Tag Archives: shrubs

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.

Konza Prairie Trip Part 2 – Tree and Shrub Encroachment

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A couple years ago, I wrote about some work from Kansas State University related to woody plant expansion in prairies.  Many of us who work with prairies constantly wrestle with questions about trees in prairies. Why are they encroaching so quickly these days? What prevented them from doing that in the past?  During our recent trip to Konza Prairie, we got to discuss this topic more in-depth with Jesse Nippert and other researchers at Kansas State.

Clearly, a combination of factors influences how quickly trees and shrubs enter and spread in grasslands.  One big reason is the increase in “seed rain” in some of today’s prairies.  Prairies in fragmented landscapes with numerous trees and shrubs in nearby woodlots, road ditches, shelterbelts, etc., are deluged with seeds from those woody plants.  The vast majority of those seeds fail to establish, but the high number of seeds coming in means that some will find opportunities to grow.

Smooth sumac clones like this small one are expanding at Konza Prairie, especially in areas burned less frequently than every other year.

Smooth sumac clones like this small one are expanding at Konza Prairie and many other sites in central North America.

Other factors may include the higher rates of carbon dioxide in today’s atmosphere and higher amounts of nitrogen deposition (from industry and agricultural facilities, for example), both of which tend to favor woody plant establishment.  In addition, we are in a relatively wet climatic period if you look at the geologic record.  While there have been droughts, including severe ones, in recent years, those droughts are nothing like the multi-decade severe droughts that can be seen in the relatively recent geologic records for the central United States.  Long and/or frequent droughts favor herbaceous plants (such as grasses and wildflowers) over trees.

However, Jesse Nippert’s research into the way trees, shrubs, grasses, and forbs (wildflowers) compete for water belowground provides some additional insight into the march of woody plants into prairies.  As we started talking about roots, Jesse confirmed something I’d heard from Dave Wedin at the University of Nebraska; even though grasses can have very deep roots, most of their water use is actually very shallow – within the top 25 cm of soil.  Jesse says the reason those grasses persist during very dry periods is not because of their deep root systems, but because they can continue to grow and function when available soil moisture is very low.  Forbs also pull a lot of their moisture from shallow roots, but utilize slightly deeper roots (50-75 cm deep) during droughts because they can’t compete well with the ultra-efficient fine-rooted grasses at the upper levels.

It turns out that understanding root competition might help us better understand woody plant encroachment as well.  In many parts of Konza prairie, clonal shrubs such as rough-leaved dogwood and smooth sumac have expanded rapidly over the last several decades.  As Jesse and his students have studied this phenomenon, they have concluded that an important factor behind this expansion is the strategy those shrub clones use to acquire water.  While grasses and forbs are mostly using water from the top 1/2m of the soil, shrubs pull much of their water from deeper in the soil profile, allowing them critical access to water not being utilized by their competition – especially in years when the upper soil layers are dry.

The clonal form of dogwood and sumac gives them another advantage.  As clones expand, the tillers (aboveground stems) on the outer edge of the clones have very small roots.  However, by studying the isotopic signatures of the water in those shallow-rooted tillers, Jesse can tell that they are also accessing water from deep in the soil profile.  He says this is almost surely because the older, deep-rooted plants in the center of the clone are sharing the water they acquire with the younger stems on the outside.  Not a bad strategy.

Of course, as these clones of dogwood and sumac use their water acquisition and sharing strategy to advantage and spread into the prairie, they also shade out their competition – especially beneath the tall/dense tillers toward the centers of clones.  Suppressing the growth of grassy undergrowth not only removes that competition for resources, it also helps make the clones fireproof.  Since dried grasses are the primary fuel for prairie fires, the absence of grasses beneath shrub clones means that fires can’t burn through them.  It’s not hard to see how the processes of deep water acquisition/sharing and fire-proofing can create a positive feedback loop that helps drive an inexorable expansion of shrubs into the surrounding prairie.

