Tag Archives: climate change

New Information on Tree Invasion in Prairies

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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!

Ecological Resilience in Prairies: Part 2

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This is Part 2 of a two part series on ecological resilience in prairies.  In Part 1, I interviewed Dr. Craig Allen about the basic definition of ecological resilience and then wrote about the relevance and application or resilience to prairie ecosystems.  In Part 2, I explore how ecological resilience can influence the way we restore and manage prairies, and about how much we still have to learn about how to do that.

 

Influencing Resilience through Restoration and Management

Understanding ecological resilience should help us better design restoration and management strategies that build and maintain resilience in prairies.  Using the components of resilience discussed in Part 1, it seems apparent that when restoring (reconstructing) prairies, it’s important to maximize species diversity in seed mixtures.  More importantly, prairie restoration that adds to the size and connectivity of existing prairie remnants should make the entire complex of restored/remnant prairie more resilient (see earlier post on this subject).  Finally, selecting and altering restoration sites, when possible, to include topographic and other habitat type variation – and multiple examples of each type – can also help ensure the resilience of the resulting restored prairie. 

Designing management strategies for prairies that sustains ecological resilience is trickier because we still have much to learn.  We’re far from fully understanding the various stable states prairies may exist in, or flip to, let alone where the thresholds are between those states.  In addition, the level of plasticity, or range of adaptive capacity, of prairies is a subject of great debate right now among prairie ecologists – although the discussion is not usually framed in those terms.  The real question is – How much can prairies change their plant and animal species composition and still remain “in the bowl”? 

As an example, I manage a sand prairie that was hayed annually in the mid-summer for about 20 years before The Nature Conservancy purchased it in 2000.  Over that 20 year period, the plant community in that prairie adjusted to that annual haying regime.  Species such as stiff sunflower, leadplant, and sand cherry became restricted to a few steep slopes where hay equipment couldn’t go.  Early summer grass and forb species became very abundant, but later season flowering plants were less so because they were mowed off around their flowering time each year.  The prairie was a nice quality mixed-grass prairie, with good plant diversity, but definitely had the “look” of an annually-hayed prairie. 

Stiff sunflower has increased in abundance since annual haying ceased.

When we took over the management in 2000, we let the site rest for about 5 years and burned portions of it each year during that time.  Then, we began introducing some combined fire and grazing treatments at different intensities and at varying times of the season.  As a result, the prairie looks fairly different now.  Stiff sunflower and leadplant have spread considerably through the site, re-taking lower slopes where hay equipment had earlier eliminated them.  Cool-season grasses (native and non-native) change in abundance from year to year, but warm-season native grasses are certainly more dominant than they previously were.  Overall forb diversity is about the same as it was, but the relative abundance of many forbs has changed, and those abundances now vary from year to year, rather than remaining fairly stable.

In the context of plasticity, or adaptive capacity, this prairie has demonstrated that the 20 years of haying was not enough to move the plant community into a new stable state from which recovery, if that’s the right word, was not possible.  The community was altered by that haying regime, but upon alteration of that regime, the community composition morphed to match changing conditions – without losing plant diversity.  In other words, assuming that the prairie hasn’t lost anything critical during the 11 years of our management, both the haying regime and our current management have kept the prairie “in the bowl”, though it changed appearance fairly dramatically.  Its adaptive capacity is broad enough to include the “hayed” look and the “crazy Nature Conservancy management” look.  The real test of this, of course, would be to reintroduce haying for another 20 years and see if the plant community reverted back to something very similar to the condition it was in when we purchased it. 

This sand prairie has changed species composition substantially since we switched management from annual haying to a mixture of prescribed fire and grazing.

Here’s another example from my own experience.  We have a 45 acre restored prairie (prairie reconstruction) that was seeded in 1995 by Prairie Plains Resource Institute adjacent to a degraded remnant prairie.  The seed mixture included approximately 150 species of mesic prairie plants, most of which established successfully.  We managed the prairie with periodic spring fire for its first seven years, and then incorporated it into our experimental patch-burn grazing system (light stocking rate).  During nine years of patch-burn grazing management, (6 of which were during a severe drought) the plant species composition in any one place has bounced around quite a bit due to the fire/intense grazing/rest cycles imposed by the patch-burn grazing management system.  Overall, however, the prairie has maintained its mean floristic quality within 95% statistical confidence intervals (I collect annual data which entails calculating floristic quality within 100 1m2 plots and averaging the values across those plots).   Read more about our patch-burn grazing work and results at this restored prairie and others here.

That’s not to say the prairie hasn’t changed – it has.  Some plant species have increased in frequency among my annual data collection plots, some vary in frequency from year to year, and others stay fairly stable.  However, no species has dramatically declined over the time period.  (I wish I had data on other species, particularly insects, but I don’t.)  Perhaps the most interesting, and somewhat concerning, phenomenon has been an increase in the frequency of Kentucky bluegrass in my plot data.  The increase has been fairly steady over the nine years of patch-burn grazing and data collection, and bluegrass is now in about 75% of my 100 1m2 plots, though it rarely looks dominant where it occurs.  To this point, that increasing frequency doesn’t seem to be impacting the overall diversity or floristic quality of the plant community, but that doesn’t mean it won’t at some point.  Two possibilities are: 1) Our management is allowing bluegrass to enter the plant community but remain a minor component, or 2) Kentucky bluegrass is on a steady march of increasing dominance and will eventually turn my restored prairie into the same kind of low-diversity degraded prairie that exists in the adjacent remnant prairie.  I won’t be completely shocked by either scenario, but I have hope that #2 won’t happen because of the way bluegrass is acting in the community to this point.  It’s way too early to know for sure.

