The Bench Strength of Prairies in the Face of Climate Change

In case you hadn’t noticed, the climate is changing.  Things are getting weird, and they’re going to get weirder.  Here in central North America, we’re expecting more and more intense storm events and drought periods in the coming decades.  Scientists are scrambling to figure out how to predict and facilitate the inevitable changes those crazy weather events will bring to natural systems, including prairies.

Fortunately, prairies have been training for this for a very long time.  A few months ago, I wrote a post about the resilience of prairies, and how that resilience is built largely upon the diversity within their ecological communities and the size and connectivity of prairie habitats.  Prairies that are relatively big and still have the majority of their potential plant and animal species are going into this encounter with rapid climate change with what you might call solid bench strength.

Diversity of plants and animals is the keystone to ecological resilience. The Nature Conservancy’s Nachusa Grasslands, Illinois.

In sports, teams want to have lots of available players that represent a broad diversity of skills.  Each opponent they face will have its own individual mix of power, endurance, speed, and other attributes.  A successful team can build a roster for each game that counters their opponent’s strengths, no matter what they are.  The number and quality of their players is a team’s bench strength.

Healthy prairies have great bench strength too.  No matter what gets thrown at them, they can adapt by changing their roster of species.  The speed at which they can drastically change the makeup of their “team” is impressive.  Anyone who has spent many years watching the same prairie has seen this in action, but none of us have seen prairies go through what Professor John Weaver saw back in the 1930’s and 40’s.

Weaver, one of the best known prairie ecologists of all time, had been studying 30 “large typical prairies” across parts of Nebraska, Iowa, Kansas, and Colorado prior to the start of the Dust Bowl era.  His baseline data gave him an invaluable opportunity to document the dramatic changes to the plant communities of those prairies during and after the droughts of the 1930’s.  What he recorded, along with his former student F.W. Albertson, was an incredible testimony to the dynamism and resilience of those prairies.  Their 88 page 1944 publication, entitled “Nature and Degree of Recovery of Grassland from the Great Drought of 1933 to 1940”  encapsulates the bulk of their findings in one place, and is worth a read if you have the time.

In 2012, we got a small glimpse of what Weaver and Albertson saw in the 1930’s, but our drought – while severe – only lasted one year here in Nebraska.
In 2013, the response of the prairie to the 2012 drought included some explosions of wildflowers, including shell leaf penstemon (Penstemon grandiflorus).

One of the biggest plant community shifts Weaver and Albertson documented was the widespread and dramatic death of grasses such as big bluestem (Andropogon gerardii), little bluestem (Schizachyrium scoparium), and Kentucky bluegrass (Poa pratensis), and the subsequent rise of other grasses such as prairie dropseed (Sporobolous heterolepis), sand dropseed (Sporobolous cryptandrous), porcupine grass (Stipa spartea), and most of all, western wheatgrass (Pascopyrum smithii).  Western wheatgrass populations exploded throughout the mid to late 1930’s, to the point where many prairies were completely dominated by it, to the near exclusion of other plant species. In fact, in a 1942 publication, Weaver said the following, “The large area of drought-damaged true prairie and native pasture now dominated by western wheat grass and the harmful effects of the successful competition for water of western wheat grass with species of greater forage value present a problem of much scientific interest and great economic importance.”

In other words, as they made massive substitutions within their lineups, prairies were changing so much they became almost unrecognizable, even to those who knew them best.  Weaver and Albertson watched waves of forb species they’d always considered to be of little value become stars on the field, and they and others didn’t quite know how to react.  Daisy fleabane (Erigeron strigosus), Missouri goldenrod (Solidago missouriensis), and heath aster (Aster ericoides) were all examples of wildflowers that suddenly rose to prominence in new and major ways.  The two dismayed scientists described how heath aster, a “nearly worthless native forb,” formed near monocultures across wide swaths of prairie, to the extent that it “ruined many of the prairies…for the production of hay, because of its brush-like growth.”  Others were out of their depths on this too, and Weaver and Albertson reported that “considerable native sod was broken because of the seriousness of this pest.”  In the following sentence, however, they begrudgingly added a short sentence, “Of course, it did protect the soil.”

