New Information on Tree Invasion in Prairies

One of the biggest challenges of prairie management today is the suppression of woody invaders.  Both native and non-native woody species can spread rapidly in prairie, making it difficult to maintain the open grassy habitat that most prairie species depend upon.

There has been extensive speculation about why shrubs and trees appear to be more aggressive and successful now than in the past.  Fire suppression has been a factor identified by many as a likely cause, but it’s clearly not the only factor because there are examples such as Konza Prairie in Kansas where shrubs have spread strongly under more than 20 years of regular fire application.

Fire can help suppress shrubs, but there are plenty of examples where frequent fire is not sufficient to stop their expansion.

Now, a new study from Konza Prairie may shed some light on at least some of the reasons behind the agressive expansion of shrubs in the Kansas Flint Hills and other mesic tallgrass prairies.  The research paper, written by Zak Ratajczak, Jesse Nippert and others, addresses both the initial survival of new woody plants and the subsequent spread by clonal species (such as dogwood and sumac, which spread by underground rhizomes).  It’s worth reading, and you can find a PDF here.

The question of why woody plants are able to establish more successfully in prairies now than they could several decades or more ago is still largely speculative.  Jesse Nippert explains his reasoning in an interview here.  Changing atmospheric conditions – especially higher nitrogen and carbon levels – are altering the competitive balance in grasslands to favor C3 plants over C4 plants.  Because shrubs like dogwoods (Cornus sp.) are C3 plants, higher levels of nitrogen and carbon in the atmosphere are likely giving them an advantage over C4 plants such as big bluestem and other warm-season native grasses that have historically had a competitive edge in tallgrass prairie.  This could explain why woody plants are surviving their seedling stage more now than they did in the past – but the idea still needs to be tested further.

However, while initial survival of shrub and tree seedlings is one important component of the issue, the research paper by Ratajczak et al. also addresses the subsequent spread of those shrubs – and they do so through field data collection.  They focused their work on the primary shrub species spreading at Konza Prairie – rough-leaved dogwood (Cornus drummundii).  What they found was that while most prairie plant species get the vast majority of their resources from the top foot or so of the soil profile, dogwood plants get almost half of their resources from below that level.  In other words, dogwoods are using resources – especially moisture – that most prairie plants aren’t taking advantage of.  (Yes, most prairie plants do have deep roots, but they typically reserve the use of those deeper roots for periods of drought and rely on their much more abundant shallow roots most of the time.)  Importantly, not only do “parent” stems of dogwood use deep soil water, new stems that are initiated by rhizomes (below-ground stems) do too – probably because they can pull water from their parents until they get their own deep roots established.

Taken together, the two ideas proposed by Ratzjcak, Nippert, and others provide an interesting hypothesis about how today’s shrub invasion may be taking place.  Higher levels of carbon and nitrogen in the atmosphere and/or soil provide a new competitive edge to colonizing woody plants.  That “fertilized” environment overrides the traditional advantage that warm-season grasses have over shrubs, which is that grasses are very good at monopolizing soil resources within the top foot or so of the soil profile.  Today, young woody plants are surviving long enough in that dry upper soil layer to extend their roots into deep moist soil – below where most other prairie plants mine resources.   Once those woody plants tap into that deep soil moisture, their survival is much more assured.  Woody plants that are clonal – such as rough-leaved dogwood and smooth sumac (Rhus glabra) – can then spread by rhizomes, continuing to take advantage of their ability to utilize the deep soil moisture their neighbors aren’t using.

Smooth sumac and flint hills prairie - Kansas. Konza prairie researchers have found that shrub invasion in upland prairies has much less aggressive than in lowlands. Is this because deep soil moisture is less abundant in uplands, reducing the competitive edge to those shrubs?

In addition to the carbon/nitrogen levels and deep soil moisture that both favor shrubs, anyone who has conducted prescribed fires in prairies containing large clones of dogwood or sumac knows that those shrub patches can inhibit the growth of grasses around their edges, reducing the amount of fuel for fires.  In other words, shrub patches can reduce nearby fire intensity – thus greatly reducing the effectiveness of one of the most important threats to their survival.  You really do have to admire their strategies, don’t you?

I think the hypotheses proposed by Ratajczak, Nippert, and their colleagues could explain a good portion of the puzzle.  Atmospheric conditions have certainly changed over recent decades, and that could explain why trees and shrubs have an easier time getting started in grasslands now.  However, the competition for deep soil moisture shouldn’t be much different now than it was historically.  We know there were at least some shrubs in historic prairies – why didn’t they grow into gigantic unstoppable clones?  What controlled their spread that isn’t doing so now?  Was the historic abundance of browsing animals high enough to control those clones?  Are the fewer browsers today simply overwhelmed by the increased number of new clones that are successfully establishing?  Are there other factors we’re not even considering yet?

