To protect intact grasslands, start with the seed

Research highlights the importance of seed sources as a driver of grassland vulnerability to encroachment

As woody plants like eastern redcedar continue to expand into treeless grasslands, it is critical for land managers to understand what underlying processes drive encroachment. When trees move into formerly tree-free grasslands a cascade of negative effects follow, including lost forage production, reduced water supplies, increased risk of severe wildfire, increased risk of vector-borne disease, and significant losses of grassland biodiversity. Fortunately, new science and emerging conservation strategies are helping managers better protect the large, intact grasslands that remain, including the world’s second-largest intact grassland found in Nebraska’s Sandhills.

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Two recent papers from WLFW-affiliated researcher Dillon Fogarty and colleagues at the University of Nebraska shed light on how eastern redcedar trees (Juniperus virginiana), one of the most prevalent woody species encroaching into grasslands across the Great Plains, spread across grasslands and whether precipitation precludes woody encroachment in more arid grasslands as previously believed.

We sat down with Fogarty to better understand his research and its implications for managing woody encroachment in the Great Plains.

Explain why woody plant encroachment is happening in the Great Plains and why it’s a relatively recent phenomenon.

There have always been two things that have regulated woody encroachment in the Great Plains – the sensitivity of grasslands to woody encroachment and the exposure of grasslands to encroachment.

Sensitivity is the relative ease at which woody plants establish and the rate at which they spread and take over. Exposure is driven by seed sources, seed dispersal, and seed persistence in grasslands.

Historically, the Plains had very low sensitivity to woody encroachment. They were a fire-dependent ecosystem, and they burned quite frequently. That frequent fire acted as a barrier to encroaching trees kept the Plains treeless at large scales. As we’ve excluded fire as a land management practice, we’ve largely removed that historical barrier to trees and increased sensitivity across all of the Plains’ grasslands. There are other factors that influence grassland sensitivity, like climate, soil conditions, and herbivory, but fire is the most important.

On the other side we have exposure. Exposure is driven by proximity to seed sources. Historically, the Great Plains had little exposure to trees because trees were rare and limited to very few areas with low fire occurrence (e.g., rocky escarpments). With tree planting, we’ve massively changed the distribution of seed sources. That is, we’ve increased grassland exposure to encroaching woody plants.

Today, we have both factors working against us. We’ve increased exposure to seed sources and removed controlling factors, like fire, from the Great Plains. Historically, you could have gone miles and miles in the Plains and not seen a single tree. Now, you’re hard pressed to find a single treeless horizon.  

How big a problem is woody plant encroachment in the Great Plains?

It’s a biome-wide problem. It’s one of the top drivers of grassland loss in the Great Plains. As a biome, the Great Plains has the potential to exist in different states. For approximately 10,000 years, it’s been grassland. Woody encroachment is the process of it transitioning to a woody-dominated ecosystem, which results in a wide range of impacts to Great Plains communities.

As we go from grass to an eastern redcedar-dominated state, we see up to a 75 percent loss of forage production, which directly affects ranchers’ livestock and wildlife. We have impacts on water quantity and quality. An increased risk of vector-borne diseases like Rocky Mountain spotted fever and West Nile virus from ticks and mosquitos have been linked to woody encroachment. And there’s huge losses to biodiversity, specifically to grassland-dependent birds and mammals. Wildfires in encroached areas are getting larger and more severe due to the buildup of volatile woody fuels. Juniper trees [eastern redcedar] are major contributors to hay fever; they produce a lot of pollen and are problematic for sensitive populations.

These impacts aren’t just theoretical; there is science behind all of this.  

Why haven’t past approaches to stop or slow down woody plant encroachment worked?

Past approaches have been reactive. We’ve waited for impacts and then stepped in to cut trees, but we’ve ignored the leading edge of the problem. So, while we’re focusing on the most infested areas, where, by the way, it’s the more expensive and time-consuming place to work, that leading edge is advancing and moving into our treeless grasslands. These past approaches don’t address the problem at big scales, which is what we need to prevent the biome from collapsing.

One of the things that has limited us at the science-to-management interface is that we’ve had a poor understanding of woody encroachment as a landscape process or, in other words, as a spatial process. We’ve understood things like how fire and herbivory impact woody plants, but that spatial aspect has been lacking. And that’s really important from a management perspective because it supports more targeted approaches.  

