Synthesizing the 2022 Conserving Fragmented Prairies Workshop Part 3 – Grazing, Invasives, and Expression

Really quick – we’re hiring a Conservation Coordinator for the Platte River Prairies. The position includes both administrative and outreach work. We need someone to help with the logistics and administration of our conservation work and to organize and assist with our many outreach efforts. If you’re interested, click here for more information. The application period closes on August 31.

Want to come work with us at the Platte River Prairies in Nebraska? Apply at the link above.

This is the final of three posts on the July 2022 Conserving Fragmented Prairies Workshop co-hosted by Prairie Plains Resource Institute and The Nature Conservancy’s Platte River Prairies. The first post focused on prairie reconstruction (restoration) – the conversion of crop land to high diversity prairie vegetation. The second post focuses mostly on issues related to the control of trees and shrubs but ends with a conversation about local ecotype seeds. This last post covers grazing and invasive plant management.

Grazing

We talked a lot about grazing during the workshop, but much of those conversations have been covered elsewhere in this blog, so I’ll try to keep the summary brief.  Cattle and bison grazing are both valuable options to consider in prairies because of the selectivity of the grazers and the ability to manage that selectivity to achieve particular objectives related to plant composition and habitat structure.  As managers, we can decide the frequency and timing of grazing events, as well as the intensity.  That’s a lot of flexibility to play with and grazing can increase habitat heterogeneity for animals and to manage the competition between plant species.

Cattle make active decisions about every bite of food they take in. As managers, we can use their selectivity to advantage and affect both the habitat structure and plant composition of prairies.
Here’s an example of the selectivity of cattle. Purple prairie clover is a highly palatable prairie wildflower but under the grazing conditions used in this prairie is being largely grazed around. Grasses were eaten right up to the edge of the plants.

That said, grazers are sentient animals and don’t always act predictably, so we talked about the flexible and tolerant mindset needed by managers who use grazing animals.  Prairies are resilient enough that if cattle or bison do something we don’t like one year, we can change plans and the prairie will recover just fine.  That unpredictability can also be helpful, though, because we can learn a lot from watching how both cattle and prairies act and respond to each other.  Over time, we can reduce the unpredictability by better understanding and predicting what happens.  

Even so, cattle and bison will still do their own thing and it might not always make sense to us.  On one end of the spectrum, cattle can be trained to eat certain plants, including things we want to see less of.  On the other, they’re both very good at maximizing their nutritional needs and will graze different plants at different times of the season to regulate their diet.  In addition, just like us, individual cows and individual herds can develop favorite plants, as well as favorite locations, and it can be tough to break those habits.

We looked at sites where grazing has facilitated the invasion of smooth brome and Kentucky bluegrass in a restored (reconstructed) prairie but also seems to be suppressing the negative impacts of those grasses. You can read more about one example in this prior blog post.  The grazed portion of the prairie has higher plant diversity than the ungrazed area, but differences in the relative abundance of individual plant species. Overall, we are satisfied with the results of the grazing because it creates much better habitat heterogeneity while maintaining high plant diversity and isn’t extirpating any plant species.  However, we will continue watching closely to see how things change over time.

While most prairie plants withstand grazing bouts just fine, one example of a plant species that’s usually found at lower abundance under grazing is common milkweed (Asclepias syriaca).  Both cattle and bison enjoy grazing milkweeds and tend to decrease their abundance in prairies.  Since milkweeds are host plants for monarch butterflies (and many other species) that is obviously notable and worth consideration.  I’ve been collaborating with Tim Dickson at the University of Nebraska-Omaha for several years on research projects related to this topic. 

Common milkweed is a favorite food of cattle, despite the sticky white latex inside it. These cattle ate all the flowers off the plants less than a week after entering this pasture and stripped the leaves a week later.

We still have milkweed plants in our grazed prairies, but they’re definitely less abundant than in nearby ungrazed sites.  Monarchs don’t need dense patches of milkweed, but they do seem to thrive best when that milkweed is at a higher abundance than it is in most of our sites.  We’re trying to understand how cattle graze milkweed at different times of year and how long of a rest period (exclosure from grazing) milkweed plants and colonies need to thrive.  This is no small matter when it comes to monarch conservation since Nebraska has about 23 million acres of grassland, most of it grazed.  Increasing milkweed abundance in even a portion of those acres (and those in other nearby states) could have a huge impact on host plant availability for monarchs.

