Douglas-fir are Encroaching on Unique Grasslands! Three Methods to Keep This from Happening

By Rolando Beorchia

September 2021

When I talk with friends about restoring grasslands and meadows, I frequently talk about cutting down trees. Naturally when we think of open meadows and grasslands, we think of the absence of trees, but have you ever thought about what it takes to keep grasslands open? Well, the short answer is this: it takes lots and lots of manual labor!

Douglas-fir (Pseudotsuga menziesii) are one of the biggest threats to the survival of meadows in the Pacific Northwest. You wouldn’t think it by looking at them, but they are a bully of a tree…always throwing shade on the weaker herbaceous plants and making meadow edges such an unpleasant place to be that meadow species can’t stay there anymore.  Not only that, but each Douglas-fir tree on the meadow perimeter drops its seed into the meadow, and the next generation of trees comes up a little farther into the opening. After decades of this process, grasslands can be greatly reduced in size (or even disappear) due to this continual inward march of trees. So what is the answer?  There are three ways IAE and our restoration partners combat Douglas-fir encroachment in order to maintain open grassland and meadow habitats: oak release, perimeter tree removal, and corridor creation.

Figure 1. View of meadow edge and Doug-fir encroachment at Horse Rock Ridge, located 10 miles south of Crawfordsville, Oregon (Horse Rock Ridge is managed by the Bureau of Land Management’s Upper Willamette Field Office).

Figure 2. Oregon white oak release at Twin Prairie, located approximately five miles east of Cottage Grove Lake, Oregon (Twin Prairie is managed by the Bureau of Land Management’s Upper Willamette Field Office).

Oak release: Many of our restoration project sites have Oregon white oak (Quercus garryana) present. Oregon white oak provides habitat and food for many important animals, and open-grown oak trees are often desired in meadows and grasslands. However, Oregon white oak grows much more slowly than Douglas-fir, and can easily be surrounded and shaded out by the faster-growing species, causing the oaks to become unhealthy and even die. In order to prevent this, we cut down the Douglas-fir in the vicinity of a desired oak tree and open up the canopy for healthier oak growth. Figure 2 shows a few oak trees that were being crowded at Twin Prairie, a Bureau of Land Management site, and the recently felled Douglas-fir waiting patiently to be removed. Most of this debris will either be piled for later burning or scattered into the woods for eventual fungal decay (we do our part to keep those wild fungus well-fed!).

Perimeter tree removal: Not only do we think about how to free space for desired oak trees, but we also ponder how the forest edge and overall shape allows for or hinders the movement of critters throughout that landscape. If rare butterflies don’t have the ability to fly from one section of prime habitat to another, then there isn’t much point to create island habitats for them. Working to reduce encroachment along meadow edges by cutting trees poking out into the meadow is a regular practice used to maintain meadow habitat. We do this by hand cutting small seedlings or felling young trees and moving debris into the understory. This not only immediately increases the size of the meadow, but also slows the inward march of Douglas-fir.

Figure 3. Corridor creation at Henry Hagg Lake, located 10 miles northwest of Gaston, Oregon (Henry Hagg lake is owned by U.S. Bureau of Reclamation (BOR) and managed by Washington County).

Corridor creation: Creating corridors is a visually appealing, cost-effective way of merging two meadows without clearcutting an entire stand. A great example of this is the work we are doing at Henry Hagg Lake in Washington County, Oregon, home of the endangered Fender’s blue butterfly. Two grassland areas are currently under chemical fallow to reduce the weed seed bank (Figure 3), destined to receive native seed in the near future. But these two openings were separated by a thick stand of Douglas-fir. In order to connect the two openings, we cut down a strip of trees and created a nice corridor to facilitate movement of rare butterflies, native pollinators and native wind-blown seed.

These three restoration practices are commonly used in the defense of meadows and the unique plants and animals that inhabit them. The next time you are out walking through one of your local green spaces, see if you can spot some of these techniques being used!

As always, a big thank you to our partners who help make this work possible, including the Bureau of Land Management, U.S. Fish and Wildlife Service, U.S. Bureau of Reclamation and Washington County Parks. And a special thanks to all the volunteers who have helped cut trees or haul logs and branches with us at our restoration sites.

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