Shades of Green: The Hidden Carbon Cost of Tree Planting and Care

When you work with trees for a living, it’s easy to feel good about your impact on the environment. After all, trees provide us so many benefits including controlling stormwater runoff, reducing urban temperatures and sequestering carbon. However, there is a carbon footprint involved in their production, planting, pruning and ultimate removal. Trees play the long game – it takes decades before they provide the ecosystem services plant them for.  The way we work with trees can increase or decrease the time before they become “carbon neutral” and lengthen the number of years they can provide vital ecosystem services to us all.

Two studies have analyzed this issue. The U.S. Forest Service study of 2014 looked at the Million Trees Los Angeles program (MTLA). The more recent How Green Are Trees? study funded by the Horticultural Research Institute (HRI) examined the life cycle of Red Maple (Acer rubrum)  in the Chicago area. Both studies looked at the carbon emissions used in nursery production, delivery and planting, pruning and removal of street trees.

The Forest Service study found that the MTLA trees (various species) would become net carbon reducers after 40 years, assuming they remain for 100 years. The HRI study was slightly more optimistic, projecting a range of 26-33 years to carbon neutrality for Red Maple in the Chicago area. One of the main differences between the studies is that the Forest Service considered mulch and root decomposition as part of the emissions associated with tree work and the HRI study did not. The Forest Service Study also used longer transport distances and more intensive irrigation in their model as appropriate for the Los Angeles setting. It is typical in California to irrigate new trees longer than one year, while providing one year of irrigation, if that, is more common in the rainier Midwest climate.

While the studies differ, both conclude that reducing emissions associated with tree work reduces the number of years after planting that the tree attained carbon neutrality and prevents the urban forest from actually becoming a carbon emitter.

Some strategies for reducing emissions recommended by the two studies are listed below. No municipality or organization can incorporate them all, but we can certainly make improvements in some areas of our operations:

  • Choosing species that
    • Attain a large mature size, but not larger than the space warrants.
    • Grow at a moderate rate (slow growers take longer to become carbon-neutral, fast growers may need more pruning and have shorter lifespans)
    • Are climate-adapted and drought-tolerant.
  • Sourcing trees from nurseries close to the planting site.
  • Using smaller, fuel-efficient trucks or trucks that use lower-carbon fuels.
  • Organizing work so the minimum amount of driving is necessary in a given day.
  • Using equipment with the minimum horsepower required for the job and reducing idling time, or using battery-operated equipment.
  • Providing the most hospitable soil environment for root growth so trees can reach their maximum size as soon as possible. This may include using suspended pavement on new developments, providing larger openings for trees, or ensuring that any imported soil is similar in texture to the native soil to avoid any drainage problems.
  • Planting and backfilling manually rather than using mechanized methods.
  • Minimal or no tree staking.
  • Minimal irrigation using a hose or irrigation bag instead a water truck.
  • 5-year pruning cycles and minimal pruning.
  • Conducting periodic “windshield” surveys differently: using a hybrid car, setting up a system of public feedback, or merging surveying activities with other activities to reduce emissions.
  • Utilizing waste wood from tree removal and stump grinding, either by using the wood itself to make furniture or structures, or by using it as feedstock for biopower production.

After reading both studies, some additional recommendations come to mind. What else can you think of?

  • Utilizing the labor of community volunteers – insurance requirements and individual skill levels generally necessitate manual rather than mechanized work.
  • Planting smaller specimens – #5s and #15s are easier to move and plant by hand and also require less water to establish.
  • Wherever possible, specifying tree forms other than “standards” to minimize tree staking. Possible forms may include “multi-trunk,” “columnar,” “bush,” or “low-branching.” With pre-inspection at the nursery, it’s possible to find specimens that could be trained as standards with some early pruning, assuming that standard form is necessary in the site.
  • Planting at the correct time of year for the region to reduce irrigation time. In California, we recommend planting in autumn at the beginning of the rainy season.
  • Structural pruning in the first 5 years after planting to minimize pruning and reduce the risk of failure, lengthening useful life.
  • Recycling tree support materials through thoughtful installation and removal techniques. For example, pressure-treated lodgepole stakes can be re-used provided they are removed as soon as the tree becomes self-supporting and in such a way to prevent breaking them. Tree ties can be re-used provided the nails or screws holding them to the lodgepoles are not driven deeply into the wood.
  • Tougher tree protection ordinances – preserving existing trees is much “greener” than removal and replacement.

It’s important to be aware that everything we do has an environmental impact, even when it’s “good” work. If we’re lucky, we’ll live long enough to stand in the shade of the trees we’ve planted, and breathe the clean air they provide us.

 

Ellyn Shea is a consulting arborist in San Francisco.

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