Healthy Forests to Resist Invasion

This work was supported by a grant from the Legislative-Citizen Commission on Minnesota Resources (M.L. 2010, Chp. 362, Sec.2, Subd. 6c.) "Healthy Forests to Resist Invasion"

Description and Background

In Minnesota, invasive plants cause considerable ecological and economic damage, and their control is difficult to achieve in a long-term cost-effective manner. Although not immune from invasion, healthy forests may be somewhat resistant to invasion; therefore management aimed at maintaining, restoring, or enhancing key forest characteristics might be a useful strategy for slowing forest invasion. This type of preemptive tool could help maintain diverse forest systems and might be cheaper and more effective in some instances than trying to remove invaders after they are present. Consequently, the goal of this research was to better understand whether forest characteristics, especially those amenable to management, can be effective deterrents to plant invasion. To accomplish this goal we established 67 forest study sites to assess invasive plant species and a set of key indicators relevant to invasion, including disturbance history; degree of tree canopy cover; native plant diversity; levels of light and soil resources. This was done in order to determine links between forest attributes and plant invasion, and attempt to discern cause and effect. Information learned in this study can aid in the development of land management prescriptions that incorporate the current invasive status of the plant community and the health and integrity of the ecosystem, which will serve as an indicator of vulnerability to invasion.

Location of Sites

During the first field season of the project (summer 2010), we identified ten sites using GIS data from the Minnesota DNR (Forest Inventory Module and Minnesota Biological Survey) and discussions with DNR area foresters and private landowners. By the end of the final field season, we surveyed a total of 67 sites with a balance between stands with no recent disturbance (38 sites), sites with recent timber harvest (17 sites), and sites with recent invasive species management (12 sites). This total gave us a substantial number of data points from which we could examine the potential effects of site characteristics on invasibility.

At each of the 67 survey sites we established 16 sample plots in a 4 four by four grid. Each sample plot was 10 m in diameter and the plot centers were 15 m apart. Within each 10 m plot we identified and measured the diameter of all woody species (trees and shrubs) that were at least 1.3 m tall. At the center of each 10 m plot, we established a subplot with a diameter of 2 m. In this subplot, we identified and estimated cover of all smaller trees and shrubs (<1.3 m tall), and also all herbaceous plants.

Within each forest stand, the 16 sample plots were set up in the first area encountered that had a relatively homogenous topography and was large enough to encompass the four by four grid. In each plot we collected data on plant species identity and cover for all native and introduced trees, shrubs, and herbaceous plants. We also collected voucher specimens for all species encountered and deposited these in the Bell Museum Herbarium at the University of Minnesota. In addition, we measured light levels, duff layer thickness, the amount of bare soil, slope, aspect, and earthworm presence. Soil samples were also collected for analysis in the laboratory for pH, soil texture, and nutrients

After the field surveys were complete, it was obvious that the most frequent and abundant native plant species in the oak forests we focused on was common buckthorn (Rhamnus cathartica). As a result, we focused our analyses on common buckthorn since we consider this to be the most pernicious invader of forests in southern Minnesota at this time. Introduced honeysuckle (Lonicera sp.), garlic mustard (Alliaria petiolate), barberry, (Berberis thunbergii), and multiflora rose (Rosa multiflora) were present in a few sites but they were rarely abundant.


The field surveys suggested that the most important site characteristics in determining the abundance of buckthorn were light levels, leaf litter, and evolutionary diversity of the resident plant species. Buckthorn abundance was lowest in sites with lower light, more leaf litter, and higher diversity.

Given these findings, pre-invasion management to reduce invasibility should maximize diversity to leave less space for invaders. One way to do this is by controlling the density of white-tailed deer, a species that has severe impacts on plant growth when populations are high. Another is to limit disturbance by using winter timber harvest and low impact trail construction techniques that minimize soil disturbance and keep sunlight relatively low. Maintaining an intact litter layer also appears to be particularly important.

