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You are here: NRS Home / Research Programs / Forest Disturbance Processes / Invasive Species / Emerald Ash Borer / Effects and Impacts / Assessing the role of riparian forests on EAB dispersal
Emerald Ash Borer

Assessing the role of riparian forests on EAB dispersal

Research Issue

Ash is a major component of Northern hardwood forests. In the 24-state region covered by the Northern Research Station (NRS), Forest Inventory and Analysis program (FIA), there are approximately 5.7 billion ash trees (greater than 1 inch in diameter), which account for 7 percent of all hardwood species found on forest land. The predominance of ash throughout the NRS makes the EAB a major threat to the health, structure, and composition of the region’s forests. Understanding patterns of EAB dispersal is an important part of early detection, risk assessment, and mitigating the impacts of this exotic insect. Michigan, in addition to being the origin of the North American EAB infestation, has the largest number of ash trees in the NRS. Over 50 percent of Michigan’s ash trees are located in riparian forests. Riparian forests, which are often high in species diversity, provide many ecologically important benefits; however, they are also very vulnerable to disturbance. American elm, once found alongside ash, has largely been removed from Michigan’s riparian overstory as a result of the virulence of Dutch elm disease. EAB has the potential to have a similar effect on ash species. Since ash is still a dominant feature of riparian forests, there is a need to study the potential effects of EAB on riparian ash and the potential effects of riparian ash on EAB dispersal. Identifying the concentration, distribution, and arrangement of riparian ash, as well as its ability to support EAB populations will be important in evaluating the role of riparian forests in the dispersal of EAB.  This research is based primarily on study sites in Michigan’s Southern Lower Peninsula (SLP).

 Our Research

First, the distribution of riparian ash forest types in Michigan’s Lower Peninsula was mapped using U.S. Fish and Wildlife Service, National Wetlands Inventory (NWI) data and a land cover dataset developed for the Lower Peninsula (IFMAP). Land cover data identified a gradient of increasing forest fragmentation from north to south. As riparian forests may play the greatest role in the dispersal of EAB in areas with higher fragmentation, the SLP became the focus of the study. FIA data was then used to estimate the abundance of ash in the SLP, in terms of biomass; biomass was compared by physiographic classes (riparian vs. upland). To identify the proximity and spatial arrangement of riparian and upland ash forests, forest area (more specifically, the arrangement of forest patches) in 3 SLP watershed subbasins was analyzed and compared using fragmentation indices. The primary indices used were designed to measure the degree of forest patch isolation and forest patch connectedness. Lastly, measurements of tree diameter, taken from FIA plot data, were used to estimate ash phloem area and determine the potential number of EAB adults that could be produced per m2 of phloem area (production potential). EAB production potential was compared for riparian and upland forests in the SLP. 

Expected Outcomes

Results from this study provide evidence that, in comparison to upland forests, riparian forests are more likely to contain EAB infestations, influence the direction and rate of EAB dispersal, have higher EAB population densities, and experience greater impacts due to EAB infestation. These results will aid in enhancing detection methods, risk assessments, and in effectively allocating limited resources and management efforts. 

Research Results

  1. In comparison to the Northern Lower Peninsula, forest land in the SLP is highly fragmented. However, the SLP has a higher concentration of ash biomass and a wide distribution of riparian forest types that contain ash. Many riparian ash forest types were oriented in narrow, sinuous bands. While riparian ash forest types make up a small percentage of total forest area in the SLP, ash biomass was more likely to occur on riparian plots than on upland plots. Riparian plots also contained twice as much mean biomass per hectare.
  2. Analysis of forest fragmentation indices showed that forested riparian ash patches were less isolated and less fragmented than upland patches, i.e. riparian forests were closer together and more highly connected to one another than upland forests were to other upland forests. Thus, the spatial distribution and pattern of riparian ash abundance in the SLP may influence the direction and rate of EAB spread by allowing EAB to quickly increase dispersal along narrow, connected riparian corridors that contain high ash biomass.
  3. Riparian ash forests were found to have a significantly greater potential to produce adult EAB than upland ash forests. A higher degree of species diversity (i.e., non-ash hardwoods) in upland forests is likely to contribute to the lower total production potential found among upland ash forests. In general, riparian forests contain more ash than upland forests and have more phloem area. Greater phloem area increases the potential tree area available to support the development of EAB. Higher total production potential in riparian forests indicates that EAB has a greater reproductive ability in riparian forests. EAB-induced damage may be more significant within riparian forests since populations of EAB may build more rapidly and spread more quickly. Therefore, the rate of EAB spread may be greater in riparian forests.

    In addition to physiographic class (i.e., riparian vs. upland), total EAB production potential was also influenced by size class. Medium- and large-diameter stands, where the majority of stocking is in trees larger than 12.5 cm, have more available phloem area. As a result, these stands can produce more EAB compared to small-diameter stands. EAB presents a significant risk to medium- and large-diameter stands, as these stands are areas of potentially high EAB density.

Crocker, Susan J. 2006. An assessment of the connectivity and susceptibility of riparian ash in the Southern Lower Peninsula of Michigan:  Implications for the dispersal of the
emerald ash borer, Agrilus planipennis (Coleoptera: Buprestidae). Syracuse, NY: State University of New York, College of Environmental Science and Forestry. 57 p. M.S. thesis.

Crocker, Susan J.; Moser, W. Keith.; Hansen, Mark H.; Nelson, Mark D. 2007. The spatial distribution of riparian ash: implications for the dispersal of the emerald ash borer. In: Mc Roberts, Ronald E.; Reams, Gregory A.; Van Deusen, Paul C.; McWilliams, William H., eds. Proceedings of the seventh annual forest inventory and analysis symposium. Gen. Tech. Rep. WO-77. Washington, DC: U.S. Department of Agriculture, Forest Service. 155-160.

Research Participants

Principal Investigators

  • Susan Crocker, Entomological Specialist, U.S. Forest Service, Northern Research Station

Research Partners

  • Mark H. Hansen, Research Forester, U.S. Forest Service, Northern Research Station
  • Mark D. Nelson, Research Forester, U.S. Forest Service, Northern Research Station
  • W. Keith Moser, Research Forester, U.S. Forest Service, Northern Research Station
  • Deborah G. McCullough, Michigan State University
  • Nathan W. Siegert, Michigan State University

Last Modified: 01/08/2013