Riverwoods Preservation Council

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 Protecting the Woodlands
 RPC Woodland Health Study Progress  Report


 March, 2009

 

 

(Photo courtesy of Greg Mancuso)


In September 2008 the RPC collected field data on trees, shrubs, and perennials on fourteen Village sample sites in partial fulfillment of the Riverwoods Woodland Health Study. This study is sponsored by the RPC with matching contributions from the Village. The fourteen sample sites used in the study are on both public and private properties and were selected to be representative of the historically wooded portion of the Village.

What are our preliminary findings?

Shrubs Our study found almost a complete absence of shrubs taller than one meter. Those that we did find were considered invasive, such as tartarian honeysuckle, or were a result of active management by the property owner. Absent from the field data were instances of many shrub species known to be native to this area – dogwood, viburnum, choke cherry, and hazelnut – shrubs that provide food and habitat for birds and other animals.
Bare Ground Percent bare ground is good way of visualizing the habitat and is an easily understood measure of ecosystem health. Bare ground is the absence of plant growth immediately above the ground layer. Generally, the more bare ground, the more degraded is a site. Of fourteen sample sites twelve showed a percent bare ground of 60-90% – a clear indication of the scarcity of ground layer plants and shrubs. Significant bare ground and an absence of shrubs is often described as a loss of structure – rather than the woods being composed of canopy, saplings and small trees, shrub, and ground-level tiers it is primarily composed primarily of just one, canopy. This loss of understory appears to be a result of the closing of the canopy with consequent loss of ground layer light, and may be compounded by deer browse.
Oak to Maple As observed elsewhere in the Chicagoland area, the oak/hickory woodlands are changing – oaks are not regenerating in the Village’s historic oak woodlands while other species are becoming more dominant. This is indicated by an almost complete absence of oak saplings in the eleven oak sample sites. They are being replaced primarily by sugar maple, hop hornbeam, elm, and ash. The causes most often attributed to this common phenomenon are: loss of fire, closing of the woodland canopy and resulting increase in shade, over-abundance of white-tailed deer, and overall cooler and wetter conditions compared to those observed several hundred to several thousand years ago.
Floristic Quality Floristic Quality is a measure used by ecologists to determine the relative diversity and quality of a natural area such as woodland. Floristic Quality is quantified using the term Floristic Quality Index (FQI), and in this study we applied FQI to the perennial (herbaceous) plants on the woodland floor. Of the fourteen woodland sample sites three rated an FQI of ‘good’. These sites are all located on property adjacent to the Des Plaines River. The FQI of another three sites rated ‘poor’, the remaining eight rated ‘fair’. This degradation is typical of woodlands throughout the Chicagoland area.

What’s next?
We haven’t finished our study. In May we will collect more field data, but this time we will document the diversity and abundance of spring plants, especially spring ephemerals. These plants are less sensitive to canopy closure because they do their active growing before canopy trees leaf out. These plants are also useful in another way – white trillium, in particular, are considered an indicator species for browse habits of white tail deer. Patterns of growth of white trillium may suggest the level of browse.

In addition we will measure available light on the canopy floor. Preliminary results indicate that loss of light is a major factor in limiting understory plant growth. Light measurements will enable us to compare our observations with studies written by ecologists.

UPDATE: The first phase of the Woodland Health Study was completed after this article was published. You can see a PDF version of the report if you Click Here. For more discussion of woodland health, Click here.

© Riverwoods Preservation Council- - Page last updated: December 2009