These areas, while once comprised of native soil and vegetation, have become "brownfields", generally only supporting less desirable plants like thistle, nutsedge, knotweed, and crabgrass.
Brownfields are generally created over a long period of time by various human activities (construction, development, chemical pollutants, etc.). The soil in this are is altered in such a way that many trees have a difficult time establishing. These sites are used by urban foresters to identify tree species and varieties that will adapt to even the harshest of urban conditions.
The following map (created with ArcGIS) gives an overview of the recently planted brownfield site in Saint Paul. Located on the southwest corner of Phalen Blvd. and Payne Ave., this site is a collaborative research project between the Saint Paul Park & Recreation's Forestry Division and the University of Minnesota's Urban Forestry & Horticulture Research Institute.
Many of the species planted at this site are of interest in other areas of Saint Paul and Minneapolis, representing possible replacements for threatened trees like ash.
Others are tried-and-true species that have worked on other locations but never put to the test in a place like this!
If trees survive (and thrive!) in this location they become prime candidates for use throughout the city, especially on tough disturbed sites.
Watch this site for updates as the project progresses!