The Ecology of Cities
by Steven Crum, PhD Candidate, UC-Riverside
Dr. Darrel Jenerette’s Landscape and Urban Ecology Lab
At the Jenerette Lab, we are recruiting citizen scientists from coastal, inland, and desert southern California to help us gather data on park and street-side trees. Citizen scientists will collect leaf samples from cities around southern California to see how different urban environments affect trees. Climate change will impact urban tree species differently, as some trees do not prefer high heat environments. This project will help us understand which tree species will thrive in warmer environments, and which will provide the most cooling benefits. The sheer volume of samples that are need for this project would not be possible without the help of citizen scientists. Since we have received so much support for our efforts, I thought I would take the time to describe two areas of research in our lab that I find particularly interesting.
Cooling Benefits of Trees
Starting in the mid-20th century, large cities in the Unites States began heating twice as fast as surrounding rural and natural areas, especially in the southwest, mostly due to the urban heat island (UHI) effect. The UHI is created by increasing built surfaces and decreasing vegetation, creating higher temperatures in city centers compared to surrounding rural and natural areas. Rising urban temperatures are predicted to cause shifts in wildlife distributions, increases in pathogen and disease spread, increases in global greenhouse gas emissions, increases in energy consumption associated with air conditioning, negative impacts on human health from heat related illnesses, and deterioration of air and water quality. The UHI can be mitigated through vegetation, especially trees.
Over the past two years our lab installed around 500 small wireless temperature sensors, right, in street-side trees. Counterintuitively, we found that trees reduce urban air temperatures most at night. During the day trees shade built surfaces, including side-walks and streets. Since shaded surfaces absorb less heat in the day, they warm the air less at night. Additionally, this effect is not equally distributed across city neighborhoods in part because low-income areas have fewer trees.
This summer we will pair air temperature with surface temperature measurements in urban environments to see how built surfaces and trees affect the UHI. We will measure surface temperature with thermal infrared imagery, pictured right, which, I am almost certain, is the same technology used by the invisible creature in the movie Predator!
Urban Greenhouse Gas Emissions
Our lab is also investigating a phenomenon that has global-scale consequences, CO2 emissions. Greenhouse gas emissions, including CO2, are produced by a wide variety of sources. Human-caused emissions range from fossil fuel combustion to land-use change. Like its name implies, land-use change is the conversion of natural ecosystems into cities or agriculture—changing plant and soil composition. Because plants absorb CO2 and soils release CO2, land-use change hinders the environment’s ability to manage climate change. We have found that urban land-use conversion in southern California accounts for increases in soil CO2 emissions by 200% to 15,000%! Additionally, increasing temperatures with climate change will likely increase urban soil CO2 emissions more than that of natural ecosystems. On the other hand, drought will likely have larger emissions impacts on natural ecosystems, since they are dependent on rain and not irrigation inputs.
Cities are hot spots of environmental change. They alter land-use, biodiversity, green house gas emissions, and water and nutrient cycles. For urban residents global environmental change is magnified by changes in the local environment. We hope our efforts, and those of citizen scientists, will help society anticipate and mitigate for the changing ecology of cities.