Principal investigators: Michael Saunders, professor, Forestry and Natural Resources, Purdue University (msaunder@purdue.edu); Michael Jenkins, professor, Forestry and Natural Resources, Purdue University
Co-authors: Christopher Webster, professor, College of Forest Resources and Environmental Science, Michigan Technological University; Robert Morrissey, owner, Branch Scientific Editing; Thaddeus Swart, research assistant, Forestry and Natural Resources, Purdue University
Emerald ash borer (EAB; Agrilus planipennis), a jewel beetle native to northeastern Asia, has led to the decline of ash species (Fraxinus spp.) in forests throughout the Central Hardwood Forest Region. Using long-term datasets predating EAB introduction, researchers aim to identify forest responses to overstory species loss resulting from emerald ash borer. This study explores the impacts of ash mortality on forest regeneration and how these shifts in regeneration may contribute to future structural and compositional changes.
In 1996-97, permanent monitoring plots were established across Indiana state parks and nearby reference areas on north-facing slopes of mesic, closed canopy, hardwood forests. These locations were remeasured in 2010-11 and in 2023. Species level data were collected for the seedling, sapling and overstory layers on plots in parks that contained ash trees in 1996-97.
Species responses were found to be highly variable across parks. Blue ash (Fraxinus quadrangulata) has largely survived attack by EAB and was widely present as living trees throughout this study. One positive outcome is that invasive species were not as common as expected in the seedling layer (Figure 1).
While considerable ash regeneration was observed within the seedling layer in the 2010-11 resample, in 2023, the recruitment of those seedlings to the sapling layer was found to have largely failed. Instead, shade-tolerant and invasive species have become established in the sapling layer, with few ash seedlings successfully recruited into the larger size class (Figure 2).
The seedling layer had variable composition and density across parks but generally contained high densities of ash and sugar maple (Figure 1). Most parks were dominated by the same groups of species in 2010-11 and 2023 and show only slight deviation between sampling periods, except for Potato Creek State Park.
Analysis suggests that ash may become functionally extinct in Indiana, even in late successional forests. Early findings prompted further investigation into the implications of ash species loss on the ecological health of closed canopy hardwood forests.
In 2024, researchers concentrated on documenting the ecological impacts of emerald ash borer (EAB) on the Davis-Purdue Research Forest (DPRF), a 21-hectare relict forest remnant located in eastern Indiana, where EAB has caused widespread mortality in canopy ash trees since 2012, drastically altering stand dynamics and threatening biodiversity. Work focused on determining the fate of EAB-induced canopy gaps and how that disturbance may be influencing tree regeneration and overall species composition.
Findings indicate that shade-tolerant species such as Acer saccharum (sugar maple) dominate the understory, while historically dominant canopy species, including oaks and ash, are rarely regenerating. Gap dynamics, influenced by factors like gap size and decay class, revealed trends in lateral infill and seedling density, though some results were statistically insignificant.
This study underscores a shift toward shade-tolerant species, particularly those that grow in environments with moderate moisture levels, in response to EAB-induced disturbance. This shift has implications for long-term forest composition and management.
Despite similarities to other invasive pests, such as Dutch elm disease and chestnut blight, EAB impacts are unique in their near suppression of all ash, contrasting with elms’ ability to persist in smaller size classes.
“This research demonstrates the long-lasting ecological changes invasive species can induce,” primary investigator Mike Saunders said. “This emphasizes the need for further study to guide forest management and conservation strategies in the face of such disturbances.”
Goals:
Using long-term datasets predating EAB introduction, researchers aimed to identify the response of forests to overstory species loss resulting from emerald ash borer. The study explored the impacts of ash mortality on forest regeneration and how these shifts in regeneration may contribute to future structural and compositional changes.
Methods:
Permanent monitoring plots were established across Indiana state parks and reference areas in 1996-97 on north-facing slopes of mesic, closed canopy, hardwood forests. These were remeasured in 2010-11 and in 2023. Species level data were collected for the seedling, sapling and overstory layers on plots in parks that contained ash trees in 1996-97.
In 2024, researchers investigated EAB-induced canopy gaps in the Davis-Purdue Research Forest (DPRF) using data from prior censuses and field measurements. All ash trees over 60 centimeters diameter at breast height were mapped and canopy gaps were measured using the Runkle ellipse method. Quadrats were established at 5-meter intervals to assess seedling and sapling density. Gaps were classified by size, single- or multi-tree nature and decay class of dead ash trees. Spatial data were collected via drone imagery and analyzed with ArcGIS. Linear models assessed the relationships between gap size, regeneration, and edge tree basal area. Statistical analyses included ordination and similarity testing.
Key Findings in 2024:
Findings indicate that shade-tolerant species such as Acer saccharum dominate the understory at Davis-Purdue Research Forest, while historically dominant canopy species, including oaks and ash, are rarely regenerating. Gap dynamics, influenced by factors like gap size and decay class, revealed trends in lateral infill and seedling density, though some results were statistically insignificant.
Partners/Collaborators:
- Indiana Department of Natural Resources
- Davis Purdue Agricultural Center for accommodating researchers during field work activities
- Christopher Webster, professor, College of Forest Resources & Environmental Science, Michigan Technological University; and Robert Morrissey, owner, Branch Scientific Editing, who provided advice and historical data
- Jinha Jung and the Institute for Digital Forestry for access to the UAS-based imagery for the site
- Thaddeus Swart, research assistant, FNR, Purdue University