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ArboristWESTERN

Fall 2008

Trees and the Clean Air Act: Strategic treeplanting in SacramentoGreg McPherson

“Trees fight air pollution in severalways…less spending for energy, re-duced atmospheric carbon dioxidelevels, cleaner stormwater, increasedproperty values, improved physical &mental health, a greater sense of placeand a more beautiful environment.”

Trees, the EPA, and cleaner airThe Clean Air Act requires that allstates with unhealthy levels of air pol-lutants submit a State ImplementationPlan (SIP) that describes the methodsthat will be used to meet federal airquality standards. Until recently, SIPshave been aimed mainly at reducingpollutant output at the source: powerplants, factories, and vehicles. Re-cently, the Environmental ProtectionAgency (EPA) has begun to encour-age new, innovative measures to fightair pollution and trees are being con-sidered as one part of the solution.

Sacramento’s air quality problemVehicles, including cars, trucks, buses,trains, and agricultural and construc-tion equipment, cause the majority ofSacramento’s air pollution with mostof the remainder emitted by powerplants. Vehicles and power plantsgive off nitrogen oxides (NOx) andvolatile organic compounds (VOCs),which combine in the presence of sun-light to produce ozone, Sacramento’sbiggest air quality problem. The Sac-ramento metropolitan area is amongthe ten most ozone-polluted areas inthe country, exceeding the acceptedlevels as many as 40 days a year de-pending on weather conditions.

Vehicles and power plants alsoproduce small particulate matter (PM)and carbon dioxide. Particulate mat-ter has been linked to increased sever-ity of asthma attacks and can haveserious consequences for the elderly,children, and people suffering fromlung and heart disease. Carbon diox-ide contributes to global warming.

How trees can helpCalifornia has long been in the fore-front of the battle to fight air pollu-

tion with strict vehicle emissions laws,high fuel efficiency standards and in-novative programs to reduce pollut-ants at the source; and importantstrides have been made. Still, becausethe population of California is grow-ing so fast, there are more and morevehicles on the road, and more andmore energy is needed. Innovativeways to clean the air are necessary.

Trees fight air pollution in severalways. First, they absorb or interceptpollutants on their leaves, branches,and trunk. Second, by lowering airtemperatures, trees reduce our needfor energy for air conditioning, which

reduces the production of pollutantsat the power plant. Third, by shadingstreets and parking lots, trees also re-duce temperatures of parked vehicles,which reduces the amount of pollut-ants leaking from hoses and fuel tanks.

Although trees have many posi-tive effects on air quality, some spe-cies also give off their own VOCs, oneof the building blocks of ozone. Theseare known as biogenic volatile organiccompounds or BVOCs. The impact ofBVOCs on air quality is not well un-derstood: scientists are not sure how

BVOCs react with NOx or what im-pact the temperature-lowering effectsof trees have on the reaction. In ourstudies, we consider BVOCs to havethe same effect on air pollution asother VOCs.

The projectThe Center for Urban Forest Researchis working with the Sacramento TreeFoundation and the Sacramento Met-ropolitan Air Quality ManagementDistrict to study the feasibility of us-ing trees to clean the air. We have re-cently completed a preliminary studythat:

� Assessed the current urban forest.� Modeled the changes in air pol-

lutants that could be expectedfrom five different planting sce-narios.

� Developed a tree planting planfor inclusion in the State Imple-mentation Plan.

For each scenario, 1 million treeswere planted by 2018 and only spe-cies known to be low emitters ofBVOCs were included; the five sce-narios varied according to the num-ber of new versus replacement trees.

Arborist

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WESTERN

Fall 2008

The resultsAir quality results varied accordingto the scenario. If the goal of the plant-ing is to reduce BVOC emissions, thenthe scenario in which all 1 milliontrees were planted as replacements forhigh emitters had the greatest impact,lowering BVOC emissions by about 1ton per day (tpd). On the other hand,this scenario had the least impact onother types of pollutants, as it merelyinvolved replacing fully grown treesthat are working to absorb air pollut-ants and provide shade with younger,and therefore, smaller and less effi-cient replacements.

The scenario in which all 1 milliontrees were new had the greatest over-all impact on air quality. In this case,BVOC emissions were increasedslightly by 0.13 tpd, but other pollut-ant reductions were substantial:� Nitrogen oxides: 0.24 tpd� Particulate matter: 1.2 tpd� Ozone: 1.5 tpd

Clean Air Act regulations requirethat the Sacramento metropolitan areareduce its VOC emissions by 12 tpdand NOx emissions by 21 tpd fromforecasted 2018 levels. The urban for-est-related changes projected herewould help meet air quality goals,achieving as much as 8% of the re-quired reductions in VOCs and 1.1%of the required reductions in NOx,depending on the scenario.

The new trees won’t just be clean-ing the air. They will also be provid-ing the residents of Sacramento andthe surrounding area with numerousbenefits: less spending for energy, re-duced atmospheric carbon dioxidelevels, cleaner stormwater, increasedproperty values, improved physicaland mental health, a greater sense ofplace, and a more beautiful environ-ment.

Greg McPhersonProject Leader, Research ForesterCenter for Urban Forest ResearchPacific Southwest ResearchStationUSDA, Forest Service, Davis, CA

Chinese pistache—Pistacia chinensis

CHINESE PISTACHE IS AN ATTRACTIVE TREE SUITABLE FORlawn, patio, garden, or street planting. It is a deciduous tree with amoderate growth rate to a height of 50 to 60 feet. Some varieties

are noted for their exceptional fall color. It can be grown in full sun and isadaptable to various moisture conditions. It is tolerant of lawn irrigationas well as of very little water in deep soils. Chinese pistache is resistant(not immune) to oak root fungus.

Insect pests:� aphids� leaf-feeding caterpillars: Obliquebanded leafroller, Western tussock

moth� soft scales: Black scale, Brown soft scale, European fruit lecanium,

Frosted scale

Disease pathogens:� Armillaria root rot, Root and crown rot� Botryosphaeria canker and

dieback� Leaf spot diseases: Alternaria

leaf spot, Alternaria blight� Powdery mildew, Gray mold� Verticillium wilt

Environmental disorders� mineral deficiencies� natural senescence� mineral imbalances� poor water management

Adapted from UC IPM On-line, Statewide Integrated Pest Manage-ment Program: How to Manage Pests: Pests in Gardens and Land-scapes—Landscape Plantshttp://www.ipm.ucdavis.edu/PMG/GARDEN/plantmenu.html