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.

Prairie Word of the Day – Disturbance

It’s been too long since the initial installment of what was intended to be a regular feature of this blog – the Prairie Word of the Day.  Since one blog post does not make a series, I figured I’d better at least write one more.  I will try to add more to the series relatively soon. 

If you read this blog or any other source of information about prairie ecology or management, you’re likely to have seen the term “disturbance” used in some context (e.g., “ecological disturbance,” “disturbance regime,” “disturbance in the force”).  Ok, that last one is from a different context, but nevertheless.

Outside of prairie ecology or other ecological conversations, a disturbance is often something that requires contacting the police.  However, in almost every case, prairie managers would appreciate you not calling the police in response to disturbances in a prairie.  So what does the word “disturbance” mean in an ecological context?

Fire is one of the big three historical disturbances to which prairies are adapted to and rely on.

Fire is one of the big three historical disturbances to which prairies are adapted to and rely on.

It’s probably easiest to think about a disturbance in a prairie as something that disrupts the lives and processes of prairie organisms and creates a shift in the competitive balance among those species.  Historically, the big three disturbances in grasslands were fire, grazing, and drought.  Fire and grazing are still used as management strategies today, along with mowing/haying and, sometimes, herbicide treatments.  Drought (and other significant weather events such as a flood) is not something managers prescribe or apply to a prairie, but it certainly has significant impacts.

Both “natural” disturbances and those prescribed by prairie managers are critically important to the health of prairies, despite the fact that they have significant impacts on species.  In fact, many animals, plants, and other organisms often die as a result of disturbances.  Prairies, however, more than most other ecosystems, are not only well suited to survive disturbances, they are defined by and rely on those disturbances.  Without fire and drought, for example, prairies wouldn’t even exist – they would be woodlands instead (and who wants that??).

Every time a fire burns through a prairie, grazers chomp off much of the vegetation, or an extended drought turns a prairie brown in mid-summer, the competitive balance in a prairie is altered.  Depending upon the timing and intensity of a fire, for example, it’s likely to kill some trees and “top-kill” others, forcing them to restart from buds at or below the surface of the ground.  Fires can also kill or suppress the growth of other plant species, especially those most actively growing at the time of the fire.  At the same time, fires create opportunities for other plants – especially those in direct competition with the ones suppressed or killed.  Plants that thrive best in full sunlight and don’t do well under layers of thatch or beneath tall plants also respond very favorably after a fire.

These cedar trees died in a recent fire, but the vegetation around them is regrowing strongly. In fact, the cattle (in the distance to the top right) in this prairie will be attracted to that growth and graze this part of the prairie much harder than unburned parts.

These cedar trees died in a recent fire, but the vegetation around them is regrowing strongly. In fact, the cattle (in the distance to the top right) in this prairie will be attracted to that growth and graze this part of the prairie much harder than unburned parts – adding a second disturbance to the first.

Of course, fires can also kill or injure wildlife or insect species (not to mention microorganisms and other life forms) that are aboveground at the time of the fire and can’t evade the heat and/or smoke.  The season in which a fire burns plays a big role in which organisms are affected.  Dormant season fires can kill insects and other species that overwinter in vegetation or along the surface of the ground, but growing season fires cause even more fatalities because so many more species are active during that time of year.  Even animals that survive the fire itself may have to travel to unburned areas in order to find suitable habitat, and those trips can be hazardous as well.

Despite the immediate negative impacts of fire on individual insects, wildlife, and other species, however, periodic fire usually has longer-term benefits – even for those species that suffer fatalities.  Without fire – or some other disturbance that removes vegetation – prairies can become so overgrown that most species can’t survive there.  Suffocating layers of thatch, encroachment by trees and shrubs, and the potential for diseases and predator populations to build up over time are all conditions that can be found in prairies that go undisturbed for long periods of time.  In addition, if the same habitat and growing conditions prevail year after year in a particular patch of prairie, some species will thrive but others will not – leading to a loss of species diversity and overall prairie health.

Without going into details here, the impacts of drought and grazing on prairie species are similar to those of fire – each event will favor some species over others.  Prairie plants are well adapted to periodic burning, grazing, or drought.  Perennial plants have numerous buds at their base, from which they can grow new shoots after they’ve been burned or cropped off, and most can enter dormancy during a severe drought in order to conserve resources for better days.  Shorter-lived plants often rely on disturbances to create opportunities for them to grow with less competition from nearby perennials.  Those annual and biennial plants produce copious amounts of seed that fall to the ground and wait until the next time conditions favor their germination and growth.

Every vertebrate and invertebrate animal species relies on a particular set of habitat conditions.  Some thrive when vegetation is short and sparse, and others prefer tall dense vegetation.  The highest diversity of animals and insects is usually found where perennial plants have been recently weakened by fire, grazing, and/or drought, and short-lived plants are thriving while those perennials recover.  In any case, habitat conditions are tied to disturbances – some conditions are created during or in response to a disturbance, and others (tall dense vegetation) are created when a disturbance hasn’t occurred for a while.

Prescribed fire, along the grazing and mowing, are important ways for prairie managers to introduce disturbances into grasslands in order to maintain prairie health and biological diversity.

Prescribed fire, along the grazing and mowing, are important ways for prairie managers to introduce disturbances into grasslands in order to maintain prairie health and biological diversity.

As prairie managers, our job is to provide habitat and growing conditions for as many prairie species as possible.  We burn, graze, and mow prairies at prescribed intervals (and usually in patches, rather than across an entire prairie) in order to maintain a competitive environment in which all those prairie species can survive.  Whenever we burn a prairie or introduce large grazers to it, we know that some species will be harmed by that action, but that others will flourish because of it.  As long as we allow a patch of prairie to recover between disturbances and create a patchwork of habitats (some recently disturbed and others not), the prairie community will remain diverse and resilient.

Prairie management is complicated, and it can be difficult to make decisions and take actions that you know are going to negatively impact some species.  On the other hand, the other choice is to do nothing.

And that would be disturbing.