There are plenty of questions left to answer, but it’s great that we’re moving in the right direction.  Besides the work of Ratazcjak, Nippert, and their colleagues, there are several other projects I’m aware of that are working to investigate the issue of woody invasion of prairies.  There are certainly plenty of us interested in their results!

41 thoughts on “New Information on Tree Invasion in Prairies

  1. Dr. Clenton Owensby at Kansas State University’s range department did elevated atmosphere carbon dioxide studies for years in the Flint Hills, at least 25 years ago I believe. How do his results compare with the source you sited?

    • Good question. I know OF his work, but don’t know the details. Maybe someone from K-State or someone else familiar with the work can address that for us. Since the researchers I mention in the post are from the same university and research site, I’ll assume they’re incorporating Owensby’s work into their thinking… If it’s like everywhere else, CO2 levels have been rising for quite a while. Hard to know when/where those levels start to become relevant to plant competition and many other factors.

      • I do not see Owensby sited in the article. I know he was doing work in the field with canopies that had elevated CO2 at various levels.

          • I’m not aware that the work Clenton was involved in included effects of CO2 enrichment on shrub/grass interactions. One of the practical reasons would be that the research utilized open-top chambers where CO2 levels could be elevated. They would have needed chambers much bigger than the ones I remember them using if they were to encompass whole shrub/herbaceous communities.

            Try going to Google Scholar and conducting a search using the keywords “owensby” “CO2” and “enriched”. You’ll be able to see a lot of the articles published on this topic by him and others, and many of them you’ll be able to download.

  2. The work at Konza is useful expecially for larger landscapes. I think woody encroachment in small prairies have other important contributing factors as well. The increase in those woody plants in the surrounding matrix increases the volume of seeds bombarding smaller prairies as compared to large landscape prairies. The perimeter to area ratio is also greater on small prairies increasing the influence of the surrounding matrix on those little remnants. Add on the possible influence of climate change and greenhouse gasses and it makes for a real challenge to figure out how to keep these remenants open.

    • Good prespective, Sherry.
      “Scale, scale, scale” and “It’s a combination of these hypotheses” – recurring refrains as we get to know more about natural phenomena.
      (Hmmm, I suppose all refrains are recurring.)

  3. Interesting hypotheses. While not debating that atmospheric changes have taken place, I do have a little trouble with the idea that the changes of the magnitude that we’re seeing in our recent history are enough to swing the pendulum so far in one direction or the other. I can imagine historically (in geological, not human, time) the atmospheric changes have been far greater in one direction or another, which would result in a rather drastic degree of biome instability.

    Personally (and thanks for running a site where I am free to say such word), dramatic changes require dramatic causal factors, and nothing has been more dramatic historically (in human time) than the change in scale and removal of large herbivores. I accept the large role that fire played, but I can’t see widespread conflagrations happening with the frequency needed to be the main determinator of woody invasion.

    • Ted – I don’t have any doubt that herbivore changes have been important too. Whether the changes in their populations match up on the timescale with the uptick in woody plant aggression is another problem. I’m not arguing either way. Lots of potential causes, and the most important thing is our response, I suppose.

  4. I am new to this blog, quite interested in this subject. I am not a scientist, but from what I read it appears that herbivores are the major driver of the whole thing. See David Western’s work on this in Africa, as well as Owen-Smith’s. Many sources for N American info, my favorite is Guy Robinson, a personal acquaintance. Paleo-ecologists seem to agree that the spread of people on the planet coincides with extinction of megafauna and appearance of a huge leap in deposit of fossil charcoal, indicating either a causal or coincidental shift from herbivores to fire as a (or the?) major factor in maintaining grasslands and shrublands. When one looks at the widespread distribution of grasslands and savannas east of the Mississippi even in historic times (blueberry barrens and Kennebunk plains of Maine, the many intervales (river valley meadows), Hempstead Plains of Long Island, oak savannas of Chesapeake Bay and the Carolinas, Kissimmee Prairie in Fla, Black earth prairies in Alabama, and many many more) it is hard to say available water was the defining factor. Elk and bison were recorded in many of these habitats. Imagine their impact along with the many smaller grazers and browsers, from deer to ground squirrels to ground hogs to rabbits. One of the Turner Ranch scientists has a good paper documenting how range of prairie dogs shrank with demise of bison, indicating multi species interdependence and impacts. Effects of grazing seem powerful enough to explain vegetation distribution patterns on a scale considered in this post. Happy to hear opinions otherwise.

