By Amy Grotta and Brad Withrow-Robinson, OSU Forestry & Natural Resources Extension

Through a creative and carefully planned harvest, this mature forest has undergone a structural diversity makeover.

Continuing our series, we’ll now look at steps that woodland owners can take to enhance structural diversity in their forests.  Recall that “structural diversity” refers to the amount of three-dimensional variation in the forest. In other words, a structurally diverse forest has trees of different sizes arranged in uneven patterns across the site.

Why do we care about structural diversity? Structural diversity is important to creating an “older” or “natural” forest look that many people want on their property.

Structural diversity is also a very important part of wildlife habitat. By creating variation in the tree canopy, and creating layers of vegetation at many levels within a forest, you are creating micro-environments. These micro-environments, each with its own set of light, temperature and moisture conditions, allow different sets of plants and animals to flourish. You are setting the stage for biological diversity to develop. Raptors may perch in the upper branches of the tallest trees, while small birds that nest in thickets may find a home lower down. Light-loving shrubs will fill in the sunny openings, while mosses and lichen will be happy in the cool shade of taller trees.

To create structural diversity, we rely on all the same tools in our forest management toolbox that we use to achieve other things in our woods: planting, thinning, harvesting. We just apply them a little differently to produce a different outcome.   Many of the opportunities described below and illustrated in the graphic at the right will be familiar from our last post, since structural and species diversity often go hand in hand.

Preparing to grow a new forest: don’t clean it up too much

After a timber harvest, there will be a lot left behind: limbs and treetops (slash), and sometimes logs with too little value to take to the mill. Part of site preparation involves getting these residues out of the way to make room to plant new trees. Often slash is piled and burned, but leaving those logs and even a few of these piles creates instant structure. They will decompose over decades, providing a shelter for critters and slowly evolving into a substrate for new plants. Check out this article for some great how-to’s and cautions.

Planting a new forest

Since a diversity of tree and shrub species can also greatly enhance future structural diversity by creating different textures or layers in the forest, the steps at planting outlined in our previous post can do double duty.

Tending a developing forest: thin early, thin often and change it up

While a diverse forest is an appealing image to many small woodland owners, what they often actually have is a uniform forest of trees roughly the same size and spacing. In these situations, thinning is the best opportunity to enhance structural diversity.

From top to bottom, examples of young, intermediate and mature conifer forests with low structural diversity

You can use thinning early in the life of a forest to create both horizontal and vertical structure (recall the cookie metaphor for definitions). Some specific ideas for making your forest more structurally diverse through thinning include:

  • Thin early to promote layers of plants on the forest floor.
  • Mix it up with a variable density (rather than uniform) thinning strategy:
    • Leave some areas dense or even unthinned.
    • Thin some other areas heavily to grow big trees and promote regeneration of new trees.
  • Be messy. Leave snags and dead wood behind.
Grand fir started to seed in this stand after an earlier thinning. Structural diversity could be improved by removing a small patch of overstory trees (like the three in the foreground), releasing the trees beneath them.

Opportunities in an older, maturing forest are to create gaps and leave legacies.   If you find a group of young, shade-tolerant trees coming up on the forest floor, you can create a small gap to give them room to grow. Or you can cut a patch and plant it with shade tolerant species such as western hemlock or grand fir to create another canopy layer. Both of these things could be part of a regularly planned thinning harvest.

 

Any harvest is a chance to enhance the structural diversity of the future forest. Leaving dead wood (both standing and down) is particularly beneficial for wildlife.  If doing a clearcut harvest, you can also leave some live trees, in groups or islands when possible.  Good candidates include trees that are hard to access, have defects, or are intermixed with snags you wish to protect. Even if they blow down, the downed wood is another important structural legacy.

We hope this series has given you some new ideas about how to enhance your woodland diversity and enrich your experience as a landowner. There is lots more information out there to help you work out the details.  Here are a few:

This is an excellent publication from WSU Extension, Diversifying Forest Structure to Promote Wildlife Biodiversity in Western Washington Forests (written for our neighbors to the north, but just as applicable to western Oregon).

Wildlife in Managed Forests: Oregon Forests as Habitat published by Oregon Forest Resources Institute

Woodland Fish and Wildlife Publications

Alternative Forest Management series in the OSU Extension Catalog

 

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