Woodland Study: Canopy Thinning Treatments

The research component of the Arboretum's Woodland Restoration Project is vital to our success. Ecological restoration is a young field, and there are many things we don't yet know about how woodlands react to techniques currently in use. We must scientifically test these practices to be sure restoration efforts are effective. The research results can then be used to improve outcomes of future restoration activities.

In October 2007, in a 30-acre woodland northwest of the Big Rock Visitor Station (P-13) in The Morton Arboretum's East Woods, researchers began a long-term study to answer how increasing light levels through canopy thinning can impact the sustainability of oak woodlands. This research will assist regional land managers in understanding how much canopy thinning is required to improve biodiversity and woodland sustainability. Researchers want to determine:

• What is the impact of different thinning treatments on understory diversity and growth patterns of planted trees and shrubs, as well as their natural recruitment?

The first measurements will be taken in 2009, and then again in 2010.

Why this Research Matters

Oaks are the dominant trees within our region and play a major role in woodland and savanna ecosystems. Unfortunately, oaks are on the decline. They are not reproducing, gradually disappearing from our forests. We need to learn more about the optimal conditions oaks require so that we can help their recovery.

We know that oaks require more light than other tree species in order to regenerate naturally. Current restoration practices call for canopy thinning and controlled burning to increase available light. We aim to further understand the most effective methods to restore woodlands of our region. Some documentation exists on these techniques, but we are looking to advance this by coupling research and management together.

Methodology

This project aims to look at the effect of different thinning techniques on planted trees and shrubs as well as natural oak regeneration and ground layer diversity.  Four different thinning techniques (thin, gap, thin + gap, control) were replicated in 6 plots, with each plot consisting of three circular nested subplots.  Each subplot contained a 250 square meter treatment zone on the interior of a circular nested plot, a 8.9m buffer extension surrounding the treatment zone, as well as a 5m control zone surrounding the buffer zone. Plots were established in fall 2007 and GPS coordinates obtained for each center point. The three subplots were clustered to minimize landscape differences.

Thinning treatments took place in winter 2007-08 with ground crews from The Morton Arboretum when the soil was frozen to minimize soil disturbance. Thinning treatments were conducted in each of the 6 plots by felling trees to ground level within the 250 square meter treatment zone. A slab of each canopy tree was cut from ground level and stored at the Arboretum.

Planting of tree saplings (Quercus alba) and shrubs (Corylus americana) commenced in spring and fall 2008. Plant material was obtained from a local nursery which specializes in local source material. Two trees and two shrubs were installed in each zone of all subplots at random distances and bearings from the center point of each subplot.  All trees and shrubs within each zone needed at least 90 degrees of separation to minimize their effects on each other.  One tree and shrub from each zone will be protected from large mammals with 48" tall fencing.  Plants were given supplemental water when initially planted and during periods of low precipitation during summer 2008.  GPS coordinates of each tree and shrub will be obtained during winter 2008-09 when the GPS receiver can obtain signals through the tree canopy.  Growth measurements will be gathered by Arboretum staff and volunteers in March and November 2009 of each tree and shrub, along with mid and late season defoliation notes, and number of stems per shrub.

Invasive shrubs (Lonicera maackii, Rhamnus cathartica, R. davurica, R. frangula, Rosa multiflora) and vines (Celastrus orbiculatus) were removed by volunteers and Arboretum staff from 1996-2007 throughout the site, but aggressive control of resprouting stumps and seedlings throughout the entire site began in spring 2008 and will continue into 2009.  Control measures consist of grubbing and hand-pulling small woody, resprouting stumps and spraying herbicide (glyphosate and triclopyr) on cut stumps and stems of invasive woody species. Hand pulling of herbaceous invasive species, primarily garlic mustard (Alliaria petiolata), was initiated in spring 2008 with Arboretum staff and volunteers.

Restoration Resources:
Read about oaks in Tree Collections

The Arboretum woodland conservation researcher is Kurt Dreisilker, Manager of Natural Resources.