While Weaver and Albertson considered heath aster to be “nearly worthless” it plays an important role in the prairie, and is an important food source for pollinators in the fall.

Exactly.  While the strategy was foreign and frightening to those who hadn’t seen prairies dealing with these kinds of conditions before, those prairies were just doing what they’ve done many times before – making whatever roster adjustments were necessary to keep functioning at a high level.  In addition to forb species they denigrated as weeds, Weaver and Albertson noted that many wildflowers with “large storage organs”, including bulbs and corms, also greatly expanded their population size during the dust bowl years.  This included species like Violet wood-sorrel (Oxalis violaceae), bracted spiderwort (Tradescantia bracteata), windflower (Anemone caroliniana), and wild garlic (Allium canadense).  Those species and others increased the size of the patches they’d occurred in previously, but also were found “in many new locations.”  Other native forbs that became superabundant in some prairies, especially early in the dust bowl years, included prairie ragwort (Senecio plattensis), white sage (Artemisia ludoviciana), and yarrow (Achillea millefolium).

Windflower (Anemone caroliniana) was one of the wildflowers with “large storage organs” that proliferated during the droughts of the 1930’s.

As rains started to return in the early 1940’s, Weaver and Albertson watched with amazement and renewed optimism as plant communities started “recovering”, which of course meant they were returning to a composition more familiar to the people observing them.  Grasses were often the first to rebound in prairies, including big bluestem, which initially formed large and lush monocultures in many places.  Wildflowers that hadn’t been seen for seven years or more, suddenly appeared everywhere, including blue-eyed grass (Sisyrinchium campestre), which grew “more thickly than if the stands of 7 normal years had been combined.”  Downy gentian (Gentiana puberula), which had been considered rare prior to the big droughts, became much more common in the early 1940’s than Weaver and Albertson had ever seen before, with abundances of “15 or more plants in a space of a few rods”.

Stiff sunflower (Helianthus pauciflorus) returned fairly quickly to “normal abundance” by 1943, as did many others, including silverleaf scurfpea (Pediomelum argophyllum), cream wild indigo (Baptisia bracteata), and buffalo pea (Astragalus crassicarpus).  Prairie violets (Viola pedatifida), pussy toes (Antennaria neglecta),  and others came back more slowly, but returned nevertheless.  Importantly, those returning species didn’t appear to be traveling from long distances.  Instead, they simply re-emerged, either from seeds or underground buds, from where they’d been sitting on the metaphorical bench, awaiting the call to step up to the plate again.

Buffalo pea (Astragalus crassicarpus) and many other wildflowers recovered from the long droughts at a speed that amazed Weaver and Albertson.

The prairies we know today have been through a lot.  In Nebraska and surrounding states, we have specific documentation of the kinds of extreme roster changes prairies can and have made to adjust to the world around them, thanks to the work of John Weaver and F.W. Albertson.  If you have a favorite local prairie, and I hope you do, it’s important to remember that the way it has looked for as long as you’ve known it is only a small sample of what it’s capable of.  Smart teams don’t reveal their secrets before they need to.

As we work to keep prairies healthy through this period of rapid climate change, it’s both useful and reassuring to remember what they’ve been through before.  Today’s prairies certainly have additional challenges to deal with today, compared to the dust bowl days (more invasive species, more landscape fragmentation, etc.), but many should still have sufficient bench strength to make the adjustments they’ll need to make in the coming years.  Our responsibility is to provide management that helps prairies sustain their plant and animal diversity, as well as to protect prairies from additional conversion to cropland or other land uses.  Where possible, restoring prairie habitat around and between prairie fragments can also help build resilience.  In short, we have to allow prairies to do what they do best – adapt and adjust.  Prairies are wily veterans and they’ve been in this game for a long time.  It’s a good bet they’ve still got a few tricks up their sleeve.

Conservation Grazing in Iowa

I got the chance to spend a couple days in Iowa last week, talking about conservation grazing with staff of the Iowa Department of Natural Resources.  They invited me to join a two day workshop discussing various ways to use grazing for conservation objectives.  My main role was to kick off the meeting by providing various examples of objectives that can be addressed through grazing.  Beyond that, I was asked to participate in the remainder of the workshop and contribute thoughts and ideas as appropriate.  I am grateful to have had the opportunity to participate, and came away with a better appreciation for the challenges faced by Iowa prairie managers.

