One of the biggest jobs of a prairie steward is to manage the competition between plants, ensuring that no species becomes too dominant and no species is pushed out of the community. In our prairies, much of our effort is directed toward some of the stronger grass species, including big bluestem, indiangrass, smooth brome, and Kentucky bluegrass. Left unchecked, those grasses (and a few others) can monopolize both light and soil resources and reduce plant diversity. Our management targets those grasses with fire and grazing, often using season-long defoliation by cattle or bison to weaken the competitive ability of those grasses, opening up space and resources for other plants to flourish. Our long-term plant data show that we’ve been able to maintain species richness and a full complement of plant species with this kind of management.
When those major grasses are weakened, one of the most obvious responses is a flush of “weedy” vegetation that quickly takes advantage of the soil and light resources that have become available. Research has shown that growing season defoliation temporarily causes grasses to abandon some of their roots (until defoliation stops and the grasses recover), opening up space for nearby plants to grow larger and more abundant. However, there are still many questions about the actual physical responses of grass roots to defoliation, and gaining a better understanding of that could be really important to prairie managers. Researchers at Kansas State University are actively working on those questions right now. Dr. Jesse Nippert, in particular, has done a lot of work on this subject, including some work on prairie shrubs that I wrote about a few years ago.
Last week, a couple of Jesse’s graduate students, Seton Bachle and Marissa Zaricor, were at our Platte River Prairies, collecting data on roots under grazed and ungrazed conditions. In addition, Seton brought along a nifty tool called an air spade, which uses compressed air to dig into prairie soil with enough force to expel soil particles, but not so much that it tears apart the roots of plants (with the exception of the tiny rootlets at the tips). Seton and I started talking about a year ago about the possibility of getting the air spade up here so we can look for visual evidence of grazing impacts to roots. Marissa and Seton are both doing very in-depth (ha!) measurements of plant root responses, but I also wanted to see what’s those roots really look like. The air spade seemed like a great way to do that.
For this initial trial, we chose a part of the prairie that was burned this spring and was being grazed intensively by cattle as part of our patch-burn grazing management. Abundant rain this year has meant that the cattle aren’t keeping the grasses as short as we’d really like, but we were still able to find some big bluestem plants that have been cropped pretty short. As a comparison, we went across the burn line to part of the prairie that hasn’t had much grazing pressure in recent years and, because it is unburned, hasn’t had much attention from cattle this year either. As a result, we were (ok, Seton was) able to excavate around the roots of big bluestem plants that had been grazed off to just a few inches of leaf height, as well as ungrazed plants with leaves around 12 inches high.
As Seton started blowing soil away from the roots (and I photographed the process with my camera and our drone), one of the first things that became obvious was the relatively shallow depth of the main root mass. The work of J.E. Weaver and others has shown that prairie plants, including grasses, have some very deep roots. However, more recent work, including that of Jesse Nippert of Kansas State, Dave Wedin at the University of Nebraska, and others, has shown that those grasses don’t appear to actually use those deep roots for much. In fact, grasses tend to concentrate the vast majority of their root masses in the top foot or so of the soil profile, effectively monopolizing most of the moisture and nutrients there. Forbs tend to pull most of their resources from below that, and shrubs work at even greater depths. I’ll write about this more in a future post, but for now, just trust me when I say that this is abundant evidence for this (and many more questions being pursued). Prairie grasses can have deep roots, but it’s the incredible root density at shallow depths that they most rely on, even during drought.
With the air spade, we could pretty easily see that most of the big bluestem roots were in that shallow depth, and only a few extended down below that. However, as Seton pulled out fully-excavated clumps of big bluestem shoots and roots, my initial reaction was one of disappointment. There didn’t seem to be any obvious difference in the density of roots or size of the overall root mass between the grazed and ungrazed plants.
My immediate thought was that because these plants had only been exposed to grazing for about a month, maybe there hadn’t been enough time to see changes in their root masses. In addition, it might be that some of the roots were no longer active, but were still connected to the root mass for now. We’ll be repeating this excavation process later in the season, and might see differences then that aren’t yet obvious. In addition, we’ll look at some roots of grasses that were heavily grazed all of last season and see what those look like. Still, I was a little disappointed not to see a bigger visual difference.
However, when Seton and Marissa looked at the roots, they pointed out something I hadn’t initially seen because I was so focused on root length and density. The diameter of most of the roots of the ungrazed bluestem appeared to be considerably larger than those of the grazed plants. We were working with a small sample size, but among all the plants we dug up, that size difference seemed to be pretty consistent.
Marissa explained that thicker roots have more carbohydrates stored in them. Plants that have been defoliated, and are trying to regrow shoots, have to pull carbohydrates from their reserves to do so – pulling them out of their roots and putting them into aboveground growth. Whether those roots kind of deflate as the carbohydrates are pulled from them or stressed plants just create skinnier roots is something Marissa and Seton are hoping to learn from their work. Regardless, carbohydrate storage plays into competitive ability. Grasses rely on their storage capacity to fuel growth and withstand further stress, so differences in root diameter could be part of the answer to why grazed grasses are less competitive. Seton and Marissa plan to examine some cross sections of the roots we dug up to see if they can see more under a microscope than we could by just looking at the roots with our naked eye.
Seton and Marissa’s actual scientific explorations will give us much better answers to questions about grazing impacts on grass roots than simply looking at a few samples, but it was fun to see the actual roots themselves. While the differences between grazed and ungrazed plants weren’t as stark as I’d expected, I’m still looking forward to our next effort later this summer – especially because all I have to do is photograph the results of the hard work Marissa and Seton are doing!
If you’re interested, here is a short 1 minute drone video showing the excavation process. You can also check out Seton’s science website here.
Special thank you to the Nebraska Environmental Trust for funding our drone purchase through a PIE (Public Information and Education) minigrant, administered through the Nebraska Academy of Sciences.
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.
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.
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.
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.
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.
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.