By Jennifer Sherry, Ecosystem Analyses
Try to imagine your city without it’s trees.
You probably wouldn’t want to live there without them.
Although most of us do realize the aesthetic value of trees in urban
areas, we are only vaguely aware of the handful of other environmental benefits
that they provide. As the Denver
metro area develops and sprawls across the plains, so too does our “urban
forest”; millions of trees have been planted here over the last century and a
half. Since we live in a semi-arid
environment (historical average
rainfall in this area is only 10-15 inches), this
“green infrastructure” would not have survived without human care and the
development of irrigation. Native
trees include cottonwoods, willows and little else, and require a generous
supply of water, so prior to settlement these trees could only survive in close
proximity to streams and creeks. The
urban forest that exists today has been thoughtfully created by our
predecessors, yet they too were probably unaware of the myriad of environmental
benefits that trees in urban areas provide.
Collections
of historical photos demonstrate this previous lack of tree cover.
Yet trees were desired, evidenced by the many large shade trees that line
the streets of historic neighborhoods. We
can only speculate on the motive behind such massive tree plantings.
Fore example, an estimated 300,000 trees have been added to Boulder’s
landscape since settlement. Some of
these reasons might be:
Figure 2:
As seen from the Yount Flour Mill at the mouth of Boul
For
whatever reason trees were planted, the current residents of the Front Range are
still receiving the benefits of these settler’s forethought.
Little did they know that this urban forest would provide so much more
than simple aesthetic improvement. It is now recognized that their tree
plantings have provided us with measurable environmental enhancements including
air pollution removal, stormwater runoff prevention and water quality
improvement, carbon storage and sequestration, and energy savings realized when
trees shade buildings and cool the air through evapo-transpiration.
In April 2001, American Forests (a non-profit agency whose
“mission is to improve the environment with trees and forests”)
released the findings from the Regional Ecosystem Analysis for
Metropolitan Denver and Cities of the Northern Front Range, Colorado.
This analysis combined both the use of large-scale satellite imagery
classification, aerial photography, GIS and computer modeling to quantify the
environmental benefits of the urban forest.
The sampling of 39 metro sites in different land use categories
(Residential, Commercial, Industrial, Public/Recreational) allowed the
estimation of the percentage of area covered by tree canopy, building,
impervious surfaces, and grass. At each site, tree’s species ID, trunk
diameter, health, and crown diameter were cataloged.
The entire data set was then entered into CITYgreen, a software package
created by American Forests that performs the environmental modeling within a
GIS, to quantify the benefits previously mentioned. Refer to Figure 1 for an
example of a sampled residential site.
Figure 1: Residential site with 60% canopy cover in Boulder, CO.
The following environmental benefits provided by trees were
modeled:
·
Energy
Savings:
Trees help to cool the air inside buildings by directly shading windows,
roofs, and air conditioning units, and indirectly cool the air outside through
evapo-transpiration (similar to the way a swamp cooler works).
This leads to lower energy bills and reduced energy needs at the power
plant. The energy savings model
calculates the geometry of the shade the trees provide to determine whether it
falls upon a window or air conditioning unit, and then calculates the dollar
amount and kilowatt hours (kWh) saved by this tree shade. This model also determines the amount of carbon
conserved (measured as “avoided carbon”) when reduced region-wide energy
consumption leads to reduced consumption of fossil fuel for power.
·
Stormwater
Runoff Prevention: Trees benefit water
in urban areas in two ways: both by increasing water quality, and by decreasing
stormwater runoff. Between
storms pollution emitted from vehicles is deposited on roadways and impervious
surfaces; rainfall washes these pollutant off and this runoff is delivered to
nearby waterways. Not only is the
water contaminated, but efficient removal of stormwater from the roadways
creates high peak flows in the streams, which can displace aquatic flora and
fauna and lead to bank erosion. Trees’
leaves and needles have a high surface area and can hold a tremendous amount of
water, and the pervious soil and root system beneath the tree also stores large
amounts of water. By retaining
rainwater, trees and grass prevent it from becoming stormwater runoff. Trees also filter pollutants prevent it from harming
aquatic life, improving overall water quality.
Multi-layered canopy surfaces of trees slow the impact of falling
rainwater, and also help to reduce erosion.
The stormwater runoff prevention model evaluates several parameters
(perviousness of the soil; slope of the site; percent of canopy, grass, and
impervious surfaces; and amount of precipitation) in order to calculate the
volume of stormwater retained by each site.
