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Vol. 14 No. 1 - January 2008

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 (BVOCís, primarily monoterpenes and isoprene) that are involved in ozone formation and carbon monoxide formation. BVOCís 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 havenít 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 Polyís 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 & OíConnor, 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. Globalís 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.

1Allan 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: apdrew@syr.edu

2Richard Smardon, Ph.D., Professor of Environmental Studies, SUNY College of Environmental Science and 13210 USA Forestry, 1 Forestry Drive, Syracuse, NY, USA, Email: rsmardon@esf.edu

3Graduate Students at SUNY/ESF

*Contact Richard Smardon for full paper including figures and tables.


This article has been reproduced from the archives of EnviroNews - Newsletter of ISEB India.


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