Invasive Plant Management

We had some great discussions about invasive plant management.  That included conversation about grass-selective herbicides like Clethodim, which many managers are using to good effect on species like reed canarygrass and other invasive grasses.  For much more on those topics, I encourage you to read the Grassland Restoration Network blog about work going on in Illinois.  Two posts in particular might be helpful.  The first talks about the Flint Creek Savanna restoration project and the second compares the effectiveness of Glyphosate to Clethodim under different circumstances.

Since I missed a lot of the invasive plant discussions from our workshop while I was leading tours on other topics, I’m going to avoid trying to summarize the specifics of those discussions.  I’d love for anyone who was in those groups to share what you picked up in the comment section below.  One big point I do want to mention has to do with evaluating the success of invasive plant control work.  I think that we sometimes focus too much on measuring the abundance, density, or cover of invasive plants and forget to focus on the reasons we’re worried about those invaders in the first place.

It’s crucial to have specific objectives for your prairie because that will tell you which invasive species are affecting those objectives and also help you evaluate whether your control work is successful.  For example, if plant diversity is the primary objective, an invasive grass is only a problem if it reduces that plant diversity.  In the case I mentioned earlier where grazing has increased the presence of smooth brome and Kentucky bluegrass, measuring the spread or frequency of occurrence of those grasses would tell us our work is failing, but measuring plant diversity tells a different story. 

At that same site, we see big peaks and valleys in sweet clover abundance from year to year, but because we don’t see any correlation with overall plant diversity, we’re not spending any time controlling sweet clover (other than with cattle, who really like to eat it).  Our objectives might not be your objectives, though, so you might read our results very differently and that’s exactly why it’s so important to have clear and specific objectives.

Sweet clover is visually abundant in this photo but our data consistently shows no effect of sweet clover pulses on the abundance or diversity of other plants in prairies around here. Sweet clover abundance isn’t our ultimate goal, so measuring the number of clover plants wouldn’t tell us what we really want to know.

Prairie Expression

A couple workshop participants commented on a phrase that was thrown out several times during our conversations. As we talked, we periodically talked about the variability with which plant and animal species ‘expressed themselves’ in prairie communities. That idea of species expressing themselves is certainly evocative, isn’t it? In the context we were using it, the phrase refers to the way species become more abundant or more visually-apparent in particular years or situations.

A big example is the way plant species might be absent or uncommon at a site for several (or many) years until a weather event or management treatment creates the conditions that allow it to suddenly thrive. Annual sunflowers (Helianthus sp.) appear at our Platte River Prairies in great abundance after a spring fire, and even more so when that burn is grazed afterward. Fourpoint evening primoses (Oenothera rhombipetala) and shell leaf penstemon (Penstemon grandiflorus) both put on big shows a year after a site is grazed intensively. Dotted gayfeather (Liatris punctata) doesn’t increase in abundance the year after a grazing bout, but plants significantly increase the number of flowering stems produced.

Sometimes, plants appear after years of us not noticing them because a long-term management regime changes. Well-known examples (within the community of prairie managers) include species like eastern gamagrass (Tripsacum dactyloides) and compass plant (Silphium laciniatum) appearing in abundance when grazing systems change – even after not being seen for decades. This is true even in situations where prairies are being closely monitored by people looking for those plants. Are they surviving without putting out aboveground growth? Or just putting out such little growth they go unnoticed?

When we were growing prairie plants in nursey beds years ago, we noticed a strange phenomenon with sensitive briar (Mimosa quadrivalvus). We put out plugs of the species, which bloomed and produced lots of seed for a couple years but then completely disappeared the next. Gone. Bare ground, with no leaves or stems showing. We debated whether to replant but decided not to. The following year, the plants were back like nothing had happened. Seeing that helps me not worry about the peaks and valleys in sensitive briar abundance I see in prairies now, except that I still don’t know why it happens.

Sensitive briar is a perennial plant but seems to be much more abundant in some years than others. I don’t understand why.