We didn’t talk about this in Kansas, but my experience is that fire-proof shrub clones are an important avenue for the establishment of trees as well.  Many tree seeds are deposited into those shrub patches by birds that see those shrubs as convenient and prominent perching sites.  If those seeds are able to germinate and establish within those clones – and they often can – the resulting trees can grow without fear of the fires that would otherwise threaten them.  Hiding in the middle of big shrub clones also gives those trees a chance to grow in relative safety from marauding prairie land managers…

Because much of Konza prairie has been managed under a variety of long-term fire regimes (1,2,4,10, and 20 year frequencies), Kansas State Researchers have some pretty good data on how fire frequency affects shrub expansion as well.  Essentially, prairies burned every year or every other year do not have encroachment by dogwood or sumac, but prairies burned less often are being gradually overtaken by shrubs.  Interestingly, the fastest expansion appears to be in prairie watersheds managed with a fire frequency of every four years (which is also about what the estimated average fire frequency was for that landscape during pre-European settlement).  While it might seem counterintuitive that a four year fire frequency allows for faster woody encroachment than a 10 or 20 year frequency, the explanation appears to lie in the way shrubs respond to fire.  Fire seems to stimulate radial growth in dogwood and sumac, meaning that the plants put an emphasis in growing horizontally rather than just vertically after they are burned.  Under very frequent fire, this is apparently immaterial, probably because the shrubs never get enough rest between fires to take advantage of that radial growth.  However, when they are given 3 years to recover between fires, that radial growth response after each fire means that burning actually stimulates faster expansion of shrub clones.  Under a 10 year fire frequency, that extra radial growth only occurs once every 10 years, so the overall expansion is actually slower than in under a four year fire regime.

Small research plots like these at Konza Prairie help demonstrate the impacts of various fire frequencies. The treeless grassland to the left is burned annually. The wooded area to the right is burned every 20 years. Both started out looking the same.

Small research plots like these at Konza Prairie help demonstrate the impacts of various fire frequencies. The treeless grassland to the left is burned annually. The wooded area to the right is burned every 20 years. Both started out looking the same.

Before you jump to the conclusion that burning every year or two seems the obvious best strategy for shrub control, remember that woody plant suppression is only one of many objectives for prairie management.  I’ll address some of the other, less positive, effects of frequent fires at Konza in an upcoming post.

As I said earlier, there are multiple factors that affect the rate of tree and shrub encroachment on prairies.  Seed rain might be as important as anything, and climatic conditions, increases in nitrogen and carbon dioxide levels, and fire suppression are all likely contributors as well.  However, the way plants compete belowground, particularly the deep water use strategy of clonal shrubs such as dogwood and sumac, also seems to play an important role.   Frequent fire application can be one way to prevent encroachment, though it comes with other baggage (see upcoming post…) and may not help remove shrub patches once they’re established.  At Konza, they took some of the every-20-year-fire-freuency watersheds and started burning them annually to see if they could get rid of the shrubs and trees.  Thirteen years later, those patches are still there, though the individual stems are much smaller.  It seems that while frequent fire might help prevent woody plant establishment, frequent fire alone might not be able to reverse it – at least not on a very fast timeline.

Woody plant encroachment is one of the biggest challenges we face in prairie management today.  A solid understanding of the mechanisms behind that encroachment should help us design more effective strategies to combat it.  Shredding, burning and herbicide application are all useful tactics, but figuring out the timing, frequency, and intensity of those applications will be critical.  We need to use the various competitive strategies of grasses, forbs, and shrubs to our advantage.  As an example, some recent work by Dirac Twidwell (University of Nebraska-Lincoln) seems to indicate that burning under more extreme heat and drought conditions than we typically feel comfortable with might be one way to really tip the scales away from woody plants.  The feasibility of that will be limited in some landscapes, but surely there are other innovative tactics that can help.  If we work together and aren’t afraid to try some new ideas, we can figure this out.