Got Nature? Blog

Walk through many forests in Indiana today, and you might not notice what is missing until you look up. Ash trees, once a common part of these woodlands, have rapidly disappeared due to the invasive emerald ash borer. Since its arrival in the early 2000’s, this small beetle has killed millions of ash trees across the state (Figure 1).

Figure 1: Dead ash tree killed by emerald ash borer

But what happens after ash trees are gone? Do new trees grow in their place, or do forests begin to change in unexpected ways?

To answer these questions, researchers revisited long-term forest monitoring plots across Indiana that were first established in the 1990s, before emerald ash borer was present. By comparing forest conditions before and after widespread ash mortality, we can begin to understand how these ecosystems are responding to one of the most destructive invasive forest pests in North America.

Even though young ash trees are still present in many forests, their numbers are dropping quickly (Figure 2). In some areas, ash seedlings have declined by more than 70 percent over the past decade. While a few saplings remain, very few are surviving long enough to replace the large trees that have been lost.

This suggests that ash is unlikely to return as a major component of Indiana forests anytime soon. As the remaining seeds in the soil are used up and fewer young trees survive, ash may become a much rarer species on the landscape.

As ash declines, other plant species are filling the gaps, but not always in the way we might expect.

Instead of fast-growing canopy trees taking over, many forests are becoming dominated by shade-tolerant species that thrive in low-light conditions. Native plants like spicebush and pawpaw are increasing in abundance, often forming dense layers in the understory. These species can make it difficult for other trees to establish and grow.

Sugar maple, another shade-tolerant species, remains common as seedlings but is not always successfully growing into larger trees. This creates a bottleneck where young trees are present, but few make it into the next stage of forest development.

At the same time, invasive plants are becoming more common in many forests following ash loss. Species such as bush honeysuckle and multiflora rose are increasing, especially in areas where ash trees have died.

These invasive plants can outcompete native vegetation, further limiting the ability of forests to regenerate diverse tree communities. Once established, they often persist and spread, creating additional challenges for forest management.

When large trees die, we might expect more sunlight to reach the forest floor, helping new trees grow. However, in many Indiana forests, the loss of ash is creating only small openings in the canopy rather than large gaps.

Figure 2: Ash regeneration in the understory.

These smaller gaps favor shade-tolerant plants rather than species that need full sunlight to thrive. As a result, forests are gradually shifting toward communities dominated by shrubs and slow-growing, shade-adapted trees.

Ecologists often refer to this process as mesophication, a transition toward cooler, shadier, and more moisture-loving forest conditions. Over time, this can lead to forests that are more uniform and less diverse.

The loss of ash is more than just the disappearance of a single group of trees: it is part of a broader pattern of change driven by invasive species, altered disturbance patterns, and shifting forest conditions.

Without intervention, many forests may continue to develop dense understories dominated by a small number of species, including invasive plants. This can reduce tree diversity, limit wildlife habitat, and alter how forests function over time.

While forests are resilient and will continue to grow and change, they may look very different from those of the past.

Landowners, land managers, and the public all play a role in supporting healthy forests. Monitoring for invasive species, promoting a diversity of native trees, and managing dense understory growth can help improve forest regeneration.

As research continues, understanding how forests respond to the loss of species like ash will be critical for guiding conservation and management efforts in the years ahead.

Resources:
Planting Your Tree Part 1: Choosing Your Tree, Purdue Extension – Forestry & Natural Resources (FNR) YouTube Channel
Tree Planting Part 2: Planting a Tree, Purdue Extension – FNR YouTube Channel
Find a Certified Arborist, Trees are Good-International Society of Arboriculture (ISA)
Video: Step by Step to Find an Arborist, Trees are Good-International Society of Arboriculture (ISA)
Professional Forester, Indiana Forestry Woodland Owners Association
District Forester, Indiana Department of Natural Resources, 10+ acres
Trees and Storms – The Education Store
Why Is My Tree Dying? – Got Nature? Blog, Purdue Extension – Forestry and Natural Resources
Tree Risk Management – The Education Store
Tree Pruning Essentials, Publication & Video, The Education Store
Tree Wound and Healing, Got Nature? Blog, Purdue Extension – FNR
What are Invasives Species and Why Should I Care?, Got Nature? Blog, Purdue Extension – FNR
New Hope for Fighting Ash Borer, Got Nature? Purdue Extension-FNR
Invasive Pest Species: Tools for Staging and Managing EAB in the Urban Forest, Got Nature?
Emerald Ash Borer, Purdue Extension-Entomology
Emerald Ash Borer, Animal and Plant Health Inspection Service (USDA)
Emerald Ash Borer Invasion of North America: History, Biology, Ecology, Impacts, and Management, Daniel A. Herms, Deborah G. McCullough. 2014. Annual Review Entomology. 59:13-30
Emerald Ash Borer Tools & Resources – Purdue Extension Entomology
Subscribe – Purdue Extension-FNR YouTube Channel

Dr. Mike Jenkins, Forestry & Natural Resources Professor
Purdue Department of Forestry & Natural Resources

Dr. Mike Suanders, Professor of Ecology and Natural Resources
Purdue Department of Forestry & Natural Resources