Seed availability is another important factor in determining the spread of invasive species. We documented a strong positive relationship between landscape scale "propagule pressure" and local buckthorn abundance.

Reducing common buckthorn seed sources is a promising first step in successful pre- and post-invasion management. Pre-invasion, reducing seed availability is particularly important where common buckthorn is not yet abundant in natural areas but present in nearby towns. A community effort to remove common buckthorn from residential areas would be one of the most effective ways to slow its spread into surrounding forests. Post-invasion, focusing first on removal of mature adult plants before removing smaller individuals will improve the success of a management program. Collaboration with neighboring landowners is necessary since any attempt to remove buckthorn from an individual forest stand will ultimately fail without reducing seed availability in the surrounding area.

All data from the 67 field surveys are in a relational database. The database has a table for site descriptions that includes latitude/longitude coordinate, disturbance history, stand size, earthworm status, soil pH, soil texture, the amount of bare soil, leaf litter depth, duff depth, and light levels. There is also a table with survey plot descriptions (that includes abundance data for each species of tree, shrub, and herbaceous plant) and another with abundance of invasive plants across the landscape around the survey site (estimated from driving surveys). Plant taxonomy follows the Bell Museum of the University of Minnesota Herbarium and is accessed via a drop down menu. Other drop down menus access tables for other aspects of site and plot level descriptions. The structure of the database allows for custom queries that can extract data in various forms for analysis. This database can be used by managers to determine the abundance of buckthorn in survey sites close to a particular forest stand of interest. If you are interested in this database, please email

Workshop Presentations

On August 14, 2013, we hosted a symposium to bring together managers and researchers to share the latest information on invasive plants in Minnesota forests. In addition to talks based on this LCCMR project, other speakers presented information about buckthorn invasion on the prairie-forest border in west central Minnesota, garlic mustard (another common plant invader in Minnesota's forests) as a driver of species invasion, management of buckthorn from a foresters perspective, and management efforts to control other common invasive plants.

Links to YouTube video recordings of all the symposium talks can be found below:

Laura Van Riper, Terrestrial Invasive Species Coordinator, MN DNR, 'The Minnesota DNR's long-term perspective on garlic mustard and buckthorn prevention and management'

Peter Wyckoff, Associate Professor of Biology, UofM-Morris, 'European buckthorn invasion: insights (and incitement?) from West Central Minnesota'

Alex Roth, PhD, UofM, 'It takes two: explaining the success of buckthorn and earthworms in Minnesota's deciduous forests'

Sascha Lodge, PhD, UofM, 'From the hedgerows to the forests: Seed availability is more important than disturbance history'

Laura Phillips-Mao, Postdoctoral Research Associate, UofM, 'Garlic mustard as a "back-seat driver" of change in Minnesota's woodlands – implications for restoration & management'

Paul Kortebein, Senior Manager for Forestry & Horticulture, Three Rivers Park District, 'Keeping Murphy-Hanrehan Park Reserve wild (but not that wild)'

John Moriarty, Senior Manager of Wildlife, Three Rivers Park District, 'Buckthorn Battle at Battle Creek'


Roth, A., T. Whitfeld, A. Lodge, N. Eisenhauer, L. Frelich, P.B. Reich. 2014. Invasive earthworms interact with abiotic conditions to influence the invasion of common buckthorn (Rhamnus cathartica) Oecologia 10.1007/s00442-014-3175-4

Whitfeld, T., A. Lodge, A. Roth, P.B. Reich. 2014. Community phylogenetic diversity and abiotic site characteristics influence abundance of the invasive plant Rhamnus cathartica L. J. Plant Ecology 7: 202-209.

Whitfeld, T., A. Roth, A. Lodge, N. Eisenhauer, L.E. Freich, P.B. Reich. 2014. Resident plant diversity and introduced earthworms have contrasting effects on the success of invasive plants. Biological Invasions 16: 2181–2193.


Information on other aspects of the 'Healthy Forests' project are linked below, along with further information, resources and outcomes of the project.