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Recommended Urban Forest Mixtures
to
Optimize Selected Environmental Benefits*
By: Domm3, A. Drew1,
R. Greene3, E. Ripley3, R. Smardon2, and J.
Tordesillas3 Introduction
Urban forests are all the trees and
other vegetation that grow in places where people live, work and play. This
includes trees on public and private land, along streets, in residential areas,
parks and commercial developments, and in other locations within a community.
Their proximity to people provides substantial environmental and recreational
benefits to urban dwellers (McPherson, 2000). One of these environmental
benefits is improving air quality through deposition of pollutants to the
vegetation canopy, sequestration of atmospheric CO2 in woody biomass,
and reduction of summertime air temperatures and associated ozone formation.
Urban forest carbon sequestration
is an environmentally acceptable and cost effective way to reduce atmospheric
carbon concentrations. As urban forests are usually owned and managed by local
governments, and are not subject to commercial harvest, they can become
permanent reserves and thus, can be permanent repositories of carbon. As the
growth of urban forests increases, so does their potential to store atmospheric
carbon.
However, urban forests can reduce
air quality through emissions of biogenic volatile organic compounds (BVOC’s,
primarily monoterpenes and isoprene) that are involved in ozone formation and
carbon monoxide formation. BVOC’s can also exacerbate smog problems.
Fortunately, these compounds are temperature-dependent and increased urban tree
cover is believed to lower overall VOC levels and therefore reduce ozone
formation (Cardelino and Chameides, 1990).
Although researchers haven’t fully
determined the net effect of urban forests on air quality, results from modeling
studies indicate that benefits can be substantial (Nowak, 1994.). Additionally,
forests are one of the most cost-effective means of mitigating urban heat
islands (Akbari, et al. (Eds.), 1992) Objectives
The objectives of the present study
are:
To evaluate current contribution of
the Syracuse USA urban forest to carbon sequestration, volatile organic compound
emissions, and energy conservation;
to predict forest species mix with
recommended management options that will maximize selected urban forest
functions.
The first objective will give us a
baseline to make comparisons against. It will allow us to determine if
improvements can actually be made to the urban forest mixture within some
guidelines used by forestry professionals. The second objective will allow us
to make recommendations for the future urban forest mix in Syracuse USA. We
chose to focus on increasing carbon sequestration, which is an indicator of
removal of atmospheric carbon dioxide, reducing VOC emissions (isoprene and
monoterpenes), which contribute to the production of ground-level ozone, and
demonstrating energy savings from proper placement of shade trees. Methods
In order to determine an
optimal forest mix based on selected urban forest functions, maximization of
carbon sequestration and minimization of VOC emissions, USDA Forest Service
UFORE data (USDA City Urban Forest Data Draft) for the city of Syracuse were
analyzed. Specifically, data pertaining to the carbon sequestration levels of
current urban forest species was analyzed. The carbon sequestration data and
the leaf biomass for each tree species was used to calculate an average carbon
sequestration per unit leaf biomass for each species per year. This information
was used to determine the most effective trees at carbon sequestration. In order
to determine VOC emission rates for specific species, data from two different
papers was utilized (Guenther et al., 1994; Benjamin et al., 1996). This data
was combined with the leaf biomass data for each species to create numbers that
could be used to compare relative rates of VOC emissions for the trees being
studied. The CO2 and VOC data sets were merged to create one data
set that compared both carbon sequestration and VOC emissions. This was used to
determine the optimum trees to use for both of these functions.
To determine an optimal urban
forest species mix, management recommendations by Nowak were utilized (Nowak,
D. In-Person Conversation). The following characteristics were focused on: 1)
relatively large (at least 25 ft.), 2) long-lived (greater than 50 yr.
lifespan), 3) disease resistant (e.g., eliminated American Elm (Ulmus
americana) based on susceptibility to Dutch Elm disease), and 4) native or
non-invasive species (e.g., eliminated Tree-of-Heaven (Ailanthus
altissima) because of invasive quality). In addition to the management
recommendations, a species, genus, family ratio of no more than 10% of one
species, no more than 20% of one genus, and no more than 30% of one family for
the optimal species mix was utilized (Santamour, 1990). To select the urban
forest mix based on the newly created CO2 and VOC dataset, management
recommendations, and urban forest diversity ratio, the Selectree tool provided
online by Cal Poly’s Urban Forest Ecosystems Institute was utilized (http://selectree.calpoly.edu). Finally, energy conservation recommendations were based on a study
done for the Greater Toronto Area by the Heat Island Group at Lawrence Berkeley
National Laboratory (Lawrence Berkeley National Laboratory, 2001). Given the
temperature similarities between Toronto and Syracuse, the general energy
conservation recommendations based on urban forestry could be easily applied to
Syracuse. Results
Using the data taken from
the UFORE Study (USDA City Urban Forest Data Draft) for tree populations in
Syracuse, NY, USA and carbon sequestration, and VOC emissions data from several
different papers (Guenther et. al. 1994; Benjamin et al., 1996), relative values
for carbon sequestration and VOC emissions were calculated for each tree
species.
The results of changing the urban forest mix varied
greatly depending upon what the goal was. All results are in percent change
from current conditions which were weighted averages amounting to 0.568 mt
carbon sequestered/yr per mt leaf biomass and 8.134 ug VOC emissions/g leaf
biomass per hr.
