The Myth of Self-Sustaining Prairies

Here’s a question I get asked occasionally:  “At what point will my prairie become self-sustaining?”

There are lots of ways “self-sustaining” can be defined, of course, but usually the person is hoping that at some point they can just step back and let the prairie do its thing with very little or no human input.  In other words, they hope the prairie will function like a machine.  Once you have it tuned up correctly, it’ll hum along just fine with only occasional inputs of fuel or maintenance.

Ah, that it would be so easy.  Unfortunately, there is a short answer to the question, and it’s a disappointing one.  The answer is, “It just doesn’t work that way.”

Here’s the short explanation of that short answer:

A prairie with no management at all accumulates thatch from each successive year of plant growth, and if not removed, that thatch eventually builds up to the point at which only a small number of plant species can survive.  Unfortunately, the most dominant of those surviving species tend to be either trees/shrubs or invasive plants.  In the eastern half of Nebraska, smooth brome tends to be a primary winner, along with tree species such as Siberian elm and eastern red cedar.

Besides the issue of thatch build-up, there are just too many threats, particularly from invasive species and trees, for prairies to maintain their species compositions and ecological functions without human management.  This is particularly true with tallgrass prairies in an agricultural matrix.  The degree of vulnerability to invasion depends upon soil type and the surrounding landscape.  Some soil types seem favor invasives more than others – oftentimes, high soil nitrogen levels can favor exotic grasses, for example.  The degree of invasive species pressure on a prairie is also influenced by the abundance and proximity of those invaders in the neighborhood around the prairie . However, all prairies (that I’m aware of) have some degree of vulnerability to invasive species.

Active management, such as the application of prescribed fire, is needed to prevent excessive thatch buildup and to help suppress invasive species.

That’s the short answer.  A longer and better answer is that tallgrass and mixed-grass prairies are not “climax communities” in the classic sense.  In my early ecology classes, I learned that terrestrial plant communities move through a process called succession from bare ground to some final stable state – usually a forest.   Bare ground is colonized by opportunistic species, which are eventually pushed out by longer-lived grasses and wildflowers.  Those grassland species are then replaced by various generations of tree species, each topping out the other until a final set of tall long-lived trees becomes dominant and creates a stable community.  Disturbances such as fire or severe weather events might set back succession temporarily, but the process keeps moving toward that climax community.

Prairies don’t fit that successional model very well.  Prairies are maintained (and defined?) by disturbances such as fire, grazing, and drought.  Without some combination of those ecological processes, prairies turn into woodlands.  Because of that, some might argue that prairies are simply an ephemeral stage of the longer successional process, and not really a stable ecosystem.  Others might argue that the whole idea of ecological succession is overly simplistic and not representative of the way all ecosystems function.

Without getting into that larger argument, the real point is that if we agree prairies are important, and we want to maintain them, active management is necessary.  Some people point to expansive prairies in Great Plains landscapes and wonder if those prairies could maintain themselves without humans if given the chance.  After all, lightning-caused fires and roaming herds of bison should be able to take care of things without interference from people, right?  In reality, we don’t have any historical precedent to back that up.  Today’s prairies have only been around since the last ice age –  about 10,000 years (less in the east, more in the west.)  During that entire time, people have been active managers of those prairies.  Fires set by Native Americans were much more abundant and extensive than lightning-caused fires.  Bison herds, and many other herbivores, responded to those fires by focusing grazing in those recently burned areas.  That intensive fire/grazing disturbance interacted with and compounded the impacts of long droughts, floods, and other weather-related events.  Cumulatively, those major disturbances maintained the integrity of prairies.

Along with climate and fire, bison (and other grazers/herbivores) were a major force that shaped prairies. However, people and their activities were also an important component of the process.

There’s really no way (and no reason) to separate people from prairie.  Regardless of the intent or motivation of the people who manage prairies – historically or now – their actions have tremendous impacts.  Similarly, inaction by people who control prairies has tremendous impacts as well.  In the natural resource management world, the phrase “No management is still management” is well-worn but nevertheless true.

Of course, defining the need for continual human management – even in the absence of today’s new challenges such as invasive species – doesn’t solve the problem.  What kind of management is needed?  How do we know when to do what?  The answers to those questions are complex, still being debated, and the primary subject of this blog, my book on prairie management, and myriad discussions among prairie managers around the world.

Some people who agree that prairies require some level of active management still search for a relatively simple management recipe to follow.  Annual haying or burning or two to three-year rotations of fire or grazing are examples of management regimes that are commonly used and advocated for.  This is really just a small step up from the idea that prairies should maintain themselves.  In this case, the argument is that prairies should be able to maintain themselves if we just provide them the right basic disturbance framework.

