Impervious Surfaces and Vegetation

Connecticut Studies

The Connection Between Impervious Surface and Vegetation

Editor’s note: This research was conducted as part of a Doctoral Dissertation by Yale student Richard Karty, with support provided by the Connecticut Sea Grant College Program. When his thesis is turned into a journal article, we will provide the paper on the Literature portion of this site.

The amount of impervious surface in a region can serve as a shorthand or surrogate measure not only of water quality, but of other aspects of ecosystem health as well. For example, a study in New Haven County showed that weedy, invasive and non-native plant species are mostly restricted to places having a high proportion of impervious surface, and conversely, that plants typical of relatively undisturbed forest interiors are restricted to areas with little impervious surface. This is something that everyone knows intuitively—dandelions aren’t found deep inside state parks, and orchids don’t grow in parking lots—but until now it hadn’t actually been measured with respect to impervious surface area.

The threshold level of impervious surface above which the forest interior species become rare was found to be around ten percent. This is remarkably similar to the threshold of impervious surface for water quality degradation, as indicated by a number of national studies. And above a mere seventeen percent impervious surface area, weedy plants are predominant to the exclusion of the more sensitive species. This can be seen in the graph below, which shows the average amount of impervious surface in which each species was found.

The average amount of impervious surface in which each species was found.

Typical suburban Connecticut sites of low, medium and high impervious surface area can been seen in the photographs: a narrow road in a heavily wooded suburb (right), wide swaths of road and driveway in a less-wooded subdivision (below), and a tiny streamside fragment in a parking lot (below right).