Category Archives: Prairie Plants

Using Defoliation of Dominant Grasses to Increase Prairie Plant Diversity

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 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.

Aiding and Abetting Invasive Species

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HAPPY (?) NATIONAL INVASIVE SPECIES AWARENESS WEEK

It’s bad enough that invasive species are taking over our prairies and other natural areas.  Why do we insist on helping them do it?

When my dad was in high school in the 1960’s, he planted trees and shrubs behind the pond in my grandpa’s pasture as part of a 4H conservation project.  The species he planted – Russian olive, eastern red cedar, honeysuckle, and multiflora rose – were all promoted as conservation trees at the time but considered to be invasive species today.  The honeysuckle and multiflora rose didn’t make it, but we’ve still got a strong population of Russian olive and cedar trees today, and now that I’m taking over the management of the pasture, I’m making plans to eradicate them.  I give my dad a break on this one because it was a long time ago.  Unfortunately, less has changed in the last 50 years than you might hope for.  (See my earlier post on invasive trees in prairies)

Eastern red cedar trees are native to Nebraska, and so should probably be labeled as “aggressive” rather than “invasive” – though they can certainly take over grasslands, they’re only a problem where fire has been removed from the ecosystem.  Russian olives, on the other hand, are clearly invasive species.  Many thousands of dollars are being spent trying to remove them from riparian areas around Nebraska and many other states.  Why, then, are we still planting Russian olives??  They are still listed and sold as conservation trees in Nebraska, and several agencies have long been providing cost-share dollars and promoting both the planting AND eradication of Russian olive trees (a situation that is just starting to change).

A Russian olive tree resprouting after a late-spring prescribed fire. Russian olives and most other deciduous trees are particularly difficult because of their capacity to re-sprout.

Siberian elm is another species that is being both promoted and controlled.  The species was first introduced to the United States in 1860.  When Dutch elm disease started wiping out American elms in the 1930’s, Siberian elm gained popularity because it was much less susceptible.  It was also promoted as a substitute for privet (another introduced invasive species) in hedge plantings.  Today, Siberian elm is widely recognized across the country as an invasive species (it is an official noxious weed in New Mexico) and is one of the most aggressive trees invading prairies in Nebraska and other states.  Its abundant wind-dispersed seeds allows it to quickly overwhelm grasslands (and grassland managers) with numerous small saplings.  They are common urban trees – but largely reviled because of their brittle branches and unattractive crown shape.   Notwithstanding its record, however, this ugly aggressive non-native tree is still being promoted as a conservation tree in Nebraska and elsewhere, and the U.S. Department of Agriculture’s Plant Materials Center in Kansas is actually developing an “improved” cultivar for use in the western Great Plains (see page 82 of this report for details).

Unfortunately, Russian Olive and Siberian elm are only two of many examples of introduced species that are simultaneously viewed as useful and invasive by different constituencies.  Nebraska’s newest official noxious weed is Japanese Knotweed, which has numerous hybrids (most of which are invasive) that can still be purchased through garden catalogs.  Sericea lespedeza, a horrible invasive plant in the southern prairies (and a noxious weed in Kansas and Colorado) is still promoted as a forage and wildlife plant by Auburn University.  In fact, sericea’s status as a “crop” is preventing the development of biocontrol agents that may be the only hope of controlling its spread.

Invasive grasses such as smooth brome, tall fescue, and reed canarygrass are being promoted as forage crops for livestock at the same time that huge amounts of money and effort are being spent to keep them from overrunning prairies.  Perhaps most frightening, Monsanto and Scotts have developed strains of Kentucky bluegrass and creeping bentgrass (both invasive grasses in prairies) that are resistant to glyphosate herbicide (aka Roundup) for use in turfgrass applications.  The possibility (probability) that those Roundup Ready species will either escape from planted areas or hybridize with naturalized populations is apparently being ignored – in spite of the fact that the bentgrass has already escaped from test plots in Oregon.  There are plenty of other examples of introduced ornamental or agricultural plants that we’re assisting with their invasion of natural areas, but the list is too long and depressing to present here.

