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What are the environmental costs and benefits of urban forests?
Tree planting programs are being implemented in many cities because of their potentially beneficial effects on local and global climate. Trees remove greenhouse gases from the atmosphere which can mitigate global warming, and may have a cooling effect on local temperatures through shading and the evaporation of water. However, the direct uptake of greenhouse gases by urban trees is very small compared to total greenhouse gas emissions from vehicles and energy generation in cities. In addition, urban trees have potential maintenance and environmental costs that include fuel usage, fertilizer application and associated pollutant emissions, and irrigation – the latter being particularly significant in semi-arid regions facing water shortages such as southern California. In fact, planting trees in semi-arid environments provides both environmental costs and benefits. There have been very few direct, comprehensive studies of these tradeoffs in urban forests, as most measurements of tree processes have been collected in natural ecosystems, where tree form and function may differ dramatically from urban forests. We are making direct measurements of water use, physiology, carbon sequestration, and plant-environment interactions of trees in the Los Angeles urban environment. By working with collaborators in economics, urban planning, and remote sensing, our goal is to use these data to quantify environmental costs and benefits of trees and the potential impacts of one million more trees in the city of Los Angeles.
We are also studying other types of urban landscaping throughout the LA Basin to quantify ecosystem services and environmental impacts of lawns, managed urban soils, and both native and imported vegetation. In addition to direct measurements of plant physiology and soil fluxes, we are measuring and mapping the stable isotope and radiocarbon composition of urban plants and soils. Isotopes are different forms of elements that provide information about a variety of biological and physical processes. For example, most carbon in the environment has a mass of 12, but there is a rare form of carbon with an extra neutron to form a mass of 13, and an even rarer form with a mass of 14. By measuring the ratios of rare to heavy isotopes of carbon and other elements we can calculate pollutant uptake in plants, as well as many aspects of plants physiology and how they are impacted by altered urban environments.
Our study sites include:
(click the links for photos)