Animal populations can express themselves under various conditions too. Small mammal populations famously go through boom and bust cycles, often in synchrony (or correlated with) predator numbers. Invertebrates, too, are known for those huge swings in abundance, and that annual variability makes short term studies of their responses to management nearly impossible. Lots of invertebrate species also express themselves on annual cycles, too, suddenly popping onto the scene at a particular time each year. Many spend most of the year underground or as difficult-to-see larvae or nymphs and then morph into adults all at once, giving us a big (usually brief) show.

Thinking about the various expressions of prairies can lead down lots of fun paths. There are intriguing scientific explanations of the phenomena, of course, many of which we don’t yet understand. In addition, though, there are artistic aspects to the idea of prairie expressions, and it’s tempting to ascribe emotions or other attributes to prairies and prairie communities. For example, I often refer to the ‘wildflower parties’ that happen a year after a prairie is burned and grazed. Plants that are usually suppressed by dominant grasses are released from that tyranny for a year or two and respond with great vigor. Similarly, late summer often brings a frenetic energy created by hordes of pollinators at the peak of both their abundance and the abundance of their food sources. As flower numbers dwindle in the late fall, that frenetic pollinator energy takes on a more desperate tone as those invertebrates rush to squeeze everything they can out of the tail of the season.

Anyway, we talked about prairie expressions at the workshop. We also talked about lots of other topics, not all of which made it into these three summary posts. For those of you who attended, please add your observations, questions, and comments below. If you didn’t attend, your questions and thoughts are also welcome!

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About Chris Helzer

Chris Helzer is the Director of Science for The Nature Conservancy in Nebraska. His main role is to evaluate and capture lessons from the Conservancy’s land management and restoration work and then share those lessons with other landowners – both private and public. In addition, Chris works to raise awareness about the importance of prairies and their conservation through his writing, photography, and presentations to various groups. Chris is also the author of "The Ecology and Management of Prairies in the Central United States", published by the University of Iowa Press. He lives in Aurora, Nebraska with his wife Kim and their children.

5 thoughts on “Synthesizing the 2022 Conserving Fragmented Prairies Workshop Part 3 – Grazing, Invasives, and Expression

  1. Thanks Chris for sending this series. I haven’t had the chance to read it all yet but I’m sure it will be helpful in my little 2 acre project.

    • Hi Margy,
      That’s a bigger question than I can cover in a quick reply, but I’ll start. The answer depends upon whether you are asking about the absence of cows or the absence of all big grazers (including bison). Since we don’t have many bison around these days, cows have replaced them as the big grazers in most prairies.

      Big grazers manipulate vegetation structure in really important ways, creating habitat heterogeneity that is crucial for supporting the kind of animal diversity we want and need. Their grazing also shifts the competitive balance within the plant community, reducing the dominance of some of the bigger grasses and making space for lots wildflowers to compete more effectively.

      Here’s a post I wrote last year that expands on that habitat theme a little: https://prairieecologist.com/2021/09/07/picturing-habitat-heterogeneity/

      So, if we didn’t have big grazers on the prairie, we’d have less diversity of habitat, which would affect animal diversity. We’d also have different plant communities that would be less dynamic (same species dominating in most places most years) and less diverse (at least in the prairies around here). We’d also have a lot less ability to manipulate all of those factors and to adapt, as neeeded.

  2. In my loess hills remnant, I’ve reproducibly seen May burns cause leadplant to bloom profusely the same year, while the frequency of slender leaf false foxglove (a hemiparasite of grasses) seems to significantly diminish, only to reappear quite robustly the next year. Pasque flowers have been abundant the year after burns as well. These correlations are pretty interesting to witness over years.

  3. Fascinating results and observations. In watching sweet clover come and go over decades in western Dakotas and eastern Montana I also have not observed any significant changes in native plant composition. Almost seems it occupies a niche not colonized by native species but that would seem unlikely. Some years in grazed areas sweet clover appears to be exclusively utilized by cattle for weeks. There has to be some effects of that, perhaps benefiting native warm season species, but not Big Bluestem by your data. Perhaps a combination of warm season species then, Little Bluestem, Prairie Sandreed and others.

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