Urban & Community Forestry Models


Carbon and Trees

Carbon is the basis of all organic compounds and all organic material contains carbon.  It is important to understand how carbon in its various forms is used and transported through Earth's ecosystems and how its presence, surplus, or deficit might effect those ecosystems.
Carbon and the energy that it produces is moved and transformed through the carbon cycle, where it is present as both a gas and as a solid.  As a gas, it is found in the atmosphere as carbon dioxide, carbon monoxide, and methane and is removed from the atmosphere by plants through the process of photosynthesis.  In photosynthesis, carbon is converted into a solid (sugar) that can be stored or be released back into the atmosphere through respiration.  If it is stored as a solid, it is deposited to form reservoirs, or carbon sinks. Examples of carbon sinks include trees, soil, and the ocean.  

As a natural process, the carbon cycle maintains an equilibrium between the amount of carbon  in gaseous forms in the atmosphere and the amount stored in carbon sinks and as sugar for plants to use as energy.  However, human activity greatly disrupts the balance of the carbon cycle (as it does the processes of most biogeochemical cycles).  Burning fossil fuels releases carbon in the form of carbon dioxide (CO2) and humankind's increased dependence on fossil fuels over the past century has caused the amount of carbon dioxide in the atmosphere to increase dramatically.  Also, human activities such as deforestation, agriculture, and development have disturbed important carbon sink systems worldwide, also contributing to the increase of carbon dioxide in the atmosphere.  The diagram below depicts the processes of the carbon cycle with specific numbers alloted to the amounts of carbon (in petagrams) in the carbon sinks, or pools, and the fluxes in carbon in petagrams per year.  

carbon cycle
Image from The GLOBE Program at www.globe.gov

So why is the increased amount of carbon as CO2 in the atmosphere a concern?  Why does it matter if the carbon cycle's equilibrium is disrupted?  The answer is climate change.  Carbon dioxide is a major greenhouse gas, as is methane, nitrous oxide, water vapor, and ozone.  Greenhouse gases absorb infrared radiation (heat energy) that is emitted from the surface of the Earth and radiate it back towards the planet, keeping it in the atmosphere (opposed to releasing it to space).  The general process is much like that of a greenhouse and functions in insulating the planet, causing global warming.  The image below shows how greenhouse gases contribue to climate change.  
Image from California's Division of Fish and Game website at www.dfg.ca.gov/climatechange

Trees do two very important things with carbon in the form of CO2:  they sequester it through photosynthesis and they store it as biomass.  The biomass of a mature tree is roughly 40% carbon; as the tree grows, more carbon is stored and sequestered and as it decays after it dies, that carbon is slowly released back into the atmosphere.  Thus, the net long-term CO2 storing, sinking, and sourcing dynamics of forested ecosystems is in constant flux.  Add human influence into the mix and the cycle becomes more complex.  Trees in urban and community forests have the potential to store and sequester nearly four times more carbon than individual trees in forest stands because of the open growing structure and faster growth rates.  The typical urban tree will sequester 1 ton of carbon in its lifetime. However, in consideration of net carbon benefits, urban and community forest trees also need to be maintained and managed, which involves increased CO2 emissions (chainsaws, watering trucks, and chippers, for example).  

In a 2003 study by Dwyer and Nowak, it was estimated that there are approximately 74.4 billion trees in domestic metropolitan areas and 3.8 billion in domestic urban areas and the national average tree cover in these areas is 33%.  In a 2001 report titled Carbon Storage and Sequestration by Urban Trees in the USA (Nowak & Crane), it was estimated that US urban trees currently store approximately 700 million tons of carbon with an annual rate of sequestration of 22.8 million tons.  Looking at data from 10 cities taken over the course of three years, the study determined that the national urban forest carbon storage density is 25.1 tC/ha, compared with 53.5 tC/ha in forest stands.  In terms of monetary value, these ecosystem services that urban trees provide translates into a $14.3 billion dollar carbon storage value and a $460 million carbon sequestration value annually.  