Interestingly, I have some large exclosures within this restored prairie that have never had grazing, only prescribed fire at a similar frequency to the grazed portion.  Those exclosures have very little Kentucky bluegrass in them – probably because of both the differing management and the fact that the exclosures are on the far side of the restoration from the neighboring bluegrass-dominated remnant prairie.  However, the exclosures also have much lower plant diversity and mean floristic quality than the grazed portion of the restored prairie.  Visually, they are dominated by warm-season grasses and a few large forbs (e.g., perennial sunflowers).  At this point, I prefer the grazed portion of the prairie because it seems to line up better with my management objectives of maintaining diverse and resilient plant communities (assuming it’s not slowly becoming a bluegrass wasteland). 

This photo was taken when the 1995 prairie seeding was nearing the end of its establishment period (fifth growing season). The prairie has maintained its mean floristic quality (and all of its plant species) through years of patch-burn grazing and drought.

To relate this example back to adaptive capacity, it appears likely that the grazed portion of the restored prairie has the adaptive capacity to retain its integrity as a prairie community through fairly wild fluctuations in species composition as a result of stresses from fire, grazing, and drought.  This is, again, remembering that I’m only evaluating the plant community and that the experiment is far from over.  On the other hand, it appears the exclosed portions of the prairie have lost plant diversity over time.  Whether the communities in those exclosures are still “in the bowl” or in a new stable state of lower diversity is a big question.  To address it, I’m going to open one of them to fire/grazing this coming year and exclude a portion of the currently-grazed prairie and see what happens.  If the two plant communities trade identities to match their new management regimes, I’ll know that both were still “in the bowl” and understand more about the adaptive capacity of our prairies.  If they don’t, that will be equally instructive!

The Upshot

Building and sustaining ecological resilience in prairies may be the most important component of prairie conservation in the coming decades.  Threats from invasive species, habitat fragmentation and detrimental land management practices, compounded by climate change, will make conservation extremely difficult.  Armoring prairies with ecological resilience gives us the best chance of success. 

In order to build that resilience, we first have to understand it better.  It is certainly more complex than the few simple examples I’ve provided.  There are numerous belowground processes and systems we still know relatively little about.  Even aboveground, there are many more questions than answers regarding the way species interact with each other and their environment – and what is required to maintain those interactions.  To gain a better understanding of these natural systems, we have to rely on experimentation and observation.  I think there are essentially two broad questions:

  1. What is the adaptive capacity of prairies, and where are the thresholds between the desired state and other, less desired, stable states?  This will certainly vary between tallgrass and mixed-grass prairies, and between sand prairies and black soil prairies, etc., but there will almost surely be some consistent themes. 
  2. How important is it to keep the ball moving within the bowl?  In other words, are prairies really like our human bodies, in that the more we stress and rest them, the better prepared they are for future stresses?  (Does the bowl shrink if we don’t keep pushing at the edges?)  Or do we just have to keep prairies from being stressed too far in any particular direction? 

 

We can work toward answering these questions with direct experimentation on prairies we manage (similar to my simple experiment with grazing and exclosures in the restored prairie example presented earlier).  In addition, though, we can learn much from prairies that “flip” to less desirable stable states (hopefully not the ones we’re managing!) by documenting as much as we can about what happens to them and why. 

Most importantly, I hope that thinking about ecological resilience with regard to prairies will make you look at the prairies you’re familiar with in a new way.  Seeing prairies as balls rolling around in a bowl makes watching and managing prairies a much different experience than seeing them as a stable “climax community.”  When we expect change, it’s easier for us to perceive change, and the more observant we are, the more we’ll learn.  And goodness knows we have plenty to learn.

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If you’re interested in learning more about ecological resilience, here are some relevant references that Craig Allen recommends (I’m pretty sure it’s just a coincidence that he’s a co-author on all of them).  The Gunderson et al. book reprints a lot of the classic / foundational papers on the subject.    

Sundstrom, S., C. R. Allen and C. Barichievy.  Biodiversity, resilience, and tipping points in ecosystems.   Conservation Biology: in review.

Gunderson, L., C. R. Allen, and C. S. Holling. 2010.  Foundations of Ecological Resilience.  Island Press, New York, NY.  466pp.  

Allen, C. R., L. Gunderson, and A. R. Johnson.  2005.  The use of discontinuities and functional groups to assess relative resilience in complex systems.  Ecosystems 8:958-966.

Forys, E. A., and C. R. Allen.  2002.  Functional group change within and across scales following invasions and extinctions in the Everglades ecosystem.  Ecosystems 5:339-347.

Peterson, G., C. R. Allen, and C. S. Holling.  1998.  Ecological resilience, biodiversity and scale. Ecosystems 1:6-18.