Beneath the towering beeches, wintergreen-scented birches, and bright red fall maples that dot the canopy of a northern hardwood forest, a vast array of plants makes up the understory. When land managers implement conservation practices aimed at tree health, the understory can be changed in ways that affect the forest and the services it provides.
With this in mind, a team of researchers from Canada’s Laurentian Forestry Centre investigated how soil management impacted the understory community of a northern hardwood forest. Their research was published in a September 2024 volume of Forest Ecology and Management. The team found that mixing mineral-rich soil toward the surface through scarification reduced understory diversity, stability, and resilience. These negative effects were even greater when they used scarification alongside soil amendments such as fertilizer, lime, and ash. The study, titled “Changes in understory plant communities following soil scarification and amendments in a Northern Hardwood Forest,” highlights some potential drawbacks of well-intentioned practices, providing a glimpse into the challenges of holistic ecosystem management.
Soil management practices including scarification and soil amendment are important tools for land stewards looking to promote forest health. They can decrease soil acidity, increase soil calcium, and promote seedling germination, survival, and growth. These practices can also help promote beneficial tree species in northern hardwood forests and address the increasing abundance of American beech trees, dense stands of which are highly vulnerable to devastating beech bark disease.
Despite the potential advantages of soil management for tree species composition, there are many other organisms to consider. Understory plants provide habitat and food for wildlife. They also contribute to ecosystem services by preventing erosion, cycling nutrients, retaining soil moisture, pulling carbon from the air, and supporting forest growth. Despite all these important functions, there is scant research on how soil management affects the stability and diversity of the understory. This leaves land managers with a tough question: How do you promote beneficial outcomes, such as an increase in sugar maples and yellow birch and a decrease in American beech, without unintended consequences for the understory?
To address this gap, the Canadian research team conducted an experiment. In a forest severely impacted by beech bark disease, the team cleared the understory plants, which were mostly beech saplings, as well as large sticks and logs. Then, they set up twelve plots, each with eight subplots. Within every plot, they added nitrogen fertilizer to two subplots, lime to two subplots, wood ash to two subplots, and they left two subplots with nothing added. Then, for one subplot in each of these groups of two, they scarified the soil. This experiment design allowed the researchers to compare understory communities with only scarification, only soil amendment, scarification plus soil amendment, and no soil modification at all.
After monitoring the plots for four years, the researchers found that soil scarification reduced understory plant community stability and resilience. This could be partly because scarification provides specific conditions that could favor a narrow, less diverse plant community. The team also found that adding any of the soil amendments on top of scarification further reduced understory diversity. On the other hand, when the researchers added lime without scarification, the results were promising, with the treatment increasing diversity.
The mixed results from this short-term study shed light on the important and complex role soil properties play in understory diversity and resilience. Given the potential unintended consequences of scarification and soil amendment together, the research team suggested targeted implementation of these practices might be the best strategy for balancing overstory and understory health.