    • David – welcome, and thanks for the very thoughtful comments. I don’t doubt the power of herbivores. On the other hand, I know of sites where brush seemed to be relatively low in abundance, even in the absence of fire, for decades, and then in the 1970’s or so, began an exponential increase in density (all based on aerial photos, so a little sketchy). I don’t think herbivore abundance changed during that time. That doesn’t mean the increase couldn’t have been herbivore related – with a lag time. Ryan and Sherry’s points about scale are also important. More seed rain on less area could certainly explain the exponential increase – and possibly the lag time, as seed rain and/or parent plants built up. And James M’s point about soil organisms may also be correct. I’d be surprised if any one factor provides more than a share of the cause.

      And, as I hinted at in an earlier comment, I think the real issue at hand is the way we respond. Do we give up on small grasslands because the energy needed to keep them open is more than we can afford? Do we prioritize some over others? Do we continue searching for more effective control mechanisms than we’ve thus far found? Tough questions. Enough to keep a prairie manager up at night!

      Thanks again.

      • From what I’ve seen … dogwood, sumac, and cedar are not nearly as problematic as common buckthorn. If these native invaders are removed then the prairie can be returned to ‘health’. After common buckthorn has invaded, removal tends to result in the proliferation of other non-native species like reed canary grass. It appears certain non-native species are synergetic. Instead of one plant invading, it is more like one ecosystem taking over another. Removing the buckthorn is often just the first step. You must continue to control other invasive species until complete erradication for the health of the native ecosystems to be returned. This is possible on a preserve wide basis, but would be difficult to accomplish on a continental scale. This means a less extensive, but regular, amount of effort is required to keep these species from again getting a foot hold.


  5. Good article Chris. I think Sherry has hit on a valid point in addition to the atmospheric factors. The scale of what remains may make it that much harder to keep woody species out of small prairies. I’d suggest that once established (made easier by lack of fire & grazing), the local intensity of seed rain and seed deposition by birds (which also may be more concentrated due to fewer habitats) makes the battle against woodies that much more difficult. The C3s definitely gain an advantage. I am in the midst of analyzing a new redcedar encroachment project we’ve got, and it’s interesting/disheartening to see that even with periodic fire, redcedar has recolonized prairie remnant pastures cleared just 10-12 years ago. If neighboring owners don’t manage (read: cut down) source trees, it’s a losing battle…one that has led to a 1000% increase in “source trees” in our Grand River Grasslands area in 30 years. I’m afraid the “Green Glacier” has arrived.

  6. I tend to shy away from the argument that lack of grazers is causing the invasion of woody species. This is mainly because in urban/suburban areas (where hunting is not permitted) deer levels are easily 10 times historical levels. Even with this high population of deer, which likes to eat dogwood, encroachment of this (and other) woody species continues to be a problem. The dogwood continues to spread clonally even when it has been nibbled down to a few inches in height.

    I’m not saying Bison did not have an impact; it just seems to me they selected against larger trees rather than the shrubs that have been invading prairies.

    Regarding fire, there are a number of conservative prairie shrubs that are not deterred by fire. Examples include New Jersey Tea and Lead Plant.

    I also initially thought of the altered competitive equilibrium of the C3/C4 biochemical pathways from increasing CO2/NOx levels. However, looking at various remnants, restorations, and disturbed areas (often adjacent to one another) has convinced me that soil changes are more important than the change in atmospheric gases or fragmentation.

    I really think the answer lies in biological changes to the soil. The problem invading shrubs are always more dense and more vigorous in disturbed areas. The invasion of these disturbance adapted shrubs into remnant prairies seems to indicate the prairie has been weakened as if it were sick (a description often given by Stephen Packard). I would look for changes in soil organisms first to find a reason.


    • I think the idea that bison controlled woody plants in tallgrass prairie is tenuous. Bison in the Great Plains have consistently been shown to be primarily gramivores, sometimes to a striking degree, And rubbing/horning behavior shouldn’t have mattered much to the woody shrub that is the primary focus of the Ecosphere article – rough-leaved dogwood. This species is a very prolific resprouter. A very enlightening exercise is excavating the underground portions of this plant, both roots and rhizomes, they can be substantial. Especially so, it seems, when the plant has been top-killed a few times.

      • Shrubs increase despite bison at Konza. Even if the bison cause some damage to woody vegetation, where their activities are concentrated, they reduce the load of fine grass fuels greatly. The bison watersheds that receive fire at Konza don’t come anywhere near thoroughly burning. There is a rather extensive literature from African systems with grazers and browsers, which should apply here. Grazers can break the fire cycle when in high abundance or when they focus their activities in an area, but browsers tend not to have that impact.