Staff of the Iowa Department of Natural Resources discuss conservation grazing at the Kellerton Wildlife Management Area in south-central Iowa.
Staff of the Iowa Department of Natural Resources discuss conservation grazing at the Kellerton Wildlife Management Area in south-central Iowa.

I thought I’d share some of what I covered in my presentation.  Essentially, I focused on two broad categories of prairie management objectives that can be addressed through cattle grazing.  Those are:

  1. Reducing grass dominance to increase plant diversity
  2. Increasing heterogeneity of habitat

Reducing Grass Dominance

Dominant grass species can sometimes suppress prairie plant diversity by monopolizing soil and light resources.  Two categories of prairies seem particularly vulnerable to this: 1) prairies that have been degraded by chronic overgrazing or broadcast herbicide use, and 2) restored (reconstructed) prairies.  In Nebraska and Iowa, dominant grasses can include non-native invasive species such as smooth brome (Bromus inermis), Kentucky bluegrass (Poa pratensis), tall fescue (Schedonorus arundinaceus), and reed canarygrass (Phalaris arundinacea), as well as native species such as big bluestem (Andropogon gerardii).

When attempting to reduce the dominance of these grasses, it’s important to be clear about what you’re trying to accomplish.  If the ultimate goal is to increase plant diversity, it’s not enough to just suppress the vigor of grasses.  In order to be successful, a variety of other plant species have to colonize territory abandoned by that weakened grass.  A late-spring prescribed fire can temporarily suppress the growth and vigor of smooth brome or Kentucky bluegrass, but often results in robust growth of big bluestem later that season.  Trading a dominant invasive grass for an aggressive native grass may not be success if wildflower diversity remains low.

Grazing can play an important role in increasing plant diversity by repeatedly defoliating  major grass species that limit plant diversity.  The timing, stocking rate, and frequency of grazing can all be adjusted based on the grass species and objectives at a particular site.  As an example, we sometimes combine an early spring prescribed fire with intensive grazing (through about June 1) to suppress cool-season invasive grasses such as smooth brome and Kentucky bluegrass.  If big bluestem is abundant in the same place, we’ll leave cattle in for much of the summer as well, but at a lower stocking rate.  The strategy is to suppress both the invasive cool-season grasses and the native warm-season big bluestem while allowing other plants to thrive and expand their footprint.

At low stocking rates, cattle tend to keep big bluestem closely cropped, but don’t target most wildflower species.  We usually see an abundance of new plants growing in and amongst the weakened brome, bluegrass, and bluestem during the year of grazing and the following year.  Those new plants include both short-lived “opportunistic” plants and longer-lived perennial plants.  The result is a bump in plant diversity.  If we repeat the same kind of treatment every few years, we can often maintain a richer plant community than we can with other management options such as fire or mowing alone.

The rattlesnake master plant (Eryngium yuccifolium) in the foreground of this photo was ungrazed, in  spite of being the burned patch of this patch-burn grazed prairie.  A light stocking rate meant that even this often-favored plant was being rarely grazed in this Iowa DNR prairie.  Staff were hoping to reduce the dominance of tall fescue and allow species such as rattlesnake master to increase in abundance.
The rattlesnake master plant (Eryngium yuccifolium) in the foreground of this photo was ungrazed, in spite of being the burned patch of this patch-burn grazed prairie at Kellerton WMA.  A light stocking rate meant that even this often-favored plant escaped grazing in this Iowa DNR prairie. Staff were hoping to reduce the dominance of tall fescue and allow species such as rattlesnake master to increase in abundance.

There are countless ways to employ cattle grazing to weaken dominant plants and stimulate higher plant diversity.  I’ve written about other examples previously.  You can find a couple of those here and here.