·
Air
Pollution Removal: Trees remove airborne
pollution through the process of respiration.
A tree intakes carbon dioxide, necessary for it’s survival,
simultaneously with several other air pollutants, through its stomates.
Once inside the leaf these particles are disassembled into smaller
pieces, some of which the tree can actually metabolize, while the others diffuse
into intra-cellular spaces. Street
trees are particularly important for this due to their close proximity to
vehicles, the major source of air pollutants.
The air
pollution removal model takes into account the area in acres of tree canopy for
each site and determine the amount removed of each of the following pollutants:
carbon monoxide (CO), sulfur dioxide (SO2), ozone (O3),
particulate matter less than 10 microns (PM10), and nitrogen dioxide (NO2).
· Carbon Storage/Sequestration: Carbon is the building block of all cellular life. Trees store carbon in their biomass (roots, trunk, leaves, and leaves), and carbon accounts for about half the dry weight of most trees. The amount of carbon stored increases as trees grow in size; while growing, they sequester additional carbon in order to generate new shoots and roots. Carbon is released again when trees die and decay, and a small amount is released each year when the leaves fall to the ground. Trees can be thought of as a “sink” for excess carbon in the atmosphere. Storage and sequestration are important today because humans generate tremendous amounts of carbon through consumption of fossil fuels. These activities, associated with a developed world, have led to an imbalance of sources and sinks of atmospheric carbon. Similar to the air pollution removal model, this model uses canopy acreage along with population age and trunk size to determine the amount of carbon stored in trees.
American
Forests has determined that the 7% canopy cover in the Front Range Metro Area
provide a number of environmental
benefits.
·
Stormwater
Runoff: Trees reduce the need for stormwater management facilities by 50.1
million cubic feet, valued at $44 million in one-time costs, or $3.2 million in
annual savings over a 30-year period.
·
Energy
Savings: Residents
in the Font Range spend an average of $260 per year to cool their homes, and trees
provide an average of $50/year in energy savings for the average 1-2 story
single family detached home. This
multiplies into approximately $4.5 million in annual residential savings.
Energy savings were only modeled in residential areas.
·
Avoided
carbon: kWh conserved by direct
shading results in the reduction of approximately 1.6 million tons of avoided
carbon emissions annually. The
market for “carbon credits” is in it’s infancy, however dollar values will
be attached to carbon saving in the near future.
·
Stored
and sequestered carbon: trees store
an estimated 870,000 tons of carbon and sequester about 15,000 tons annually.
·
Air
Quality: The existing tree canopy removes 2.2 million pounds of pollutants,
valued at $5.3 million annually, based on removal costs and avoided health care
costs.
American
Forests also claims that increasing overall canopy cover to 25% would
effectively double most of these values. In
order to increase canopy cover and realize these additional benefits, more trees
need to be planted, particularly in sparsely forested commercial and industrial
sectors. Although these areas may
never reach the canopy cover of residential areas simply for lack of proper
planting spaces, benefits of canopy cover can be enhanced through proper species
selection and placement.
Other
benefits of trees exist but have not been modeled by the American Forests study.
For instance, the benefits of shading streets and parking lots are many.
One, trees in parking lots provide a cool(er) place to park a car in hot
summer months by reducing the air temperatures of the interior of the vehicle.
Two, high ambient temperatures in parking lots cause the evaporation of volatile
substances from vehicles and compounds the air pollution problem.
Trees in and surrounding parking lots help reduce ambient temperatures
though evapo-transpiration and therefore help reduce this type of emission.
Thirdly, trees that shade paved surfaces extend the life of asphalt or
concrete and reduce maintenance and repaving costs.
The
Front Range’s urban forest may have originated for a variety of different
reasons, but we are still reaping the benefits of some of those trees planted
over 100 years ago, and many planted since.
In order to continue to receive these benefits, we must not only work at
maintaining our urban forest as it exists now, by replacing all removed trees
with new ones, but must also continue to plant trees in all available planting
spaces. These new trees will
grow and provide not only benefits to future generations, but some sort of
mitigation for the detrimental environmental impact of urban sprawl.
For more information regarding:
CITYgreen, visit the American Forests website at www.amfor.org
·
Urban Forestry nationwide, visit www.treelink.org
or http://www.communitytrees.org/
· Historical Photos of Colorado’s Front Range, visit gowest.coalliance.org