With advances in remote sensing products, like the Rangeland Analysis Platform, scientists and managers can now visualize and understand what’s going on across vast regions. And we’ve had this revelation that we’re chasing the problem by prioritizing infested areas. Meanwhile, all of our best grasslands are being eaten up by the leading edge of encroachment.

Your first paper examines recruitment distances for eastern redcedar to better understand how this species spreads across grasslands. Why is it important to understand how this process works?

This is where the rubber hits the road for management. Understanding how trees spread across the landscape is critical information. There’s been a lot of science about how different elements limit woody plant encroachment, like fire. But less has been focused on the spatial process of woody plant expansion. From a management perspective, this is absolutely critical if we want to have more targeted approaches like Defend the Core. We need to understand that spatial element of how species move across the landscape.

For eastern redcedar at least, birds are the primary dispersal agent, so we have these relatively predictable patterns of spread.

Why did it take so long to study recruitment?

That’s a good question. As I mentioned earlier, people, including scientists, haven’t really understood woody encroachment as a spatial process. I think part of the reason is that eastern redcedar is a native species and many people didn’t think of the spatial component as a limiting factor. That is, eastern redcedar was rarely considered as an invasive species in grasslands.

I hit on this point because scientists are really tuned in to how invasive species spread and how management and policies can prevent further spread. In the Plains, eastern redcedar has become a native invader, so it is important to understand how it moves across the landscape in order to prevent and manage the problem.

What do you mean when you say managers need to understand “recruitment as a spatial process”?

Recruitment is a key part of the encroachment process where seeds become seedlings. When mature trees send out seeds and those seeds eventually grow into mature trees that send out more seeds that eventually grow into mature trees, that is the process of woody encroachment moving across a landscape. So, recruitment distances are really important for understanding how that process works in grasslands.

Understanding how woody plants move across the landscape is at the center of new approaches for managing woody encroachment in grasslands, like the “Defend the Core, Grow the Core” management strategy. If we’re going to use spatially targeted approaches to manage grasslands, we need to understand how trees move across the land.

That’s exactly what Working Lands for Wildlife, the NRCS, and partners are supporting through the Great Plains Grassland Initiatives (GPGI). This effort puts the “Defend the Core, Grow the Core” approach into action based on new guidelines I helped develop with Dirac Twidwell in a 2021 publication called Reducing Woody Encroachment in Grasslands: A Guide for Understanding Risk and Vulnerability.  

The guide outlines a vulnerability-based approach for tackling encroachment that is more efficient and cost-effective compared to reactive approaches of the past.

What did you find in your research?

We looked at the distribution of seedlings around seed sources and analyzed how far recruitment occurs from seed sources.  

We found that more than 95 percent of recruitment occurs within 200 yards of a seed source. For example, if there is a windbreak, 95 percent of the seedlings coming from the windbreak are expected to occur within 200 yards of it.

That 200 yards is where you’re going to have the highest pressure of encroachment. This where you’re going to see a rapid transition from grassland to a woody-dominated state. Once those seedlings grow and mature, you push that recruitment zone another 200 yards, which exposes yet more grassland to encroachment.

This type of localized spread accounts for the majority of recruitment. The remaining five percent of recruitment occurs more than 200-yards from seed sources. And while the percentage is small, it’s an important part of the invasion process.

From the perspective of a large, intact, core grassland, like the Sandhills, you’ve got these long-distance recruitment events that can jump out way ahead of the majority of trees, and they can start a new node of encroachment.

So, in addition to an abundance of seedlings within 200-yards of seed sources, there are these long-distance events, say 500 yards, which can eventually create new seed sources way ahead of other recruits. As this process repeats itself, invasion speeds way up over time.

This highlights two important parts of the invasion process. First an abundance of local recruits within 200 yards of seed sources drives rapid transitions from grasslands to woody dominance. And second, long-distance recruits speed up woody plant expansion across large tracts of treeless grassland. Together, these factors make eastern redcedar a well-equipped invader.

From a management perspective, we need to account for both parts. More frequent and intensive management is needed to manage the abundance of local recruitment around seed sources. While, long-distance recruits can be managed with an early detection, rapid response approach. Together, these different tactics inform management plans that are well-equipped to defend intact grasslands.

How should land managers best use your findings to address woody plant encroachment on the landscapes they manage?

When we understand how trees spread across a landscape, we can develop management plans to counteract invasion and better account for the early stages of encroachment that are not as easy to track as mature trees.

This research helps us identify and target the leading edge of encroachment, where seeds and seedlings occur ahead of seed sources. This is where managers can be the most effective and efficient in halting the expansion of seed sources in grasslands.

