Several of our members and visitors to the Nygren Wetland Preserve have expressed concern about the removal of trees in the woods along Rockton Road. This area is designated as a “Mesic Savanna Enhancement Zone” on the restoration plan for the preserve. A savanna is defined as an area with less than 30% canopy cover (Barbour, et. al. 1999; Aber and Melillo 2001). The original savanna has grown into a closed canopy woodland over the years due to lack of fire and grazing. When the clock of succession is set back from a closed canopy to open savanna the change is dramatic. The time scale and scope required for this project is large, and the beginning stages will not be attractive or even organized.

The original management plan developed by Applied Ecological Services separated the preserve into different ecological zones based on data available from the original public survey notes, soil types, hydrology, and natural history of the area. With this information, a management plan was created by staff and approved by the Science Advisory Committee. Following the guidance of the “Management Plan for Savanna Restoration at Nygren 2009-2012”, staff and volunteers have been working on removing woody species not native to northern Illinois, and removing or girdling native tree species exhibiting invasive tendencies such as hackberry, black cherry, boxelder, slippery elm, and sugar maple. These species have the tendency to become “gap-fillers” in an ecosystem. They not only alter the light intensity reaching the forest floor, but also set in motion a shade dominant feedback loop (Martin et al. 2011). Oak species, which are mostly shade intolerant, require openings in the canopy to compete with the shade tolerant species in the understory (Cho and Boerner 1991).

In Spring/Summer 2008, no regeneration of oak or hickory were found within the mesic savanna enhancement zone prior to the opening of the canopy. Three years into the project and after removing hundreds of trees, staff and volunteers have found 26 oak seedlings and 3 hickory seedlings. All have been marked with blue tubes to ensure they are protected from further management activities and for monitoring purposes. In addition to these 29 natural regenerated seedlings, staff and volunteers have randomly planted twenty 10-inch potted bur oak and white oak seedlings in the areas that have been thinned.

It may seem counterproductive to be removing trees then planting more, but not only have we planted species in compliance with the management plan, but we also have introduced multiple successional age groups into the system. The potted oaks and shagbark hickory have a very well established root system, will be able to grow into the understory quickly, and have a better chance at filling canopy gaps as they become available.

The shrub system that appears to most as the “natural” understory was composed of bush honeysuckle, boxelder, and white mulberry, all of which are highly invasive species. The few native species found in the understory included a couple of dogwoods, a small patch of prickly ash, and one hazelnut, all of which remain. As of Fall 2011, the native shrubs are returning. Natural regeneration has included: hazelnut, grey dogwood, highbush cranberry, and elderberry. Each of these species is now found growing under the large oaks that are scattered through the savanna.

Standing dead timber is of high value, not just for multiple species of cavity nesting birds, but the ecosystem as a whole. The safety of visitors using the preserve is always the first priority for the management of any area, and standing dead timber is removed depending on its proximity to maintained trail systems. The mesic savanna enhancement zone has three of its edges bordered by trails: one trail is within the zone on the North side, and the East and West sides of the zone are bordered by trails in their entirety. Any tree that poses a safety issue must be removed.

Within the zone, there are 25 standing dead trees at this time. This number of snags more than meets the criteria for suitable habitat for cavity nesting birds. Although the number of snags per acre is dependent on the species of tree, size of tree, mode of tree death, rate or state of decomposition, and location within ecosystem, the USDA generally recommends an average of 3 to 7 snags per acre (Zack et al., USDA Forest Service General Technical Report 2002). Almost every one of the snags shows some sign of cavity use.

Bluebirds were seen nesting in the savanna this year, as was a barred owl. Most impressive was a woodcock found by staff and volunteers nesting among the woody debris on April 8, 2011. Workers leave debris on the ground as they clear the canopy, as long as it does not impede the safety of the work crews or trail users. In some instances, large logs have been moved temporarily, then returned to the site after work has been completed. Typically, only canopy tops and debris from cutting and sawing, is burned.

Keeping in mind that the composition of a savanna ecosystem is one with an understory of grasses and forbs, on June 1, 2011, a diverse seed mix was sown into areas that thinning has been completed. This seeding will take a few years to establish, and in the meantime, staff and volunteers will need to keep the raspberry and pokeweed mowed so as not to smother the young seedlings.

The Natural Land Institute staff, and volunteers are encouraged that so many citizens are taking an interest in the management of our protected properties. Our educated and trained staff welcomes feedback and appreciates comments from our members and visitors. We remind our loyal supporters that the mission of the Natural Land Institute is to “create an enduring legacy of natural land in northern Illinois for people, plants, and animals.”

In order to accomplish this mission, we must engage in restoration activities that may seem unorthodox, and at times, even counter-productive to the general public. But we assure all visitors to our properties that these activities are conducted with appropriate research, monitoring, and testing prior to their implementation. The future visitors of the Nygren Wetland will hopefully be able to walk among the oaks in the open savanna with native grass at their feet and enjoy the blooms of savanna flowers and the wildlife that associates with this incredibly diverse community.

Sources

Aber, John D. and Jerry M. Melillo. 2001. Terrestrial Ecosystems. San Diego, CA: Academic Press, a Harcourt Science and Technology Company.

Barbour, Michael G., Jack H. Burk, Wanna D. Pitts, Frank S. Gilliam, and Mark W. Schwartz. 1999. Terrestrial Plant Ecology. Menlo Park, CA: Addison Wesley Longman.

Cho, D.S. and R.E.J. Boerner. 1991. Canopy Disturbance Patterns and Regeneration of Quercus Species in Two Ohio Old-Growth Forests. Vegetatio, 93: pp. 9-18.

Dickson, James L., Richard N. Conner, and J. Howard Williamson. 1983. Snag Retention Increases Bird Use of a Clear-Cut. Journal of Wildlife Management, 47(3): pp. 799-804.

Martin, Katherine L., David M. Hix, and Charles Goebel. 2011. Coupling of vegetation layers and environmental influences in a mature, second-growth central hardwood forest landscape. Forest Ecology and Management, 261: pp. 720-729.

Zack, Steve, T. Luke George, and William F. Laudenslayer, Jr. 2002. Are There Snags in the System? Comparing Cavity Use among Nesting Birds in “Snagrich” and “Snag-poor” Eastside Pine Forests. USDA Forest Service Gen. Tech. Rep. PSW-GTR-181: pp. 179-191.