Ask the Expert: Understanding Conifer Encroachment in Sagebrush Country and the Latest Resources to Address it, with Derek Tilley

We’re often taught we should save the trees. They give us oxygen. They improve air quality. But trees aren’t always the hero. In fact, in this case, they’re taking over the West. For a decade, Derek Tilley served as manager of the Aberdeen Plant Materials Center with USDA’s Natural Resources Conservation Service (NRCS) in Idaho before transitioning to his current role as the Sagebrush Biome Technology Transfer Specialist with NRCS’s Working Lands for Wildlife (WLFW) partnership. He collaborates with NRCS staff and partners to address key threats to the sagebrush biome while supporting the sustainability of western working lands and ranching operations. In this Ask the Expert, Derek discusses the growing threat of tree encroachment in western sagebrush rangelands and introduces two new WLFW resources designed to help landowners and managers in the Northwestern Great Plains, more effectively respond to this challenge.

I always thought of trees as a good thing. Why is tree encroachment a threat to sagebrush rangelands?

Trees absolutely are a good thing, when in the right place.

Some ecosystems, like sagebrush rangelands, are meant to be mostly treeless to function properly and support the species that evolved there. 

Historically, fire was far more prevalent across these landscapes, keeping trees limited to specific areas like higher elevations, steep slopes or shallow, rocky soils. 

Now, with shifting fire patterns and increased suppression, conifers are increasingly encroaching into productive sagebrush rangelands. Over time, that shift can trigger a cascade of impacts for both wildlife and working rangelands, like an increased risk of catastrophic wildfire and reduced forage available to wildlife and livestock.  In just the Northern Great Plains (parts of Montana, Wyoming, North Dakota, South Dakota, and Nebraska), we lose 455,000 tons of forage each year– enough to feed 96,000 calf-cow pairs, which are worth nearly $195 million at auction. For producers, that’s a serious financial burden. 

If trees are such a threat, why don’t we just go cut them all down? 

Short answer: Not all trees are a problem, and not all landscapes should be treeless. 

Trees play an important role in many ecosystems, so the goal isn’t to remove them everywhere, it’s to keep them in the right places. That’s why understanding a site’s ecology and history is so important.

Without that context, randomly cutting trees can create new problems, including impacts to wildlife habitat, soil stability, and even increasing the risk of invasive species.

There’s also a practical reality: we simply don’t have the time or resources to remove all the trees in all the places where they’re causing impacts. So, it’s essential to prioritize where removing trees will have the best ecological impact.

Are there places in the West where we should focus conservation efforts to reduce the impacts and threats of conifer encroachment?

Absolutely, and this is where we have a real opportunity to make a difference.

One of the most important places to focus on right now is the Northwestern Great Plains, including parts of Wyoming, Montana and the western Dakotas. This region still contains large expanses of intact rangeland and grassland, which is exactly why it matters so much.

Yet, we can still see clear signs that much of this landscape is vulnerable to encroachment. By focusing on prevention, identifying risks early and acting strategically, we can stay ahead of the problem rather than trying to fix it after the fact.

That’s exactly why we’ve developed guides to help planners and landowners take advantage of that opportunity while it’s still in front of us.

What tools and resources do you offer to local planners and landowners to help bridge the gap between science and application?

We worked with partners across the Northwestern Great Plains region to develop two guides tailored to the region with imagery and examples that truly resonate with local planners and landowners.

The technical guide is designed to build understanding. It walks through the ecology of conifer encroachment, introduces the Defend the Core framework, and emphasizes the importance of managing vulnerability and risk, not just reacting to what’s already happened. It also provides step-by-step guidance for developing a comprehensive management plan that accounts for every acre and every stage of encroachment.

The pocket guide is the field companion, a durable, laminated 4x7 resource built to be used on the ground. It’s really a conversation starter, helping planners and landowners compare what they’re seeing in the field to real-world examples in the guide.

Together, these tools are meant to bridge the gap between science and application, giving people practical, regionally relevant resources to make better, more strategic decisions, while also equipping NRCS conservation planners and local planners to help landowners implement targeted conservation practices like, in this case, brush management, prescribed burning, prescribed grazing and pest management to reduce conifer encroachment and protect rangeland productivity.  

Are rangeland professionals receptive and finding these guides helpful?

The initial response has been really encouraging. We’ve heard positive feedback from planners, landowners, and practitioners across the region who are already putting the guides to use. 

What’s also exciting is that interest isn’t limited to just the Northwestern Great Plains. We’re hearing from folks in other regions who are asking for similar tools tailored to their landscapes.

Because of that, we’ve already started developing the next set of guides focused on the Great Basin, with a potential release this fall.

How is Working Lands for Wildlife helping people make positive things happen for conservation?

I’d say the biggest thing is that Working Lands for Wildlife is really a full-service effort.

We don’t just produce science and written resources, we also offer in-person training, webinars and hands-on support to help people apply these concepts on the ground. Our goal is to make sure folks have what they need, not just to understand the issue, but to take action.

When it comes to conifer encroachment, we want to equip people with what they need to prioritize tree removal where it will have the most short and long-term ecological impact.

To do that, we can pair resources like the guides with NRCS’s conservation assistance that offers targeted technical and financial assistance producers need to put these practices to work.

In other words, we have tools, partnerships, and the momentum to tackle these challenges in a coordinated way, and that's critical for our producers and our wildlife.

And that momentum just got a huge boost with USDA’s recent launch of the new Migratory Big Game Framework for Conservation Action, which aligns 17 western states to protect critical migration corridors for iconic wildlife. The framework targets conifer encroachment as a primary threat to game species like elk, mule deer and pronghorn that rely on open, connected landscapes for migration. Pairing this new framework and our technical guides better enables us to give producers more exact resources to precisely target where their efforts to improve and sustain productive rangelands will do the most good.

If you’re interested in learning more about WLFW’s work in the Great Plains and sagebrush country, I’d encourage you to visit WLFW.org, sign up for our monthly newsletter, and follow us on social media. There’s a lot happening!