ISSUES IN ECOLOGY NUMBER THIRTEEN SPRING 2010 en if the thinned nly rent read forest tre ment in the hat such ocus o thinnipe scale needs to be placed in the context of Figure 9.Sycamores lining Tener come of such research,the carbon cities located in grasslands and deserts.urban forests require large amounts of irrigation 6.Urban forestry water for mai enance tradeoffs,the fol Urban forestry offers very limited potential to determine the net climate impact of urban urban trees provide many co-benefits,includ- ated with planting and maintenance.3)fos insaestheticbencfcandcnronnenrl diontofartbonscquestra on proc ous an 51 mis effect of trees on local air temperature and the US ake up onl iyto be intensively managed and may require large Smdncnemlartnpuefrping 7.Bio ass energy,carbo on storage in stitution an he ant biophysi Biomass energy effect on local temperatures due both to s shad Shadin 7只0 the U.S able energy supply and%of the. day and night surface mper When energy use.Biomass energy is used primarily mass may become an important feedstock tioning.When urb n fo estsare planted over for liquid biofuels ve boh ing and the public uld ce re ing 190 teragram e regions.citics U.S.fossil fucl missions in 200 (as discussed further in Environme below).It has areas;th eO at by deforested.In such regions.trees may have rel liquid biofuel per year (offsetting 2.6 teragrams atively low maintenance requirements.In of fossil fuel carbon cmissions). The Ecological Society of Americaesahq@esa.org esa 9© The Ecological Society of America • esahq@esa.org esa 9 ISSUES IN ECOLOGY NUMBER THIRTEEN SPRING 2010 decrease carbon, even if the thinned trees are used for biomass energy. More research is urgently needed to resolve these different conclusions because thinning to reduce fuel is a widespread forest treatment in the U.S. We recommend that such research focus on the landscape scale because carbon loss in thinning needs to be placed in the context of the expected fire frequency and extent, and the potential for regener￾ation after fire. Regardless of the out￾come of such research, the carbon benefits of fuel treatments can be improved by using the harvested trees for wood or biomass energy. 6. Urban forestry Urban forestry offers very limited potential to store carbon, but we address urban forests here because of the large interest in using them to offset carbon emissions and because urban trees provide many co-benefits, includ￾ing aesthetic benefits and environmental advantages in addition to carbon sequestra￾tion. The potential for carbon offsets of greenhouse gas emissions through urban forestry is very limited for two reasons: 1) urban areas make up only a small fraction of the U.S. landscape and 2) urban forests are intensively managed and may require large energy, water, and fertilizer inputs for planting and maintenance. Urban forests can have important biophysi￾cal effects on climate. Trees have a cooling effect on local temperatures due both to shad￾ing effects and to evaporative cooling in tran￾spiration. Shading intercepts incoming radia￾tion in the daytime, which can reduce both day and night surface temperatures. When trees are planted very close to buildings, they cool building temperatures and reduce the fos￾sil fuel emissions associated with air condi￾tioning. When urban forests are planted over very large regions, the climate effects are less certain, as trees can have both warming (absorption) and cooling effects. The higher the maintenance required for urban trees, the less likely they will help miti￾gate climate change. In some regions, cities are located in what would naturally be forested areas; thus, urban forests serve to restore forests to land that was previously deforested. In such regions, trees may have rel￾atively low maintenance requirements. In cities located in grasslands and deserts, urban forests require large amounts of irrigation water for maintenance. Because of these many tradeoffs, the fol￾lowing factors must be taken into account to determine the net climate impact of urban trees: 1) the carbon storage rate of the trees, 2) fossil fuel emissions from energy associ￾ated with planting and maintenance, 3) fos￾sil fuel emissions resulting from the irriga￾tion process, 4) nitrous and nitric oxide emissions from fertilizer use, and 5) the net effect of trees on local air temperature and its impact on building energy use. These fac￾tors are likely to be highly variable by region and by species. 7. Biomass energy, carbon storage in products, and substitution Biomass energy The use of forest biomass energy prevents car￾bon emissions from fossil fuel use. In 2003, biomass energy was 28% of the U.S. renew￾able energy supply and 2% of the total U.S. energy use. Biomass energy is used primarily for electric power in the forest products indus￾try and for residential heating. In the future, biomass may become an important feedstock for liquid biofuels. If cost were not a constraint and the public supported this use of forests, U.S. forests could potentially provide energy production offset￾ting 190 teragrams of fossil fuel carbon emis￾sions per year, or the equivalent of 12% of U.S. fossil fuel emissions in 2003 (as discussed further in Environmental costs below). It has been estimated that by 2022, forest biomass feedstocks could produce 4 billion gallons of liquid biofuel per year (offsetting 2.6 teragrams of fossil fuel carbon emissions). Figure 9. Sycamores lining Sycamore Street in Los Angeles, California. Photo by Diane E. Pataki, University of California, Irvine