Aside from the carbon-related environmental services, the are myriad other benefits that city trees provide to urban populations.  These features should be considered as co-benefits, working together in a dynamic and interconnected manner towards general quality of life  improvement.  Urban tree benefits are non-exclusive and non-rival, which means that they can have an effect on multiple users without diminishing their overall value.  Some of the other benefits of urban trees include:

    1.  Environmental Benefits:
        - A mature tree (30 years old) absorbs between 120 and 240 lbs of particulate matter annually
        - A healthy mature tree can produce upwards of 260 lbs of oxygen each year
        - The Urban Heat Island Effect (UHIE) refers to the increased heat absorption and retention of built-up, concrete and asphalt-based areas (such as city centers).  The annual mean daytime air temperature of a city can be between 1.8-5.4 degrees F warmer than surrounding rural areas.  This causes increased energy demand, heat-related illness, and increased air pollution and greenhouse gas emissions.  In peak summer temperatures, shade from urban trees can lower temperatures by as much as 45 degrees F and evapotranspiration processes can result in 2-9 degree F lower temperatures.  
        - The canopy provided by urban trees absorbs and slows rainfall and similarly, the soil and root structure of an urban trees also absorbs and transpires rainwater.  These two functions in turn slow and reduce stormwater runoff down impervious surfaces in cities.
        - Trees provide rain, sun, heat, and skin protection.  
        - Trees provide habitat for wildlife.
        - In non-urban forest resources, up to 61% of total carbon stored in those ecosystems is stored in the soil environment.  No comprehensive data is available about urban soil carbon storage, but it is clear that it is significant.  

    2. Economic Benefits:
        - Having trees in front of or surrounding homes can raise property values by up to 30%.
        - A survey of real estate appraisers found that 86% of them agreed that landscaping adds value to commercial real estate.
        - Consumer behavior has been linked to the presence of trees: people are willing to drive further and spend more in "forested" retail areas.
        - The need for management and care of the urban tree resource results in the creation of "green" jobs, which are often given to urban youth.  
        - Shade from trees not only cuts down on municipal mowing costs but also protects pavement from weathering and cracking, thus lowers costs of replacement and repair.
        - A properly shaded neighborhood can reduce energy bills for a household from 15-35%.    

    3.  Social/Recreational Benefits:
        - Having trees in high density neighborhoods has been directly correlated to lower levels of crime, violent and aggressive behavior, and encourages a sense of community.  
        - Participation in urban forestry activities and urban tree care promotes community-building.
        - Hospital patients recover more quickly and need less painkilling medications when then have a view of nature from their hospital bed.
        - Attention spans and self-discipline in school-aged girls has been correlated with access to nature and trees.  
        - Urban trees provide visual and noise filters/screens.  
        - Expansion of park areas, tree-lined trails, and public open space promotes recreational use, which can lend to improvment of obesity rates by providing areas for urban populations to get daily recommended levels of activity.  

Carbon Markets and Urban Trees   

Carbon Markets deal in the buying and selling of credits for emissions of all six greenhouse gases (carbon dioxide, methane, nitrous oxide, sulphur hexaflouride, hydroflourocarbons, and perflourocarbons), either through a regulatory body or generated through emissions reductions projects.  Greenhouse gas emissions reductions are traded in the form of carbon credits; 1 carbon credit is equal to 1 metric ton of carbon dioxide equivalent (tCO2e).  In the United States, there is no regulatory (compliance) carbon market, and the Chicago Climate Exchange is the nation's only volunatry cap and trade system for all six greenhouse gases.  With emissions regulations on the horizon in the U.S., there is interest in the carbon sequestration and storage functions of urban trees and how they might play into municipal (or nonprofit) participation in emerging carbon markets.  

The CarbonPlus Calculator: Offsetting Emissions and Funding Sustainability Projects

The US Forest Service has partnered with several U.S. cities over the past few years to develop the first CarbonPlus Calculators, which basically allow users to make a financial contribution to offset their greenhouse gas emissions.  Visitors to each specific city's CarbonPlus website have the opportunity to calculate their personal or household emissions based on the structure of the U.S. EPA's Personal Emissions Calculator with added calculations of energy and other social and environmental benefits of trees in urban settings.
The CarbonPlus Calculator itself correlates emissions from energy usage, waste disposal, recycling habits, and transportation with data from each city to come up with a monetary amount that can be contributed in order to offset the emission figure.  

Aside from the actual offset, the websites of the different cities also include different features such as local action opportunities, emission reduction methods, carbon footprint explanations, information on global climate change, and specific ways in which the offset funds will be used.  For example, in Boston, the funds acquired through the CarbonPlus Calculator are used to retrofit older buildings to make them more energy efficient, to install solar panels on city buildings, and to plant trees.  

To visit the four existing CarbonPlus Calculator websites, please click on the city name below and you will be directed to that city's specific page.  



New York City


Last modified January 13 2010 09:35 AM

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