I’ve given my opinion on simple, repetitive management regimes often within this blog (see my Calendar Prairies post as an example).  I think repetitive management threatens plant diversity because there are always some plant species that are favored in a particular management regime and others who are not.  Over time, those species not favored will inevitably fade out of the community if the same regime is applied over and over.  Perhaps more importantly, animal species – including insects – with fairly specific habitat structure requirements are similarly affected.  Some species thrive under repetitive management if that management consistently favors them.  However, those animals that don’t find what they need in that management system can’t normally survive for many years in suboptimal habitat like many perennial plants can.   Animals without appropriate habitat either move or die – and in fragmented landscapes, or in landscapes where the same management is in place across the entire landscape, moving may not be a viable option.

Annual haying provides good growing conditions for many plants - especially those the bloom and produce seed prior to the haying date. On the other hand, some of the plant species favored by annual haying (including smooth brome) can become invasive. In addition, some desireable native plant species do poorly under annual hay management and eventually disappear from those prairies.

All of this adds up to one conclusion.  Diverse, functioning prairies require active, constant, and thoughtful management by humans.  There’s no getting out of that responsibility.  If we choose not to be active thoughtful managers, we are choosing to let prairies degrade, and we’ll have to live with the consequences (“No management is still management”).  Hopefully, though, most people with influence over the management of prairies will embrace their role, and be active managers – as well as active participants in ongoing discussions about the impacts of various management techniques and systems.

Though active prairie management is time-consuming, and often expensive, it’s also extremely rewarding.  Whether it’s a small backyard prairie garden, a 20,000 acre grassland, or something in-between, every year is a chance to try new things, see what happens, and learn from the experience.  More importantly, the diversity of plant, insect, and invertebrate species in well-managed prairies – large and small – is its own reward.  Who could ask for more than that?

Using Defoliation of Dominant Grasses to Increase Prairie Plant Diversity

 In many prairies, the primary suppressors of plant diversity are dominant grasses – both native and non-native.  These grasses, left unchecked, can monopolize light, moisture, and nutrients to the point that few other plant species can coexist with them.  I’m not sure why some prairies suffer from this more than others.  There is some evidence that hemi-parasitic and allelopathic plants such as pussy toes, false toadflax, and wood betony can play a role in suppressing grasses and facilitating forb diversity, but I don’t think that’s the whole answer because I’ve seen very diverse prairie plant communities without those species – or with only a few scattered populations of them.  Regardless of the reasons, we are left with many prairies that have lost – or are losing – plant diversity through domination by grasses, and we have to decide what to do with them.  Some of those prairies are restored (reconstructed) prairies that started out with high plant diversity but have since lost much of that diversity.  Others are remnant prairies that have been degraded by overgrazing and/or broadcast herbicide application.  Still others are relatively diverse remnant prairies that are slowly losing diversity as individual forbs die without reproducing.

Big bluestem. A good native grass, but sometimes so dominant that overall plant diversity suffers.

I think one of the best tools we have for combating grass domination in prairies is defoliation – the removal of above-ground portions of plants.  Defoliation has always been a major component of prairie ecosystems through fire and herbivory, and mowing and (non-lethal “burn back”) herbicide applications are additional options at our disposal today.  Plants respond to defoliation in various ways, depending upon the severity of defoliation, the frequency and/or duration of the defoliating event, the stage of the plant’s growth at the time of defoliation, and each species’ genetic programming.  The way each plant, and its neighbors, respond to a defoliation event determines which plants will gain or lose territory.  In other words, defoliation influences the competition between plants – and manipulating competition between plants is really what most prairie management is all about. 

Some of the earliest research I’m aware of on the effects of prairie plant defoliation can be found in range management research from the 1950’s and 1960’s.  The earliest paper that is often cited from that era is by F.J. Crider, who documented the effects of defoliation on the root growth of grasses.  He (and others since) found that a severe defoliation of a grass plant resulted in an immediate cessation of root growth as plants reallocated resources from root growth to regrow leaves and stems.  More importantly, those grass plants actually abandoned sections of living roots as well – shrinking the total root mass of the plant fairly dramatically.  This makes sense, since the plant has to support those roots through photosynthesis, and a severe defoliation takes away most of the plant’s ability to photosynthesize. 

A simplified look at how dominant grasses can affect plant diversity. In the top example (A) grasses have monopolized both aboveground (light) and belowground resources (moisture and nutrients). After defoliation (B), both the above and belowground parts of the grasses have shrunk, freeing up resources and allowing other plants to establish within that lost territory. As the grasses recover from the stress of defoliation, their vigor and size increase, but the new plants have a fighting chance - at least for a while - to hold the new ground they've taken.