Why do we have such trouble agreeing whether species are helpful or harmful?  A big part of the problem is the diversity of organizations and entities that work to introduce, promote, evaluate, and/or control plant species.  There is no single clear controlling entity that can designate species as being invasive or not.  Because of that, an internet search is not usually an effective way for a member of the general public to find out whether or not a species is invasive.  While there are excellent sites like the USDA’s PLANTS database that list whether the species is considered to be invasive or a noxious weed in any states, a general search for species like sericea lespedeza brings up some websites that promote it as a wonder plant and others that denounce it as a nasty weed.  What’s a person to believe?  (In this case, it’s a nasty weed…)

To further confuse things, those who continue to promote the use of invasive species often claim that the species aren’t invasive in the habitat type or geographic area they’re promoting them for.  I often hear that Russian olive trees are safe to plant as long as you only put them in uplands, and not in lowlands where they become invasive.  This argument appears to rely on the assumption that the wildlife species eating the Russian olive’s “wildlife friendly” fruits don’t move between upland and lowland habitats.  Considering a species safe in one state when it’s demonstrated to be invasive in a nearby state is equally illogical.  True, it can be difficult to predict whether a species will be able to thrive in one area or another without actually testing it, but given the cost of controlling invasive species, why take a chance?

Siberian elms and smooth brome - a pretty nasty one-two punch of invasive species.

How Do we Fix the Problems?

One part of the solution to these issues is to make the general public more aware of which species are considered to be invasive in their area.  Nebraska is currently building a set of lists that will identify known and candidate invasive species, and will rank them by severity and the appropriate response needed (e.g. – prevent their introduction, eradicate current populations, or contain the spread of current populations).  It’s surprising that lists such as this are not more widely available across the country, but they’re actually difficult to find.  Of course, while lists of invasives are helpful, they only work when they’re used.  It’s difficult to imagine many homeowners taking the time to cross check their annual garden catalog wish list against an invasive species list before ordering.  Most consumers assume that if they can buy it, it must be safe – something that is far from true – and the conflicting information found on the internet and in other sources only serves to further confuse and frustrate people.

Educating the general public about the value of using native species for both urban and rural plantings – thus avoiding the risk of introducing species that may become invasive- is another important approach.   I’m a big fan of using native plants, but I’m also realistic about both the difficulty of marketing natives and some of the complications associated with “native” plants.  One of those complications is that it’s very difficult for the public to know which plants are actually native.  For example, mixtures of wildflower or wildlife habitat seeds that are commonly sold in Nebraska and other prairie states include both native wildflower species (though often with genetic origins from far away states) and non-native species, some of which are invasive.  Invasive species that are often included in wildflower mixes include oxe-eye (shasta) daisy, damesrocket, baby’s breath, bouncing-bet, birdsfoot trefoil, red clover, chicory, common St. Johnswort, Queen Anne’s-lace, sweet clover and many others.   It’s not reasonable to expect consumers to be able to determine which species are safe and which aren’t.  (What could be dangerous about wildflowers?)

The demand side of the equation (consumers) is an important place for us to focus attention, but the only way we’re really going to solve the problem is by also working on the supply side.  Somehow, we have to build consensus and understanding between those who are fighting invasive species (e.g. conservation and agriculture groups and landowners) and those who are introducing and promoting new plants (e.g. some agencies, universities, and nurseries).  It’s certainly not going to be a quick or easy process, and it’s not going to solve the problem completely – but the only other option is to continue fighting an ever-growing list of invasive species.

Rather than sniping at each other across fences, we need to build partnerships that bring together nursery organizations and noxious weed control agencies, tree advocates and prairie biologists, and forage specialists and native plant enthusiasts.  The initial partnerships don’t have to be (can’t be?) built around the sources of greatest disagreement.  For example, nursery owners and noxious weed agents might come together because they both see the need for improved influence over nurseries that purposefully (or neglectfully) sell plants that are clearly illegal to sell – nursery owners might be concerned about reputational risk of the nursery industry as a whole, and noxious weed agents would want to prevent introductions of dangerous plants.  More difficult discussions about how to handle the sale of plants that are potentially, but not definitively, invasive can wait.  Once a rapport has been established, it’s much easier for both sides to see and appreciate the point of view of the other – and both have a stake in maintaining the relationship by finding ways to compromise.  In some cases, it might not be possible to build partnerships right away, but important first steps toward building relationships can include things as simple as inviting each other to attend meetings, join field trips, or give presentations on current and future projects.  Anything that gets people into the same space so they can see each other as people rather than adversaries is likely to be productive.

Invasive species are one of the biggest threats to prairies (maybe the biggest) and the issue is much bigger than simply attacking species as they appear.  If we can work to prevent the next leafy spurge or sericea lespedeza from being introduced or released in the first place, just about any level of effort would be worthwhile.  It’s up to all of us to raise awareness about the risks of introduced species and the benefits of native species.  Even more importantly, it’s up to us to work with those who see things differently than we do to establish relationships and common understanding.  While it’s tempting to go around with a rolled-up newspaper and whack those who appear to be acting irresponsibly, that’s not likely to be very productive.  Maybe we should start by sending them (carefully vetted) flowers instead!