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 Strateg
|Contact: Jim Erickson|
University of Michigan