Those early range science research data provide some useful context for today’s prairie management, but those researchers were primarily trying to figure out the intensity of grazing they could employ while still maintaining a dominant stand of grass.  As prairie managers, by contrast, we want to reduce the dominance of grass to increase the diversity of other plants.  We can still learn from what those range scientists discovered; we just want to employ it in a different way.  Since plants primarily compete for light, moisture, and nutrients, we want to find ways to make those three kinds of resources more available to plants other than dominant grasses.  Defoliation can reduce shading aboveground (removal of leaves and stems), while simultaneously freeing up the availability of moisture and nutrients belowground (reduction of root masses).

In prairies where dominant grass species are suppressing plant diversity, we want to defoliate those grasses in a way that forces them to cede territory to other species.  In order for that to work, the first important factor is that the defoliation has to happen during the growing season.  Defoliating a dormant plant (e.g. with an early spring burn) doesn’t have any impact on its root system, which is a critically-important part of its competitive ability.  In order to force a plant to reallocate resources away from its roots, defoliation needs to take place after the plant has already invested significant resources in above-ground growth.  This is why a late-spring burn can have a significant (if temporary) impact on cool-season exotic grasses such as smooth brome.  Burning, grazing, or mowing grasses when they are just starting to flower has the biggest impact on most species because they have invested the maximum amount in their above-ground growth by that point.  Alternatively, repetitive mowing or grazing of grasses can also have a strong – and perhaps longer lasting – impact on their root systems because every time the grasses start to regrow, they get nipped off again, forcing them to regroup and reallocate resources time after time.

The immediate result of that kind of severe and/or repeated defoliation of dominant grasses is a release of opportunistic plants that thrive under low levels of competition.  This includes many annual and biennial plants, but also perennial plants that are built to move quickly into open space.  The quick flush of these plants often turns people off of defoliation because of a widely-held misperception that those “weedy” plants are outcompeting “good” plants.  In truth, the weedy plants are only able to grow because the competition that normally holds them in check has been suppressed.  When the defoliation event is over, the dominant grasses and other perennial plants will slowly recover their vigor – at which point the weedy plants will retreat and wait for another opportunity.  Rather than indicating a problem, I use the presence of weedy plants to tell me that my defoliation treatment has succeeded in weakening dominant grasses and has opened up space for other plants to take advantage of. 

As common as the overly-dominant grass problem is in prairie conservation, there is a frustrating scarcity of research that addresses it.  However, a recently-published research project by Kat McCain and others at Kansas State University provides some very nice insight into what can happen when dominant grass species are suppressed in a restored (reconstructed) prairie.  Kat and her colleagues studied plots of seeded prairie that had become heavily dominated by big bluestem and switchgrass over time.  They found that removing half or all of the big bluestem tillers (stems) – by clipping and herbicide application – from a plot significantly increased plant diversity.  Interestingly, removing switchgrass tillers in the same way had much less impact.  Following the removal of big bluestem tillers, the researchers saw increases in the vegetative cover of some forb species, including roundheaded bushclover (Lespedeza capitata),  pitcher sage (Salvia azurea), and blue wild indigo (Baptisia australis) within those plots, as well as new establishment of forb species including whorled milkweed (Asclepias verticillata), green antelopehorn milkweed (Asclepias viridis), leadplant (Amorpha canescens), roundheaded bushclover, and heath aster (Symphyotrichum ericoides).  In other words, suppression of big bluestem competition led to increased vigor among existing forbs and also allowed new plants to establish in the territory previously held by the dominant grass.  The study bolsters the theory that grass competition is suppressing forb diversity in many prairies, but also provides information on how plant communities might respond if that grass competition is reduced.

Of course there is a difference between simple defoliation and the kind of clipping/herbicide combination used by McCain and her colleagues.  In addition, most defoliation treatments (especially prescribed fire and haying) in prairie management are non-selective, meaning that all plants are simultaneously defoliated – not just the ones we want to suppress.  Uniform defoliation likely decreases some of the benefits of suppressing grass vigor because the vigor of the plants we hope will respond is suppressed as well.  However, there will be still be plants that can take advantage of the newly available light and soil resources following a uniform defoliation treatment, and by altering the timing of defoliations from year to year, we can ensure that a variety of species get the opportunity to respond.

Haying is a good example of uniform defoliation. Every plant gets cut at the same height and at the same time.


Ideally, though, we would like the ability to defoliate only those species that are suppressing plant diversity.  One way to do that is by using a selective herbicide such as Poast, which affects only grasses (not forbs, sedges, or other plants).  Poast is labeled for control of annual grasses, but at light rates can also provide short-term burn-back (defoliation) of perennial grasses as well, and some prairie managers have seen plant diversity increase following treatments.  Because it can kill annual grasses, and calibrating the appropriate application rate with the desired result can be tricky, it’s probably best to use this treatment on restored prairie rather than on remnant prairies for now – and to test it on small patches first. 

