Building biodiversity in your forest: Forest structure

Minnesota’s forests are home to a wide variety of trees, plants, and wildlife. A healthy mix of species helps forests withstand stress, which is essential as they face the impacts of climate change. 

But biodiversity isn’t just about what’s growing in the forest, but also how it’s growing. Forests with a wide range of tree sizes, ages, and canopy openings provide what foresters call structural diversity. Promoting structural diversity in your forest not only provides a variety of habitat for wildlife but also supports ecosystem resilience and productivity as a whole.

What is structural diversity?

Structural diversity means a forest has:

• Multiple age classes of trees across species
• A mix of tree types (like conifers and hardwoods)
• Openings in the canopy of different sizes

Forests with high structural diversity support a wider range of plant and animal species. In fact, research suggests that structurally complex forests may grow more quickly because they use sunlight, water and soil nutrients more efficiently. However, this varies across different soil types and climates. 

A complex forest canopy also supports life below the surface of the soil. For example, forests with structural diversity often harbor more soil microorganisms, such as mycorrhizal fungi, which form symbiotic relationships with tree roots and help them take in nutrients. This is a great example of how soil health is closely linked to above-ground biodiversity. 

Read more about the importance of soil biodiversity.

Why structural diversity matters for wildlife

Many wildlife species need a variety of habitats to thrive. Structurally diverse forests provide more food sources, nesting spots and shelter. When you manage your woodland for structural diversity, you’re helping many wildlife species meet their needs.

• Canopy openings and early successional forests support woody and herbaceous plants that provide important food for wildlife, such as browse and berries. Fruiting shrubs and other flowering plants in these canopy openings also attract a wide diversity of pollinators.
• Gradual (or “feathered”) edges between the forest interior and canopy openings create transition zones that maximize habitat benefits.
• Cavity trees, snags, and deadwood provide food and habitat for many species of wildlife, including insects, birds, and mammals.
• Forest stands that have a mix of coniferous and deciduous tree species can provide thermal protection for wildlife during cold winters and hot summers

It’s important to note that wildlife move across a broad landscape, not just your property. When promoting biodiversity and wildlife habitat, consider your woodland as one piece of this larger forest mosaic. 

How to promote structural diversity

There are several stewardship strategies that you can use to increase structural diversity in your woodland. Start by identifying your stewardship goals, and then work with a forester to develop a plan.

Uneven-aged forest management

This approach maintains trees of many different ages. It involves selectively harvesting trees over time to open up space for new growth, whether through natural regeneration or by planting seeds or seedlings.

• Single tree selection: Removing individual trees across the stand creates small gaps in the canopy, supporting the regeneration of shade-tolerant tree species. Over time, this management strategy results in a mix of age classes and tree species.
• Group selection: Removing small groups of trees to create larger canopy gaps, usually up to half an acre in size. This increases the amount of light reaching the forest floor, supporting shade-intolerant species, and results in a patchwork of tree species and ages across the forest stand. 

Even-aged forest management

Even-aged forest management creates forest stands with a single age class by harvesting trees at the same time. Even-aged forest management can create early successional forests, such as young aspen and pine stands, which provide crucial habitat for many forest-dependent wildlife. 

• Seed tree harvest: In this method, some mature trees are left standing throughout a harvested area so they can naturally provide seed and help regenerate the next generation of trees in the stand.
• Shelterwood harvest: Trees are removed gradually over time to encourage natural regeneration. The first cut lets in enough light for seedlings to start growing beneath the remaining mature trees. Later harvests remove more of the overstory, giving the young trees more room and sunlight to continue developing.

Combining both uneven-aged and even-aged methods can create a diverse mix of forest types and age classes. By managing for structural diversity, you’re supporting not just the health of your woodland but the health of Minnesota’s entire forest landscape.