For example, managers can focus on the 200-yard recruitment zone to prevent seedlings from becoming seed sources when they reach approximately five-feet tall. Prescribed fire and hand tools to cut seedlings are examples of practices that can target the seedling stage. For the rare long-distance recruitment events, regular monitoring and simple actions like cutting down those trees is effective. Establishing regular prescribed fire is a really good strategy because it eliminates seedlings and seeds and when done on a recurring basis, it keeps encroachment in check.

In general, we’ve learned that we need to manage all stages of encroachment in order to be effective. In other words, we can’t just manage seed sources while seeds and seedlings are left on the landscape. We also can’t manage seedlings and seeds without addressing the source of the problem.

Your other paper explored whether woody plant encroachment in the Nebraska Sandhills is limited by precipitation, as previously believed, or by proximity to seed sources. Why was precipitation believed to be the main factor limiting eastern redcedar encroachment in arid grasslands like the Sandhills?

Traditionally, the scientific community attributed the geographic distribution of grasslands to climate. Grasslands were thought of as too dry to support woody plants, which is why they were dominated by grasses. In other words, climate was the primary controlling factor for woody plants.

The prevailing wisdom focused on the 100^th meridian in the Great Plains as a climactic and geographic boundary beyond which, it was too dry for trees to establish and spread, at least without people helping them. The belief that woody encroachment is not a problem in more arid grasslands was ingrained in the natural resource community.

This assumption allowed us to discount the role of disturbances like fire in maintaining open, treeless systems like grasslands. We thought that climate, not disturbance, was the main factor that limited woody encroachment in these systems.

It also provided a rationale for planting trees in grasslands without the consequences of woody encroachment. The expectation was that trees were so difficult to establish in these grasslands, even with supplemental watering, that spread was not possible.

Checking the validity of this assumption is a big reason we did this study. We wanted a better idea of where grasslands are vulnerable to eastern redcedar encroachment given expectations of precipitation-based constraints in the western Great Plains.

What did you learn from your research?

We found encroachment regardless of the climate and precipitation patterns. Essentially, where there were planted trees, there was encroachment. We used a network of 40 sites, and the only sites that didn’t have encroachment were more than one kilometer from planted trees. Which makes sense based on the recruitment study we talked about already.

We also looked at predictors of encroachment – mean annual precipitation and proximity to seed sources. Proximity proved to be the more important predictor. The closer the study site was to eastern redcedar plantings, the more encroachment there was on the landscape. There was support for precipitation as a predictor of encroachment – as precipitation decreased, so did the level of encroachment, but it wasn’t as powerful a predictor as proximity to seed sources.

In other words, if a grassland was exposed to seed sources, we found encroachment, regardless of precipitation. More arid grasslands are less sensitive to encroachment, but they are still vulnerable when exposed to seed sources.

So, proximity to seed sources plays a more important role than precipitation for arid grasslands’ risk of encroachment? 

Well, if you don’t have seed sources, you can’t have encroachment. Precipitation operates in the background. It influences how sensitive a grassland is to encroachment, but it doesn’t preclude encroachment on its own.  

What does this finding mean for areas like the Sandhills that are assumed to be protected from widespread encroachment because of their more arid climate?

The takeaway from this paper is that the Sandhills are at risk to woody encroachment and that risk is driven by seed sources. They are vulnerable. The opportunity to proactively manage these landscapes and defend intact grasslands is now. Encroachment is happening in places it has never happened before. Managers and policy makers are taking a tremendous risk when they assume “it won’t happen here.”

Based on the findings from these two papers, what is the single biggest piece of advice you have for landowners and managers working to prevent woody plant encroachment?

Both studies point out how seed sources drive encroachment. The most important thing a manager can do is maximize the distance between intact grasslands and seed sources to reduce the risk of encroachment.

I mentioned the “Vulnerability Guide” above, which is a great resource for managers, planners, and landowners looking to better understand how vulnerability leads to encroachment.

I’m super excited about a new pocket guide that we are also producing that takes all of these concepts and integrates them with a formal planning process used for grassland conservation.

It’s a pretty exciting time for grassland conservation. Despite the big challenges, there is a lot of promising work being done to save our grasslands.

Meet the Expert:

What do you like to do when you’re not studying woody plant encroachment?

Spend time outside with friends and family.

What’s your favorite dessert?

Banana split.

What show have you binge-watched most recently?

Ted Lasso.