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Ask the Conservationist

November 2009: Urban Forests & Climate Change

What's a city-dweller to do — you want to help fight climate change, but does planting trees in the city really make a difference? Can urban forests help sequester carbon and offset emissions?

Read what our conservation scientist has to say, and when you're done reading, send us your questions on any conservation subject for one of the Conservancy's 720 staff scientists. (Note: We regret that we can only answer one or two questions each month and that we cannot answer the others offline.)
Jack Capp of Fort Collins, Colorado, writes:

“I am interested in knowing how we can combat climate change and enhance carbon sequestration by planting trees, both hardwoods and conifers. I would like to help my city plant trees to help offset carbon released — can you send me information on how planting trees can help?"


Jeff Fiedler

Planting urban forests can be an effective way to combat climate change. Trees absorb carbon dioxide from the atmosphere through photosynthesis as they grow, and "sequester" or store the carbon as biomass in their trunks, branches, roots and leaves.

Interestingly, however, the primary benefit from urban trees is usually from energy savings not carbon sequestration:

  • Well-placed urban trees can shade buildings and surfaces, significantly reducing heating and cooling needs.
  • Deciduous trees can provide shade in summer and allow passive solar heating in winter.
  • Coniferous trees provide a year-round wind break.
  • Trees also indirectly cool urban neighborhoods through evapotranspiration.

While all trees sequester some carbon, the rate of sequestration varies by tree species, soil type, climate and topography of the region and management. This is why The Nature Conservancy supports policies to reduce tropical deforestation, as well as efforts to restore degraded forests by replanting native trees in appropriate locations, both internationally and domestically.

Urban trees tend to have lower sequestration rates than non-urban forests. They are typically planted more sparsely, grow more slowly and have higher mortality rates. In addition, urban trees require significantly more maintenance (e.g., pruning to avoid utility lines, which can involve use of fossil fuels), and land is more limited and costly.

On balance, while urban tree planting can provide local energy savings and considerable aesthetic benefits, as well as habitat benefits for local birds and insects if native trees are used, a comprehensive city offset program might also consider planting trees in non-urban areas elsewhere. While not providing the same immediate local benefits, the sequestration potential would likely be much higher. And lowering carbon emissions anywhere benefits everyone.

Resources:
A great local resource for you in Fort Collins is the Colorado Tree Coalition.

A statewide resource is the Colorado Carbon Fund, which is developing in-state carbon offset projects in the agriculture and forest sectors.

And, writing in the Journal of Arboriculture, Nowak et al. (2002) provide advice on maximizing the benefits of urban forests for atmospheric carbon dioxide.


About the Conservationist

Jeff Fiedler is a climate and forest specialist with The Nature Conservancy.

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