Canopy Thinning

Can thinning the forest canopy to increase light levels on the forest floor improve biodiversity and make oak woodlands more sustainable? Researchers at The Morton Arboretum and their collaborators are testing the effectiveness of different canopy thinning treatments to help guide restoration ecologists and land managers in managing woodlands for oak dominance. -term

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. It is the research component of the Arboretum's Woodland Restoration Project. This research will help regional land managers understand how much canopy thinning is required to improve biodiversity and woodland sustainability.

Researchers want to determine the impact of different thinning treatments on understory diversity and growth patterns of planted trees and shrubs, as well as their natural population increase.

Why this Research Matters
Oaks are the dominant trees in the region of The Morton Arboretum 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. We aim to further understand the most effective methods to restore woodlands of our region. Current restoration practices call for canopy thinning and controlled burning to increase available light. Some documentation exists on these techniques, but we are looking to advance this by coupling research and management together.

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, each consisting of three circular nested subplots.  Each subplot contained a 250-square-meter treatment zone inside a circular nested plot; a 8.9m buffer extension surrounding the treatment zone; and 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.

The canopy was thinned in 2007-08 by ground crews from The Morton Arboretum in the winter, when the soil was frozen, to minimize soil disturbance. In each of the 6 plots, trees were felled 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 for reference and future research.

In spring and fall of 2008, white oak saplings (Quercus alba) and American hazelnut shrubs (Corylus americana) were planted. The plants were obtained from a local nursery that 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 was protected from large mammals with 48-inch-tall fencing. 

The plants were given supplemental water when first planted and during periods of low precipitation during the summer of 2008.  GPS coordinates of each tree and shrub were obtained during winter 2008-09 when the GPS receiver could obtain signals through the tree canopy. In March and November of 2009, Arboretum staff and volunteers measured the growth of each tree and shrub, made notes on with mid- and late season defoliation, and counted the stems on each shrub.

Invasive shrubs (Lonicera maackii, Rhamnus cathartica, R. davurica, R. frangula, and Rosa multiflora) and vines (Celastrus orbiculatus) had been removed from the site by volunteers and Arboretum staff between 1996 and 2007,  but aggressive control of resprouting stumps and seedlings began in spring 2008 by Arboretum staff and volunteers.  Control measures consist of grubbing and hand-pulling new sprouts from the stumps of small woody plants; spraying herbicide (glyphosate and triclopyr) on cut stumps and stems of invasive woody species; and hand-pulling herbaceous invasive species, primarily garlic mustard (Alliaria petiolata).

Read about oaks in Tree Collections
The Arboretum woodland conservation researcher is Kurt Dreisilker, Manager of Natural Resources.
Learn more about The Morton Arboretum Forest Ecology Lab