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.
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.
Baltimore
Boston
New
York City
Philadelphia