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Guest Blog: Our urban forest's carbon footprint

Posted by Bobby Hayden at May 23, 2013 08:00 AM |

With a better understanding of the climate benefits of cities, we can properly manage and invest in their resources.

Guest Blog: Our urban forest's carbon footprint

Weston Brinkely and Lisa Ciecko

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By Weston Brinkley and Lisa Ciecko

Cities have a crucial role to play in countering climate change. Cities provide transportation choices, density, and a lower carbon footprint for residents on average. With a better understanding of the climate benefits of cities, we can properly manage and invest in their resources.

In particular, we are interested in learning more about the influence that our trees and forests can have on climate.  How do we start to understand the positive carbon footprint of our urban forest?

Fonterra Pic 1

In the summer of 2010, the Green Cities Research Alliance (GCRA) set out to measure the urban forest across the region to better understand its structure, function and values. GCRA partners include Forterra, the US Forest Service Pacific Northwest Research Station, King County Parks, the City of Seattle, and researchers at the University of Washington.

The first stop was Seattle where we visited 223 sample plots within the city limits. This is roughly 25% more urban forest assessment data than had previously been collected by the Forest Service for all urban areas in the state.  Subsequent field seasons took our team to King County Parks’ dense forestland and throughout the Green-Duwamish Watershed on a traverse across the urban to rural to wildland gradient.

Fonterra Pic 2Within each sample plot, we collected data on the type and size of all trees and shrubs, as well as plot ground and land use conditions. This information was processed using the i-Tree Eco model, with additional analysis by GCRA researchers. i-Tree Eco, the initial tool used for this investigation, was developed by the US Forest Service specifically for measuring urban trees.  These recently aggregated results show the breadth in which i-Tree has been used across the country for carbon values, as the standard in urban forest analysis.

This assessment allowed the GCRA team to develop a picture of the urban forest and its functions. From this picture a set of ecosystem services was calculated, including carbon sequestration and storage function.  i-Tree calculates not only carbon values, but is able to provide heating and cooling values, pollution interception, stormwater, and a structural value.  Though not given in detail here, those services have additional values and climate benefits.

Fonterra Pic 3i-Tree calculates carbon storage through biomass estimates for both above and below ground plant tissue. Sequestration is the annual carbon removed through photosynthesis.  Across Seattle, carbon storage in urban forest bio­mass amounts to almost 2 million metric tons of CO2 equivalent, with an additional 141,000 metric tons of CO2 equivalent sequestered in 2011 that’s equal to roughly 2% of Seattle’s carbon emissions for a year, or 7 days of emissions.

If you looked at these values per acre it would be 36 metric tons of CO2 equivalent stored per acre, and approximately 2.6 metric tons of CO2 equivalent sequestered per acre. 

Looking a little deeper into the results we see that, as you would assume, not all areas of the city are equal in their carbon storage. In fact, it’s not even close – those 36 tons per acre of storage balloons to over 160 tons per acre when looking specifically at our natural area parks.  Although the natural area parks cover over 3,600 acres and account for the much of dense forest in the city, there are other acres in the city that also fit this description.

Fonterra Pic 4i-Tree provides a dollar value for the city wide carbon – $10,900,000 in storage and $768,000 annually in sequestration. These amounts are based on an i-Tree setting of $5.45 a ton.  Is that a lot of money? California’s cap and trade carbon market most recently traded at $13.62.  i-Tree’s conservative price likely reflects the uncertain social and ecological context of these urban trees longevity.  Better understanding of the climate benefits and forest management of these trees may provide a more accurate value. There is much work to be done in the use and application of ecosystem valuation.

Our forested parklands, those with the greatest positive biocarbon impact are in a state of decline. Putting this together with our research results gives us some context – our trees have value, and they are going away.   Our declining forested parklands provide an opportunity for improving carbon storage and sequestration in our city through restoration and management.

This research is a start in understanding the positive footprint that our forests – right here in the city – have on biocarbon.  Read more about the study on the Forterra website.  It provides a full explanation of methods and results, as well as discussion of a number of ecosystem services and values. A US Forest Service Technical Report is in progress.

All photos not credited: Lisa Ciecko

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