Emerald Ash Borer Week: What Ash Decline Teaches Us About Succession and Biodiversity Planning

Following Emerald Ash Borer (EAB) Awareness Week, we’re reflecting on the significant impact this invasive pest has had on our urban, rural, and community landscapes. But even more so, we're thinking about how we can leverage this moment to drive momentum toward building more resilient local ecosystems for our future. 

Across Peterborough, the Kawarthas, Northumberland and beyond, ash species have experienced rapid decline, including the endangered black ash, whose Ontario protections now focus on areas with significant EAB-related mortality. But what we are learning from this mass decline is not simply that we need to watch for the impacts of EAB; it is that we were too dependent on a single genus and, more broadly, on too narrow a range of species to achieve sufficient ecological resilience. 

Emerald Ash Borer

Ash trees have historically provided enormous value in the urban landscape. At one time planted en masse because they were reliable performers in difficult urban conditions, ash trees, until a few years ago, could often be observed as an arboriculture staple in urban areas. Not many trees could boast the ash’s resilience, and they served a critical residential function: shading driveways, screening views, stabilizing naturalized areas, and creating mature structure in both urban and rural environments.  

Now, as hundreds of failing ash trees continue to be removed locally, communities and property owners are left with a void where these trees once stood. Suddenly, too many mature, shaded properties are left bare, hot, and exposed. Curb appeal, everyday comfort, and even property values have been impacted, and the mature trees that once formed the foundation of so many local green spaces have disappeared. 

And as we’ve watched ash decline at scale over the past several years, the subject of our focus has changed. 

No longer: What needs to come down? 

But rather: What needs to be rebuilt? 

As a community, and certainly our team as industry providers, we have a unique opportunity now to consider what we are building and how to build back more resiliently. Not through mass planting of a single species, but through diverse repopulation based on site compatibility, ecological function, individual species resilience, and what we need our trees and forests to do for us, our communities, and our ecosystems over the next 50 years. 

Where Succession and Biodiversity Planning Begins 

Local rural forest planning - planting young conifers

For homeowners, landowners, and municipalities, rebuilding canopy resilience starts with understanding what is already on site. We can do this by examining the existing species mix, identifying overreliance on any one species or genus, and considering the functions the current trees provide: shade, screening, slope stabilization, habitat, stormwater support, or long-term structure. 

Resilient replacement planting strategies may include a mix of species, ages, and functions, and is guided by site compatibility and species suitability rather than availability alone. Soil, light, drainage, exposure, mature size, pest pressure, tolerance to changing climates, local species variability, and future maintenance needs can all influence whether a tree will succeed long-term. 

An easy starting point is to ask ourselves the following: 

• What species do we already have too much of? 

• What function are we trying to replace or build? 

• What conditions does this site actually offer? 

• What species will still make sense here 10, 30, or 50 years from now? 

• Where can we increase diversity and climate resilience without compromising suitability? 

Emerald Ash Borer is not the last ecosystem challenge we will face. We’re already seeing the impacts of other stressors, from invasive species and long-term climate shift, to drought and changing pest ranges. And across the board, we are seeing the same signals: low diversity creates vulnerability. 

If we plan carefully, today’s work is not just about removing what is failing. It is to rebuild urban and forested canopies that can shade, adapt, protect, and support our local communities for the next 50 years. 

Ethan

Owner, Elevated Arboriculture

Peterborough, Ontario