  7. I wonder if there’s any research that includes shifts in bird populations. I’m not well schooled in the habits of the feathered fauna, but I imagine there is another positive feedback loop there as well.

    • Bob, it’s a good point. Birds certainly help spread the seeds of many trees and shrubs (darn robins…). I don’t know of any bird populations that have increased dramatically parallel to woody plants, but that doesn’t mean it’s not happening. There are some species of birds that have done very well with human impacts. It’d be worth having a look at some breeding bird census data to see what those look like (and winter counts as well, actually). Good comment.

      • Yes, I’ve had the thought that without the Robin’s, Cedar Waxwings, and other berry eatters the spread of many invasive species would be stopped. People have even discussed control strategies, knowing full well it would never be supported because of political reasons.

      • There is research documenting the role of birds in facilitating woody plant establishment and woody plant community succession, but not in this system that I’m aware. The Ecosphere article, however, emphasizes the spread (enlargement) of currently existing individuals – clones. This is an asexual process, so bird dispersal of seeds may not be minimally to non-relevant.

  8. The post describes strong woody encroachment even with annual burning. Actually, annually burned Konza watersheds still have very little shrub cover. It is watersheds that are burned every four years or less often that have the woody encroachment. While I don’t see how CO2 and N could not be factors in the increase of woody vegetation, proximity to other woody vegetation (surrounding properties and our unburned “20-year” watersheds), rocky breaks (which block fires on Konza), and repeated grazing in the same area consistently reducing fuel loads are all obvious factors determining where woody vegetation is on the Konza landscape.

    Nearly all fires at Konza also occur in the spring, when dropped leaves under woody thickets are packed down and less receptive to fire. So Konza work on fire effects is largely limited to fires that occur at that time.

    Chris alludes to browsers. Would elk eat dogwood? I’d like to know.

    Increased CO2 should also lead to the prediction that herbaceous C3 plants would fare better in a C4 dominated landscape….

    • The Ecosphere article barely scratches the surface in terms of the role of elevated levels of CO2 and nitrogen deposition. The research they conducted didn’t look at those issues directly but if I understand the article correctly they treated elevated levels of CO2 and N as an assumption in their model of a positive feedback loop – the elevated CO2 and N facilitate shrub establishment to a degree that didn’t occur historically.

      The podcast that is available includes much more discussion of the potential role of elevated CO2 and N in driving shrub invasion of tallgrass prairies. The hypothesis is pretty elegant and goes something like this:
      1) Tallgrass prairie as we know it is dominated by warm season perennial grasses, particularly big bluestem. Their dominance is in part due to their great efficiency in use of CO2 and N – they can maintain or even thrive when those resources are limiting to most of their floral neighbors (C3 shrubs and forbs). Another reason they dominate is because they are well-adapted to frequent top-removal via grazing, fire or drought-induced senescence.
      2) In a world characterized by historic levels of CO2, N and fire (and probably grazing as well), warm season perennial grasses come out on top in the competitive milieu. In a world where fire (and probably grazing) stays the same but CO2 and N are elevated, deep-rooted clonal shrubs do much much better than before (except when fires occur annually). Their world isn’t as limiting in terms of CO2 and N as it was before.

      Check out the podcast:

  9. How about changes in seasonality of fire? Or has Konza utilized fire during various seasons and still observed the increase in woody species?

    • At Konza, it is mostly spring fire. There are not watersheds at Konza burned less than at least every other year that are burned outside of March 1 to April 30. There are two summer burn watersheds and fall and winter burned watersheds that are burned every other year.

      Unlike a lot of management situations, fire is not prescribed so as to maximize its impact on any particular vegetation deemed undesirable.

      (I have helped with numerous Konza burns and do research there)

  10. Another fascinating topic, Chris. I would echo James’ comments regarding both grazers and C3/C4 species. Here in the Pacific Northwest, I have been puzzled for decades about how clonal shrub species were historically held in check in our lowland prairies. Native species like Symphoricarpos albus and Rosa nutkana are major invaders into some prairies, and repeated burning seems to do nothing other than make them shorter. Our prairies do not have a history of large herds of grazing animals (other than deer, which probably are more abundant now in most areas than they’ve been in many decades), which would seem to rule them out as a major contributing factor. Furthermore, we don’t have any C4 species that might be differentially affected by atmospheric changes.
    While I would agree that how we as managers respond is critical, having a clearer understanding as to why the changes are occurring can often help us craft the most effective responses. Even with such information, we may ultimately end up reaching for the herbicide sprayer (or wand), but I’d like to at least know that I’ve ruled out other alternatives that might reflect a more nuanced understanding of ecosystem function.

