested in biochar at those prices, so there is the potential for sequestering a lot of carbon that way.

How much carbon can be sequestered via biochar? The amounts per acre that have been tested range from ¼ to 8 tons/acre. Iowa has 26,256,347 acres under cultivation, and if we assume just 1 ton sequestered/acre, that is about 26 million tons of carbon that could be sequestered in this form. Story county has 306,000 acres under cultivation. Assuming 40 years, a one-time application (not an annual application) and that every farm in Story County would participate, we are looking at sequestering up to 7500 tons/year.

Since 7500 tons/year = 7.5 ktCO2e/year is not a large part of the 1200 ktCO2e/year currently emitted in Ames, I conclude that biochar is another piece of the puzzle. It will probably not provide more than 1% of the solution any time soon, but it carries significant benefits in improved soil quality.

Compare: Trees vs. prairie?

One acre of prairie or 40 trees will sequester 1 ton/year. If you have a yard area that is 30x36 feet, that is 1/40 of an acre and sequesters the same carbon per year as 1 tree. You can choose to plant it with prairie plants, or you can plant a tree, and you will get about the same benefit in sequestered carbon. In one case, the carbon ends up in the tree, and may be released when the tree eventually dies or is cut down; in the other, it ends up in the soil. Either choice is equally good in the 40 year ‘short run’. You can also combine a deep-rooted grass lawn with a tree and get twice as much carbon sequestration!

Suppose we compare 1 acre of trees with one acre of prairie. Which sequesters more? A cultivated forest may have 300 to 800 trees per acre. At 400 trees, that would be 10 tons/acre of carbon sequestered, 10 times the amount from the prairie planting. However, the trees only store the carbon until they are cut down or there is a fire, while the carbon sequestered by the prairie stays safely underground. On the other hand, the cost of planting those 400 trees is a lot higher than the cost of converting the land to prairie. I conclude it would be wise to invest in both forms of sequestration.

Nature-based offsets

Currently, the Ames CAP is not expected to include offsets among the strategies to be pursued. However, offsets do provide an opportunity to invest in nature-based solutions where they can do the most good, whether those are the restoration of peat bogs in Scotland, of rainforests in South America, or of grasslands and forests outside of Story County. The Ann Arbor, Michigan climate action plan ( https://www.a2gov.org/departments/sustainability/Carbon-Neutrality/Pages/default.aspx) contains an interesting discussion of how they are choosing offsets to assist them in reaching their goal of net zero by 2030.


Brookside Park

It is true that the nature-based solutions discussed – planting trees, increasing prairie lands, using deep-rooted plants in landscaping, and converting woody waste to biochar – seem unlikely to provide a big piece of the emission-reduction process for Ames. These might add up to a percent or so, at best. However, of equal value are their substantial co-benefits, including reduced air pollution, reduced heating and cooling demands, increased property values, improved physical and mental health, and reduced storm water runoff. So the reduction of atmospheric carbon by these approaches is a bonus on top of other clear benefits. For citizens, nature-based solutions can be an attractive way to help us achieve our goals, because they bring improved quality of life in Ames. For the city government, it is reasonable to have an initial emphasis on mitigation strategies that can have a big emissions reduction impact (transportation and buildings, for example), but including nature-based solutions in the planning, particularly when it comes to land use planning, can have a bigger effect than just the tons of carbon being sequestered.

Acknowledgments: While this blog is the responsibility of one person, who assumes responsibility for its contents, I usually run my drafts past several people for comments. For this one, I would like to thank Brendan Clough of Story County Conservation for some information and corrections to an early draft, Nancy Franz for putting me in touch with Brandon Clough, and members of Ames Climate Action Team for comments on contents and style.

Photo credits: Ames pictures courtesy of Stephen Willson. Biochar image adapted from https://commons.wikimedia.org/w/index.php?curid=87685933. Prariegrass roots image from https://tallgrassprairiecenter.org/prairie-roots-project.