Tag Archives: suppression

Learning How to Live with Shrubbier Grasslands – Part 1: The Why

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Back in 2022, I wrote a post about the increasing competitiveness of woody plants – especially clonal shrubs like dogwood, sumac, and others – in prairies. There are lots of factors that have led to more shrubs moving into grasslands, but increased levels of carbon dioxide in the atmosphere probably play the biggest role. Regardless of the reasons, more and more prairies are becoming something different than we’ve been used to.

Deciduous shrubs and trees are becoming more and more prevalent in many prairies these days.

Since writing that post several years ago, I’ve engaged in a lot of conversation with land managers and other scientists on this topic. I’ve learned several crucial things about woody plants in Great Plains grasslands:

  1. Annual fire, and maybe biennial fire, may be able to prevent woody plants from moving into prairies here in the central United States. Anything less frequent than that is unlikely to be successful.
  2. At least in the northern Flint Hills of Kansas, once those woody plants have established, even decades of annual fire may not get rid of them. Researchers at the Konza Biological Station, for example, have seen that more than 20 years of annual fire has kept shrubs short, but hasn’t reduced stem density.
  3. The season of fire is probably important, but I’ve not found any evidence that burning in the growing season vs. dormant season changes the need to burn very frequently if that’s the only strategy being used to prevent woody plant encroachment. We’ve done a lot of summer burning here in Nebraska and see immediate resprouting of shrubs. Summer burning in droughts can sometimes look promising, initially, but the shrubs seem to roar back in subsequent years.
  4. Eastern redcedars don’t resprout after being burned (or cut), so at least we know what needs to be done to deal with them. Deciduous trees and shrubs do resprout unless they’re treated with herbicide. Cutting one down and treating the stump with herbicide works a treat. Unfortunately, that’s insufficient to deal with dense stands of trees or shrubs across tens, let alone hundreds or thousands of acres. Broadcast spraying of grasslands for shrub control can kill woody plants but is catastrophic for biodiversity. So what do we do?
  5. “Use goats!”, some of you are screaming. Sure, goats can be helpful, but once-a-year, short-term goat browsing seems to have the same impact as once-a-year burning or mowing, which is that the shrubs just resprout. Multiple treatments of mowing, browsing, burning, or combinations, can more drastically reduce the height and density of shrubs, which is definitely helpful, but – again – that can be difficult to scale up. Continuous, low-density goat grazing might be a decent option if we can figure out how to keep those goats contained (at a reasonable cost).
Smooth sumac resprouting three weeks after an intense summer wildfire during a severe drought.

One of the most helpful things I’ve done is to convene a small group of smart people who have met repeatedly over the last couple years to discuss some big picture ideas. Those people, all PhD scientists and experienced grassland ecologists, include Sam Fuhlendorf of Oklahoma State University, Jesse Nippert and Zak Ratajczak of Kansas State University, Nic McMillan of the University of Nebraska-Lincoln, and Marissa Ahlering of The Nature Conservancy.

Conversations with those scientists have reinforced my thinking that prairie managers in this part of the world need to shift the way we think about woody plants in prairies. During most of my career, trees and shrubs have been the enemy – or, to put it better, they were important plants that could become problematic if I didn’t keep them at bay. Woody plants were ok in small patches, especially along the margins of grassland areas, but they could cause big problems if they started popping up out in the middle.

Well, the world has changed, dang it, and we need to change, too.

Just during my career as a prairie ecologist and land manager (30-some years, if I count my time studying prairies as a graduate student), I’ve seen changes in how deciduous shrubs respond to prairie management and spread across the landscape. There are still lots of grassland landscapes in Nebraska where woody plants are uncommon, and where it’s very feasible to keep them that way. However, there are more and more places where it’s not.

Especially in fragmented landscapes, where patches of prairies are relatively small and there are lots of woody plants nearby, trying to prevent shrubs and trees from moving into prairies can feel like poking a stick at a landslide.

Sure, annual burning may work, but there are a couple huge problems with that. In a fragmented landscape, burning an entire prairie each year risks eliminating populations of many animal species from that site. The isolation of that prairie from others means recolonization of those species is unlikely – especially if the closest other prairies are also being annually burned.

The other problem comes back to scale again. Here in Nebraska, we have 20 million acres of grassland. The idea that we could burn even half of those acres each year is ludicrous. Even if we had the will and capacity to do it (we don’t), the smoke from that many acres would be completely unacceptable. Mowing, of course, is also infeasible at that scale (not to mention limitations of topography in many places).

Currently, most of our deciduous tree and shrub encroachment is happening in the eastern third of the state, where many grasslands exist as patches within a crop land matrix. Even there, we’re still talking myriad scattered prairie parcels totalling millions of acres, so annual or biennial burning isn’t feasible at that scale.

