Ragweed pollen is one of the main causes of allergic rhinitis, the seasonally recurrent bouts of sneezing, nasal congestion and itchy eyes commonly called hay fever. There is also some evidence that allergenic pollen can trigger asthma attacks; in Detroit, the asthma rate is 50 percent higher than the rate for the state of Michigan as a whole. In Detroit, vacant lots are most common in low-income neighborhoods that have large minority populations. Katz and Carey say the results of their land-use study suggest that local production of ragweed pollen disproportionately affects such communities, which raises questions about environmental justice. The U-M studies found that vacant lots are the main habitat for ragweed in Detroit: Ragweed densities were six times higher in vacant lots than at locations around occupied homes in the city. Vacant lots contained up to 42 ragweed plants per square meter. When you consider that the tiny, drab flowers of a single ragweed plant can produce up to a billion pollen grains annually, it's clear that Detroit vacant lots are pollen factories churning out a noxious product that afflicts thousands of city residents. The vegetation survey looked at 62 vacant lots in several neighborhoods, including Mexicantown, Kettering and Core City. The researchers categorized each lot by mowing frequency: monthly, annually, biennially or unmowed. They found that 28 percent of the unmowed lots contained ragweed plants, while 63 percent of the annually mowed lots and 70 percent of the biennially mowed lots contained the weed. Vacant lots mowed at least once a month during the growing season had no ragweed plants. "When these lots are left alone completely, other plants rapidly outcompete ragweed," Katz said. When the lots are left unmowed, the common perennial plants that replace ragweed include goldenrod, milkweed, Kentucky bluegrass, chicory and aster. When a vacant lot is not mowed for several years, the young trees that can encroach include the Norway maple, silver maple, box elder, cottonwood and tree of heaven. To determine whether ragweed populations are associated with a particular land-use type in Detroit, Katz and Carey documented the amount of ragweed found in vacant lots, around occupied residences and in city parks. They also set up pollen collectors at 34 sites and analyzed the concentration of ragweed pollen grains collected at each location. They found that the amount of ragweed pollen in the air at a collection site was determined both by the abundance of ragweed plants within 10 meters (33 feet) of the collector and by the number of vacant lots within 1 kilometer (0.62 miles) of the site. That result drives home the point that pollen is mainly a local and neighborhood-level problem, even though public health officials have for decades treated it as a regional problem, Katz said. Though individual pollen grains occasionally travel hundreds of miles, previous studies have demonstrated that more than 90 percent of pollen grains travel less than 100 meters from the source plant. "Because pollen grains can travel long distances, sometimes people make the mistake of assuming that it usually does travel long distances," he said. "Our Detroit study shows that ragweed pollen is a local problem, and that's important because it means we can make local management decisions about how to reduce exposure." Leaving vacant lots unmowed and allowing some of them to reforest would reduce the allergenic pollen exposure of Detroit residents, according to Katz and Carey. Although reforestation is controversial, it is advocated by the Detroit Future City Strategic Framework, a collaboration between the city of Detroit, the Kresge Foundation and the Detroit Economic Growth Corp., among others. The U-M studies were supported by the Matthaei Botanical Garden, a Graduate Research Fellowship from the National Science Foundation, and an Ecological Society of America SEEDS Undergraduate Research Fellowship (funded by the National Science Foundation).
MEDIA CONTACT
Register for reporter access to contact detailsCITATIONS
Urban Forestry and Urban Greening