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What We Learned About Tree Planting

There’s not a lot we could write here about the threat of climate change that hasn’t already been said. Higher temperatures, more destructive storms, disruption of food systems — the list goes on. But nonprofits have risen to the challenge, whether by getting individuals to adopt greener habits or advocating for change on the global stage.

The ImpactMatters research team focused first on nonprofits that are battling climate change one tree at a time, including reforestation nonprofits (which target areas that were once forested) and urban forestry nonprofits (which plant trees in cities). The measurable environmental benefit stems from the fact that trees sequester carbon dioxide — a byproduct of burning fossil fuels and a major contributor to the climate crisis.  

We measured impact by looking at the cost to offset the amount of carbon the average American emits in a year — 18.6 metric tons (we refer to this amount as a “person-year” of carbon). Our calculation is a function of a few factors: the cost of the nonprofit’s tree planting program; how many trees it plants; how much carbon each tree sequesters, which can vary by type of tree. [This is a simplification; for a detailed explanation, read our full methodology.]

We then determined cost-effectiveness by comparing the cost to offset a person-year of carbon emissions to the “social cost” of a person-year of carbon emissions, as estimated by the U.S. government. In essence, the social cost of carbon is an estimated monetary value of the costs borne by society due to carbon emission — things like health care costs due to poor air quality, and property damage due to increased flood risk. Programs that offset a person-year of carbon emissions for less than 125 percent of the social cost are considered cost-effective (4 stars), and those that do so for less than 75 percent are highly cost-effective (5 stars). So far, we have been able to find data on 11 nonprofits working across the globe, whose annual efforts lead to a combined 202,631 personal years of carbon sequestered. 

Based on data published by the nonprofits, nonprofits earned the following star ratings:

Rating

Criteria

Number of nonprofits

5 stars

Program sequesters a person-year of carbon for less than 75% of social costs.

3

4 stars

Program sequesters a person-year of carbon for between 75% and 125% of social costs.

1

3 stars

Program sequesters a person-year of carbon for more than 125% of social costs.

7

2 stars

Nonprofit does not publish impact information

0*

1 star

Nonprofit has potential governance or financial health issues

0


* We are temporarily withholding 2-star ratings to give nonprofits an opportunity to publish data

What we learned

1. Different trees sequester carbon at different rates

They also have different growth and survival rates, but unfortunately we generally don’t have data from nonprofits on the tree species they plant. To work around this, we estimate carbon sequestration rates based on the biomes in which a nonprofit works.

2. It doesn’t matter if it’s houses or trees, rule #1 of real estate stands: location, location, location.

Climate change may be a global problem, but not every part of the globe is exactly the same. We’ve learned that reforestation nonprofits are much more effective than urban forestry nonprofits at sequestering carbon. This isn’t necessarily due to the trees themselves, but rather because urban forestry nonprofits aren’t able to plant as many trees (there’s less space), and the ones they do plant tend to grow fast, then die fast, given the difficulty of maintaining trees in a city.

3. But, urban forestry programs can still be very effective, sometimes for reasons other than carbon sequestration

Sacramento Tree Foundation, for example, was given an extra star based on a study where they demonstrated energy savings thanks to the shade cast by the trees they planted. 

What we still need to learn

1. What are all the downstream outcomes of urban forestry? 

Sacramento Tree Foundation was just one nonprofit that provided specific evidence we were able to use. It could be that all urban forestry programs result in reduced energy usage or, as some initial evidence suggests, provide benefits to health and psychological wellbeing. But to get a more complete picture of all of urban forestry’s benefits, we would need more data from individual nonprofits combined with more rigorous research from the scientific literature in general.

2. Which trees are being planted?

We know that different species of trees sequester different amounts of carbon. However, nonprofits don’t regularly report which species of trees they’re planting. Unfortunately, this incomplete data forces us to produce estimates that are less precise than we’d like. We hope that in the future, reported data will include, by default, the species of tree planted.

3. What about advocacy?

Many nonprofits that tackle climate change engage primarily in efforts like activism, research and lobbying, and less so in service delivery. Activities like activism may be highly effective, but they are much harder to measure. We don’t (yet) have a good method for estimating their impact, so we aren’t issuing ratings for them at the moment. We hope to rate them in the future.