In late 2008, the Riverwoods Preservation Council (RPC) embarked on a woodland health study, working with outside experts. The expense of the study was shared equally between RPC and the Village of Riverwoods. Individuals from the Audubon Society and Lake County Forest Preserve, as well as RPC volunteers, offered advice and helped collect field data. The study found that Riverwoods’ woodlands are suffering in several respects.
We selected six properties south of Deerfield Road and two properties north of Deerfield Road. The properties included private and publicly owned property, with some properties fenced to exclude deer. The properties also varied by degree of invasive species management. We chose properties that are typical of much of our woodlands.
Within the eight properties, we staked out fourteen “sample plots.” Our field researchers counted the trees, shrubs and understory plants found within each plot, as well as larger trees located nearby. We collected data from the plots in the fall of 2008 and the spring of 2009.
Our consultant, Applied Ecological Services, assessed each plot based on the number and variety of plants found and classified the plot by type of woodland. We used nearby Ryerson Woods for comparison, since it has similar conditions.
What is a Healthy Woodland?
A healthy woodland consists of four layers: the tallest and most mature trees (the canopy); a lower layer of trees (the subcanopy); shrubs; and vegetation covering the ground (understory plants).
Canopy and Subcanopy: The canopy comprises a variety of trees, including red oak, white oak, swamp white oak, sugar maple, green ash, basswood, American elm and hackberry. Sugar maples appear to be the most dominant tree species in the subcanopy, other than along the Des Plaines River. Young oaks and oak saplings are conspicuously absent from the subcanopy, meaning that over time oaks may disappear from the canopy. The likely causes are deer browse and shading from the canopy.
Shrubs: Overall shrub density as low, with reduced diversity in every study plot. The reduced shrub layer is likely the result of deer browse.
Understory: Ecologists use the term “floristic quality” when they speak about site quality. Understory floristic quality is based on the number, value and abundance of species. Using the formula utilized by ecologists to determine floristic quality, the plots were found to vary from very high quality to poor quality.
Additional factors of concern in the understory are the high percentage of bare ground – no plants – and the extremely low light levels. Chicago Wilderness and the Cook County Forest Preserve find that less than 15% bare ground indicates high quality areas. We found 60%-90% bare ground in a majority of the plots in Riverwoods. Scientific literature indicates that light levels on the ground below 10-20% significantly reduce growth and species composition of understory vegetation in woodlands. Twelve of the 14 plots in Riverwoods exhibit light levels below 10%.
We found understory plant species similar to those found at Ryerson, but significantly fewer specimens. The causes seem to be deer browse, shading, soil chemistry alterations caused by common buckthorn and absence of fire as a management tool.
RPC developed proposed management practices based upon the results of this study and presented its recommendations to the Village Board of Trustees.