Another way to get selective defoliation is by the use of grazing.  In an earlier post, I described our use of patch-burn grazing in our Platte River Prairies as a way to increase and maintain plant diversity.  Patch-burn grazing is essentially a technique that uses patches of burned prairie within a larger prairie to attract grazing animals, concentrating grazing activity in those burned areas while allowing other areas to recover.  Under a light stocking rate, we find that cattle – even in the burned patches – are very selective about the plants they choose to eat.  Their top choice of grasses in the spring is smooth brome, and their summer favorite is big bluestem.  These happen to be two of the top three grasses that appear to stifle plant diversity in our prairies (the third is Kentucky bluegrass, which cattle like less well).  In our application of patch-burn grazing, a burned patch of prairie is normally grazed intensively for an entire season before the next patch is burned and cattle shift their attention to that.  That length of intense defoliation has significant impacts on the plants that are grazed – and again, under light stocking rates, the primary plants that are defoliated are smooth brome and big bluestem.  Interestingly, switchgrass is much less attractive to cattle and is often left ungrazed – or lightly grazed – in our prairies.  I found it intriguing (and encouraging!) that McCain and her colleagues found that switchgrass appeared to have much less impact on plant diversity than big bluestem did.

The effects of selective grazing in a restored prairie. This photo shows the burned patch with a patch-burn grazing system where a light stocking rate allows cattle to be selective about their eating preferences. Big bluestem is cropped very short, while other grasses and forbs are ungrazed or lightly grazed. Species such as hoary vervain aren't typically grazed even under high stocking rates, but many species such as purple prairie clover (front left), illinois bundleflower (middle left) and stiff sunflower (blooming) are commonly considered to be favorites of cattle - but only at higher stocking rates.

It would stand to reason that selective grazing of big bluestem and smooth brome would favor the expansion of the ungrazed plant species growing with those grasses.  While I’ve not had the time or resources to conduct much full-scale research (help wanted!), I do have data that supports that idea.  Through annual data collection of plant species frequency, I’ve found that the density of species (the number of plant species per 1m2 plot) increases by 20 to 30 percent in the year following the burn/graze treatment in a patch of prairie – in both restored and remnant prairies. 

Data from two prairies under patch-burn grazing. In both cases, the graphs show the number of plant species per square meter over time from the year of fire and intense grazing through the two subsequent years. The East Dahms site is a degraded remnant prairie and the Dahms 95 site is a restored prairie that was seeded in 1995 with over 150 plant species. The error bars represent 95% confidence intervals.

Of course, the increase I see in species density following grazing includes many plants such as ragweed and other opportunistic species, but I also see species like purple prairie clover (Dalea purpurea), Illinois bundleflower (Desmanthus illinoiensis), and stiff sunflower (Helianthus laetiflorus) respond as well.   In addition to seeing this in my plot data, I can walk out into the prairies and see seedlings of these species around the adult ungrazed plants.  Not all of those young plants survive their first year or two, but some permanent plot data I’ve looked at shows that at least some of them do.  By the way, I see similar post-grazing increases in plant species density and establishment of species like prairie clover under patch-burn grazing with higher stocking rates (less selective grazing) as well. 

As I said in my previous post on grazing, I’m not advocating that all prairies need to be grazed.  I’m not even advocating grazing as the solution to all prairies that suffer from overly-dominant grasses.  However, as we search for answers to address grass suppression of plant diversity, grazing certainly appears to be one viable alternative that is worth more investigation.  I’m continuing to experiment with variations in the way we employ fire and grazing treatments, and will keep learning as I go.  I’m also combining seed additions with those grazing treatments to see if I can take advantage of the open space created by defoliation to help establish new plants from seed – something that appears to happen rarely without some kind of suppression of surrounding vegetation.  I’m seeing some positive results, but it’s too early to know how well it will work long-term, and I’m still tweaking seeding rates and other factors.

Whether it’s grazing, prescribed fire, haying, or herbicide application, defoliation may be the most powerful tool available to help us suppress dominant grasses and increase plant diversity in prairies – where that is an issue.  The biggest obstacle to its application is probably the fear of causing damage to prairie by burning, cutting, or grazing plants during the growing season, but I think that fear ignores the resilience of prairie communities.  We still have a lot to learn about the most effective ways to apply defoliation to achieve our objectives, but the only way we’ll learn is by experimentation.  I hope you’ll join me in testing these methods and tracking the results.  If you do, please share what you learn with the rest of us so we can all work to figure this out.