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  12. A fairly recent study, Hajny et al 2011 (Rhus glabra response to season and intensity of fire in tallgrass prairie), out of KSU highlights that only under fall/winter low intensity burns do populations of Smooth sumac actually decline. Spring burns actually stimulated increasing populations of the species. So, perhaps there’s one change we can make to combat the shrub invasion.

  13. Great topic, Chris. And thought provoking responses. I have run into the same problems controlling woody invasion (rough-leaved dogwood, smooth sumac, Osage orange, honey locust, etc.) iin NE Kansas and believe pretty much the same as you all. Timely spring rains, resprouting after fire, seed rain from nearby hedgerows, seed distribution by birds, scale, lack of grazers, and now elevated CO2 concentrations. The last factor is making more and more sense to me as I have been puzzled over the last 10 years or so by the tenacity of dogwoods and sumac (and others but not common buckthorn) persisting after prescribed burns. Since our world is rapidly changing and the prairie ecosystem is so dynamic, it is very difficult to narrow it down to one factor (of course). I am starting to wonder if the prairies are suffering, becoming unhealthy just like the pine and oak forests in some of the western states. Unhealthy may not be the right word, but certainly changing and becoming less resistant to the movement of other invasives like sericea lespedeza (Lespedeza cuneata), which seems to move unimpeded through grasslands here in eastern Kansas.

  14. My woody invader in NW Minnesota is willow, fortunately we have no sumac, red cedar and buckthorn spreads slowly. Fire will suppress the willow but only for a short time, they will be green and growing from the roots later in the season. We have had above normal rainfall every year since 1993 and areas within the grassland that are dished and poorly drained support a solid stand of willow. Because these areas hold water in the spring during the burn season control is difficult, if I get a dry stretch in the fall I will mow with a sickle mower to daylight the area. Not so many years ago moose were somewhat common on our farm and they enjoyed the young willow trees. Moose have been in decline in NW MN and we are the poorer for it.

  15. Could there be a phenolgy-climate change interaction? In southern Wisconsin, spring is definitely coming earlier, and C3 plants are able to break dormancy much earlier–compass plant is flowering about 3 weeks earlier today than the average date for the 1930s and 40s, for example ( But if C4 grass emergence and growth is more tied to day length or sunlight intensity, then the increase in the growing season length could be favoring C3 species, from non-native grasses to shrubs. I haven’t had a chance to see if there is phenological data on C4 grasses at the Leopold shack, but if so that might shed some light on this possible interaction.

  16. I’m late to the party, but just yesterday, the entire range ecology lecture was devoted to the rapid increase of Mesquite (Prosopis glandulosa, P. glandulosa var. torreyana, P. pubescens) in Texas (the first two species are the most dominant in the state). If you have ever been to Texas, Mesquite is utterly ubiquitous!

    Mesquite is also a C3 plant, and studies have shown it to respond favorably to increases in atmospheric CO2. If you drive through the Trans-Pecos region of Texas, you can really see the the increase in Mesquite densities around the large metro areas like Midland and Odessa, and Abilene and San Angelo, as well as the overgrazed and fire suppressed areas. Cities are localized and concentrated sources of CO2 outputs, so it follows that Mesquite would tend to increase radially from metro areas as a city’s carbon cloud increases.

    Humans just made an extremely tough grassland competitor much more competitive.

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  18. Maybe we tend to under estimate the contribution of atmospheric nutrient loading to changes in our few good remaining natural areas. Massive nitrogen deposition has been widely cited in Europe as a cause of change in species composition.
    In much of northern Germany, for instance the species rich heath and meadow flora has disappeared in the last 50 years, succeeded by vegetation that thrives on high levels of N. Here in Central Illinois railroad prairies that a few decades ago were rich in prairie grasses and forbs are now expanses of primarily tall fescue. Because of the few and rather small natural areas here I cannot speak to the shrub issue. However, in a nearby, very small cemetery nature preserve, described as a shrub-prairie by one author, Corylus americana has shown an almost explosive increase in coverage in just a few years. This is corn country, therefor much ferilizer use, but we have also had high precipitation and a lack of burns. Life is complicated, so is everything else. Still, high N and C levels may well be major drivers in the changes.

    • Agreed, Henry. And as you say there are so many factors that could be contributing to species composition change, it’s hard to narrow the choices and figure out what the major drivers are for each change. Management can certainly have a large impact too, but is management responses are likely strongly influenced by nutrient loading. Lots of research opportunities for a long time!

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