The Carbon Sequestration
Maximization mix was able to increase carbon sequestration by 346% more than
tripling the average carbon sequestration per unit leaf biomass, although it
showed an increase in VOC emissions by 25%. The VOC Minimization Mix was able to
virtually eliminate VOC emissions, as well as increasing carbon sequestration by
173%; however it did not take into account the overall health of the urban
forest. The Optimum Mix without Forest Management Recommendations was able to
improve upon this, again, virtually eliminating the VOC emissions, while showing
an increase of 205% in carbon sequestration. This mix also did not take into
account overall urban forest health. The Optimum Mix with Forest Management
Recommendations tried to optimize carbon sequestration and VOC emissions while
planning for urban forest health and longevity (Santamour, 1990); Nowak &
O’Connor, 2001). This mix was able to Increase carbon sequestration by 86%,
while also reducing VOC emissions by 88%. In addition, the mix was made up of
long-lived desirable trees that if planted in the correct locations can help to
reduce energy use, thus reducing CO2 emissions from power generation
stations.
The benefits of shade trees have
been demonstrated in a study of the Greater Toronto area (2001) sponsored by
Lawrence Berkeley National Laboratory. The metropolitan area of Toronto has a
population of over 4.2 million with nearly 1.5 million households. With 320
cooling degree-days and 420 heating degree-days, Toronto is one of the few
metropolitan areas where a study has been done whose geographic location is
somewhat similar to Syracuse, New York USA. The estimates of cooling and
heating Avoided Peak Power (MW) and Annual Energy Savings (M$) for the Greater
Toronto area are substantial. The reduction of cooling energy equates to a
reduction of approximately 31,000 kg of CO2 emissions, using data
from B.P. Global’s carbon calculator
(http://www.bp.com/sectiongenericarticle.do?
categoryId=9008658&contentId=7016688). Discussion:
The results of the data analysis
for carbon sequestration indicate that the carbon sequestration capacity of the
urban forest can be improved dramatically by changing the urban forest mixture.
Varying degrees of improvement can result depending upon what changes are made
in future tree planting proportions. There are two mixes that were studied that
eliminated VOC emissions entirely. Both showed a significant increase in carbon
sequestration as well. They did not, however, take into account good forest
management practices, and because of this, they may result in an unhealthy
forest, or one consisting mainly of small or short-lived trees. This would not
serve to improve the aesthetics of the city.
The carbon sequestration
maximization mix showed the greatest improvement in carbon sequestration, with
an increase of over 300%, but it also showed an increase in VOC emissions, which
may end up being inconsequential when compared to the CO2 removal.
It also did not take into account forest management practices, so it would be
difficult to recommend as a mix to use for a long term planning strategy.
The Optimum Forest Mix with
Management Recommendations was chosen as the best mix to recommend for long term
forest planning. It showed modest improvements in both carbon sequestration and
VOC emissions reduction, while taking into account the management practices
recommended for a healthy forest. It uses a variety of tree species of both
large and small trees that are long-lived. In addition, it includes trees that
will be useful for reducing energy use, such as evergreens for wind breaks, and
large deciduous trees that when properly positioned will provide useful shade in
the summer.
The study of energy savings from
trees in Toronto can be assumed to be very similar to energy savings that can be
achieved in Syracuse, due to similarities in climate and geography. Shade tree
savings provided significant energy savings due to reduced need to use air
conditioning in the summer. This equates to a significant reduction in CO2
emissions, assuming that the energy used was generated using fossil fuels.
We have shown that significant
reductions in greenhouse gases can be achieved using a few simple
recommendations in the urban forest of Syracuse. If the urban forest mixture is
changed to include more desirable trees, and the locations of certain trees are
chosen carefully, Syracuse can easily become a contributor to the reduction of
greenhouse gases in the atmosphere. Synthesis of Data Results:
The results showed significant
projected reductions in greenhouse gas output with the exception of carbon
monoxide and volatile organic compounds. This reduction will help in improving
the overall air quality in Syracuse, NY USA.
The effects of the urban forest
will serve to reduce these numbers even more, decreasing the net (total output
minus removal by the urban forest) amount of greenhouse gases and pollution
released into the atmosphere by the city. The forest mix chosen as a
recommendation by our group is projected to increase carbon sequestration by 86%
over current urban forest contributions, thus decreasing the net greenhouse
gases released by Syracuse. Unfortunately, trees take a long time to grow and
to completely replace the current urban forest mix in Syracuse with mature trees
that reflect the recommended mix may take up to 40 years. As revealed by our
research, the urban forest is contributing to carbon sequestration in Syracuse.
Further, significant increases in the carbon sequestration rate could be
achieved if urban forest canopy cover increased. This can be accomplished
through better management of the current urban forest as this will lead to
larger, longer-lived trees that are capable of sequestering more carbon. Canopy
cover could also be increased through enhanced tree planting and replacement
efforts both of which, by increasing the number of trees in Syracuse, would lead
directly to an increase in canopy cover and additional carbon sequestration.
Allan Drew, Ph.D., Professor of Forest and Natural Resources
Management, SUNY College of Environmental Science and Forestry, 1 Forestry
Drive, Syracuse, NY 13210 USA,
Email:
[email protected]
2Richard Smardon, Ph.D., Professor of Environmental
Studies, SUNY College of Environmental Science and 13210 USA Forestry, 1
Forestry Drive, Syracuse, NY, USA,
Email:
[email protected]
3Graduate Students at SUNY/ESF
*Contact Richard
Smardon for full paper including figures and tables. | 