Assuming we could somehow convince every eastern Nebraska landowner to burn their prairie every other year (there’s no chance of that), and we could figure out how to deal with all the smoke (we can’t), it still wouldn’t happen. We’d still have to deal with burn bans issued by local and state officials during drought years or whenever they feel sufficient public pressure.

Frequent burning (dormant or growing season) may be enough to stave off woody encroachment, but isn’t really feasible across millions of acres of the Great Plains.

I could go on and on, but the big point is this: excluding trees and shrubs from prairies is no longer possible in many places. It just isn’t. We can prioritize and dedicate resources to prevent encroachment in some select areas, but across much of the Central U.S., we are going to have shrubbier grasslands.

The transition from grasslands to shrubland has already happened in many parts of the Midwest and Great Plains. Ranches in parts of Texas and Oklahoma have had to shift from cattle grazing to deer hunting or other landuses. In parts of the Midwest, where many grasslands have persisted as small openings within a wooded landscape, lots of those openings have closed. Larger, drier grasslands in the western half of the Great Plains are transitioning much more slowly, but there are still examples of trees and shrubs – especially along creeks or wetlands – expanding their footprint beyond what we’ve been used to.

Deciduous shrubs in the Texas Hill Country near Austin.

All of this means we need to think about how to manage woodier prairies for biological diversity and productivity – including agricultural productivity, since grazing and other agricultural uses is what has prevented many of them from being tilled or otherwise converted to something that’s no longer prairie.

This doesn’t have to be a catastrophe. In fact, there are many prairie species that benefit from the presence of more shrubby habitat. Others won’t, but we actually have a lot to learn about what kinds of shrub height and density will affect most prairie species, and how.

How much shade will various prairie wildflower species tolerate? What about the insects that pollinate them? How do grassland wildlife species respond to different heights and densities of woody plants? For animals and plants that can’t handle even a little tree or shrub cover, how big do open areas need to be to provide them with sufficient habitat to survive?

On the land management side, if we’re not trying to eradicate or prevent encroachment of shrubs and trees, what does prairie stewardship look like? In many places, our goal will probably be to manage the height and density of shrubs. That goal will be more defined as we learn how to answer the above questions (and many more), but few of us have focused on height and density management. We’ve been trying to kill shrubs, not compromise with them.

There are a lot of deciduous shrubs in this prairie but they’re all about the same height as the surrounding vegetation. If we can keep them that way, can we maintain high grassland biodiversity and productivity?

I’m planning to dedicate a big chunk of the next decade to this topic. We’ve already started some small experiments at Nebraska sites owned by The Nature Conservancy and are collaborating with a couple researchers to dig more deeply. I hope many others will also work on this. There are lots and lots of important questions to address.

Stay tuned for more. More importantly, if you’re a land manager or scientist, please consider how you might join in the effort to learn more about and experiment with “shrubby grassland stewardship” so we can all build off each other’s work.

If you’re interested, check out part 2 of this post, which shares preliminary results of two small experiments on how to manage height and density of clonal deciduous shrubs.

Categorizing Invasive Plants

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Managing invasive plant species is often the greatest challenge faced by land managers.  Because there are so many invasives and so little time, it’s critically important to be thoughtful about how to approach them.  There is much good advice available about how to prioritize which species to focus on and how to approach those priority species.  My own approach to invasives continues to change over time.  For what it’s worth, here’s how I think (today) about categorizing invasive plants on our sites here in Nebraska.

I can put most invasive plant species into one of four categories that describe how we approach control.

  1. Thin the matrix
  2. Beat them back
  3. Nip them in the bud
  4. Live with them

 

Category 1 – Thin the matrix.

Invasive grasses, such as Kentucky bluegrass, smooth brome, and tall fescue are excellent examples of species in this group.  They are plant species that are common enough that they occur throughout most of our sites, and – if left unmanaged – can form thick monocultures that exclude most other plant species.  Eradication of these species is not possible without losing many of the other plants we are hoping to conserve.  Instead, our general approach is to reduce their dominance and limit their impact on the diversity of the plant community.  (I wrote about this approach in an earlier post, using Kentucky bluegrass as an example.)

Essentially, we try to use fire, grazing, and/or mowing to weaken invasive grass plants and open up space between them for other species to flourish.  Because these suppression strategies also have negative impacts on some native species, we are careful not to use them too many years in a row.  Instead, we apply them periodically, whenever it looks like the invasives are starting to exert their dominance a little too much.

Prescribed fire and grazing are the two best ways we attack invasive grasses such as smooth brome and Kentucky bluegrass.

Prescribed fire and grazing are the two best ways we attack invasive grasses such as smooth brome and Kentucky bluegrass.

Category 2. Beat them back.