Increasing Habitat Heterogeneity

Cattle grazing can create habitat structure that other management options such as fire and mowing can’t.  As they work to meet their nutritional needs, cattle graze some plant species (mostly their favorite grasses) preferentially.  Stocking rate, or the intensity of grazing, correlates with grazing selectivity.  At low stocking rates, cattle are free to eat only what they really want, resulting in closely cropped patches of grass interspersed with taller clumps of less palatable grasses and wildflowers.  When stocking rates are higher, cattle are forced to eat a wider range of plant species, creating a more uniformly short vegetation structure.  Both the “lower-stocking-rate-patchy-habitat” and “higher-stocking-rate-uniformly-short-habitat” can be valuable to wildlife and invertebrate species.

The ideal situation is to provide the widest possible range of habitat types within a prairie, or within a series of adjacent or connected prairies.  That way, regardless of their habitat needs, most wildlife and invertebrate species will be able to find a place to live.  Changing the location of each of those various habitat types from year to year helps keep any species (plant or animal) from becoming so abundant that it impacts other species to the point of reducing diversity.

Because of the unique vegetation structure created by grazing, a wider range of habitat types can be created with grazing than with either fire or mowing.  However, it’s also very important to ensure that grazing doesn’t have a detrimental impact on plant diversity in the name of creating wildlife habitat.  Significant periods of rest from grazing and careful monitoring of grazing impacts and populations of sensitive plant species are important.  If conservation is the primary goal, grazing should be used only when there are specific objectives to meet, not as a default strategy.

This seeded prairie has been part of a grazing system in every year since it was planted in 2003, and has maintained an excellent diversity of prairie plants.  Examples in the foreground include leadplant (Amorpha canescens).
This seeded prairie has been part of a grazing system in every year since it was planted in 2003, and has maintained an excellent diversity of prairie plants. Examples in the foreground include leadplant and Ohio spiderwort (blooming) but many others, including rare and conservative species, were abundant as well.

I’ve written much more on the topic of creating heterogeneous habitat with grazing in previous posts as well, and you can find a couple examples here and here.

Setting Useful Objectives – And Then Using Them

Regardless of the management tool(s) being employed, the biggest challenge for a prairie manager is to set clear objectives and then follow up on them.  Start by defining the outcome you want (different habitat structure, more plant diversity, etc.) and then describe precisely what success looks like.  Monitoring doesn’t have to mean spending hours on your knees with a plot frame, it just means measuring the outcome you desired.

For example, if you want more habitat diversity, you could start by listing the types of habitat structure you want (tall/dense, short sparse, patchy forbs with short grass, etc.) and how much of the prairie you’d like to be in each category.  Then, you could make a rough map of how the site looks before the treatment and estimate percentages of each habitat type.  After your grazing, fire, or mowing treatment, make another map and see if you reached your objective.

If plant diversity is important, decide how you will measure that.  This is where a plot frame and repeated sampling across a prairie can be helpful, but there are simpler ways as well.  You could pick out 3-5 small areas (less than 10 square meters) that you can find each year and then annually list the plant species you find in each area to see if that number changes over time.  You don’t have to identify all the species, just list how many there are.  If you are using grazing, it’s also important to figure out which plant species are favorites of the cattle and use that information to ensure that your management allows those plants enough rest from grazing that they can bloom and make seed every few years.

Most importantly, your objectives should drive the adjustments you make to management from day to day and season to season.  If you can define what you want, you can see if your management is moving you in the right direction.   It’s fine to change objectives as you learn, or as conditions change.  In fact, in our Platte River Prairies, while we have some broad objectives (plant and habitat diversity), we set new specific objectives and management strategies each year to respond to what we’re seeing on the ground.

Cattle grazing is just another tool that can be used for the conservation of prairies.  It’s not appropriate for all prairies or situations, but can help meet some objectives in ways that other tools (fire, mowing, herbicides) can’t.  Conservation grazing differs from ranching in that income doesn’t have to be a major part of the decision-making process each year.  On land where conservation is the primary objective, managers can decide when and how to employ grazing (or not) based purely on the conservation challenges they face.

Thanks again to the Iowa Department of Natural Resources for inviting me to their conservation grazing discussion last week.  I was impressed by the thoughtfulness and creativity of the staff I met, and I look forward to hearing more about their prairie management and restoration work down the road.