Managing forests to sustain soil biodiversity

UNIVERSITY OF MINNESOTA EXTENSION - www.extension.umn.edu

Soil is the foundation of forests. Soils are living, breathing ecosystems!

Forest soils provide many essential services: they serve as a place for trees and other plants to grow, store and filter water, cycle nutrients that are essential for plant growth, and store more than half of all the carbon in U.S. forests.

That’s a lot of responsibility, so it’s important that the soils themselves are healthy.

Healthy soils promote forest health, productivity, and resilience by supporting the basic needs of trees and other plants: water, nutrients, and air (carbon dioxide and oxygen).

Roots are the interface between the plants and the soil where water, nutrients, and gas are exchanged. If we were to take a magnifying glass and zoom into the soil rooting zone, we would see a vibrant and diverse ecosystem surrounding roots and beyond.

Feeding more than roots

Soil biota (the many organisms that live in soil) include fungi, microorganisms such as bacteria and algae, and soil fauna such as nematodes, insects, and protozoans. (Remember: there are no native species of earthworms in Minnesota and they are an invasive species. Learn more about invasive earthworms.) All of these organisms play different roles in the soil ecosystem, such as decomposing plant and animal matter, cycling plant-available nutrients, and supporting soil structure.

Mycorrhizae are fungi that form mutually beneficial associations with roots by providing plants with accessible nutrients in exchange for sugars (food) produced by the plants. The hyphae (the branching parts of fungi) essentially extend the rooting capacity of plants, and the hyphae also help to bind together and stabilize soil particles.

Without all of these different kinds of soil life, we wouldn’t have healthy soils — and then we wouldn’t have healthy forests.

So, maintaining soil biodiversity is crucial for promoting both soil and forest health. We often think about managing forests to support above-ground biodiversity in trees, plants, and wildlife — but how do we support below-ground biodiversity?

Luckily, managing for above- and below-ground biodiversity can be achieved in tandem.

How can you manage forests to sustain soil biodiversity?

The best thing that you can do to protect forest soil health is simple: keep your forest as a forest. Converting a forest to something different, like a field or a parking lot, inherently changes belowground and aboveground biodiversity.

Other than keeping your forest as a forest, the stewardship practices below can help you maintain healthy and biodiverse forest soils.

Promote diversity

Aboveground diversity is closely linked with belowground diversity, as different plant species have unique symbioses with belowground fungi. Promote a diversity of understory plant and tree species appropriate for the site.

Encourage a diversity of tree ages across your woodland, as each stage in a forest’s life cycle provides unique opportunities for aboveground and belowground life.

Retain slash

Distribute slash (the tops and limbs of harvested trees) across the site after harvesting. Some of the slash can also be piled to benefit wildlife.

This slash will eventually decompose and contribute nutrients back to the soil, which maintains soil health and biodiversity.

Keep stumps and roots

Retain the stumps and roots of harvested trees unless forest health or other concerns necessitate removal. The stumps and roots will break down and help to maintain the soil nutrient balance. 

Protect sensitive soil when harvesting 

Avoid negative impacts on sensitive soils from harvesting equipment, including compaction, rutting, and erosion. Sensitive soils, such as clay-heavy soils and peat, are more prone to long-term negative impacts from harvesting equipment. These negative soil impacts also limit the health and growth of trees.

Consider harvesting on sensitive soils during frozen conditions, and avoid harvesting during wet periods.

Don’t harvest everything

When harvesting, keep some live trees in clumps or as scattered individuals across the site. These trees will not only support the next generation of trees, they will benefit aboveground and belowground organisms.

Many of these practices are part of Minnesota’s Voluntary Site-Level Forest Management Guidelines (FMGs), which provide a menu of management practices to maintain the health of Minnesota’s forests. You can learn more about many of these practices in the FMG Quick Reference Field Guide. 

Supporting soil health and biodiversity goes along with managing aboveground forest health and biodiversity. Forest ecosystems include not only trees, plants, and wildlife, but also the diverse belowground world of soil. Being a forest caretaker also means being a caretaker of the soil.