This group includes perennial plants that radiate outward other otherwise spread from an established source population.  Species we commonly deal with that fit into this category include purple loosestrife, sericea lespedeza, and Canada thistle.  It can be tempting to jump in and start attacking (with herbicides or other approaches) the biggest thickest patches of these, but that’s rarely the smartest strategy.  Instead it usually makes more sense to work from the outside edges of an infestation toward the middle – or source – so that the problem doesn’t continue to get worse as you attack.  I wrote an earlier post on that topic as well…

An exception to the “work from the outside edge first” rule applies to species such as Siberian elms that may be spreading from a single discrete patch of parent trees.  If it’s possible to eliminate that source population by cutting down a handful of trees, it absolutely makes sense to do that first.  Next, it’s smart to target other elms that are big enough to produce seed before working on the smaller ones – rather than blindly following the rule about working from the edges of an infestation.  Rules are meant to be broken, after all.

 

Category 3. Nip them in the bud.

This category contains invasive species that are just starting to show up in our area or at a particular site.  Here, the tactic is to seek and destroy new plants as soon as they arrive, to prevent the species from becoming established.  After all, it’s always better to attack an invasive species before it gains a foothold.

Many species can fit within this group.  For us, Common reed (Phragmites australis), crown vetch, garlic mustard, and Queen Anne’s Lace (aka wild carrot) are good examples.  Sometimes, we’re not sure if a species will really cause serious problems if it becomes established at our sites (e.g., Queen Anne’s Lace) but if minimal effort can prevent that establishment, it seems  like time well spent.

Our approach to musk thistle sort of fits into this “nip it in the bud” category, but for other reasons.  Musk thistle is an officially-designated noxious weed in Nebraska, and all landowners (including us) are required to eradicate it from their property each year.  If it weren’t for the state law, musk thistle would not be among our highest priority species because it really doesn’t cause big problems in most cases (on our sites).  It is most abundant where the dominant vegetation has been recently weakened by fire, grazing, or drought, but quickly diminishes in abundance when grasses recover their dominance.  However, to abide by the law and to prevent thistles on our land from going to seed and affecting our neighbors, we do our best each year to eradicate musk thistle.

 

Tier 4. Live with them.

This last group includes species we don’t actually consider to be invasives, at least by the criteria that a truly invasive plant acts to reduce biological diversity or otherwise simplify (and thus weaken) natural communities.  Many native species are considered weeds by some of our neighbors, but we like having them around.  Prime examples include annual sunflower and ragweed species.  However, many non-native plants fall within this category as well, including common mullein, dandelions, goatsbeard, marestail, and sweet clover.  Sometimes, we spend some time collecting and/or analyzing data to help ensure that we’re categorizing these species correctly.  A good example of that was discussed in an earlier post on sweet clover.

Dandelions are a species we just live with. They're great for early season pollinators, and aren't aggressive - they just fill space when the perennial plant community is weakened (as they are doing here in the lot around our shop buildings).

Dandelions are a species we just live with. They’re great for early season pollinators, and aren’t aggressive – they just fill space when the perennial plant community is weakened (as they are doing here in the lot around our shop buildings).

Switching Categories.

Ideally, of course, we’d be able to move some invasive plant species from one category to another, so that populations of “matrix” invasives shrink to the point they are in discrete patches and we can “beat them back.”  Likewise, it’d be great if “beat them back” invasives became rare enough that we could eventually “nip them in the bud”.  Unfortunately, reality usually goes the other direction.  For example, we’re dangerously close to having to shift garlic mustard from “nip it in the bud” into the “beat them back” category (if not the “thin the matrix” category!) at a nice woodland site we manage.  We’ll see what the next year or two brings.

We hope that using more frequent fire (along with other methods) will help us prevent garlic mustard from spreading across our whole property at the Rulo Bluffs Preserve, but it's a big challenge.

We hope that using more frequent fire (along with other methods) will help us prevent garlic mustard from spreading across our whole property at the Rulo Bluffs Preserve, but it’s a big challenge.

Regardless, I find it helpful to think about our invasive plants within categories like these because we can more easily define both our control strategies and objectives.  Putting a species into the “nip it in the bud” category helps make it a top priority and we can prioritize resources toward keeping it rare.  Just as importantly, if we know that we’re only trying to suppress the dominance of smooth brome, not eradicate it, we don’t have to beat our heads against the wall in frustration because smooth brome is still present.

We spend more time on invasive plant control than on any other land management activity.  Unfortunately, that’s true of most land managers I know.  More unfortunately, invasive species numbers are going up, not down.  It’s not time to throw in the towel just yet, but it is absolutely critical to be organized and thoughtful about how we approach these invaders.

Hopefully, being organized will allow us to spend less time on invasives and more time enjoying the sites we manage!