Tag Archives: grazing

A Closer Look at Prairie Roots

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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.

Here is our sampling area, as seen by our drone. The bottom right portion was burned this spring and has been grazed fairly intensively since. The top left portion is unburned and has had very little grazing pressure.

Dust erupts out of the ground as Seton excavates with the air spade.

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.

Here is the excavation spot in the burned/grazed patch.

Here is the unburned/ungrazed excavation site.

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.

Marissa and Seton examine the roots in the partially excavated grazed site.

Seton examines some of the roots dug out of the burned/grazed site.

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.

An ungrazed clump of big bluestem on the left and grazed on the right.  You can’t see the length of all the roots in this image (they were similar between plants) but the ungrazed roots are noticeably thicker than the roots of the grazed plant.

Here’s another look at the difference in root thickness between the grazed plants (top) and ungrazed (bottom).

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.

Diversity, Redundancy, and Resilience

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Grasslands face a long list of challenges.  In many regions, habitat loss and fragmentation top that list, leaving prairies to struggle for survival as tiny isolated patches of habitat.  In addition, invasive plants and animals keep finding new footholds within both fragmented and unfragmented prairies.  Many of those invaders are aided by nutrient pollution – increasing levels of nitrogen, for example, which help species like reed canarygrass and smooth brome monopolize formerly diverse plant communities.  Most of all, the climate continues to flail crazily about, ratcheting up the temperature and tossing out more and more extreme weather events.

How can grasslands possibly survive all of that?

I’m actually pretty optimistic about the future of prairies.  Prairies are inherently resilient, and if we do our jobs as land managers and supporters of conservation, we can help ensure their continued resilience and survival.  Resilience in prairies and other ecosystems is the capacity to absorb and adapt to whatever challenges are thrown at them, while sustaining their essential functions and processes.  That resilience is built largely upon two pillars: biological diversity and the size/connectivity of the habitats that biological diversity depends upon.

Plant diversity is a key component of ecological resilience, along with the other biological diversity associated with it.  Taberville Prairie, Missouri.

We’ve severely compromised the “habitat size/connectivity” pillar in many regions of North America, but even in little prairie fragments, there is an incredible diversity of organisms, providing the countless services needed to sustain life and productivity.  In a healthy and diverse prairie, not only are all the bases covered, there is considerable redundancy built in to the system because of the number of different species present.  If one plant, animal, or microbe is unable to do its job because of drought, fire, predation or disease, another can step up and fill the role. Diversity provides redundancy, and redundancy helps ensure that prairie systems stay healthy and productive, regardless of circumstances.

It’s not hard to find examples of this kind of built-in redundancy in prairies.  In fact, you can find it within some very recognizable groups of species.  Let’s start with sunflowers.

While most people know what a sunflower looks like, you might not realize how many different kinds there are.  Here in Nebraska, we have at least nine different sunflower species, plus a lot of other flower species that look and act much like sunflowers.  Two of our official sunflowers are annuals, often classified as weeds because of their ability to quickly colonize areas of bare or disturbed soil.  The other seven species are long-lived perennials, each with its own set of preferred habitat conditions.

Plains sunflower, an annual, is a rapid colonizer of exposed in sandy prairies around Nebraska. The Nature Conservancy’s Niobrara Valley Preserve.

All sunflowers are tremendously important providers of food and shelter to wildlife and invertebrates.  There’s a reason sunflower seeds are so prevalent in bird feeders – they pack an enormous amount of nutrition into a little package.  Because of that, a wide array of both vertebrate and invertebrate animals feed eagerly on sunflower seeds when they can find them.  Sunflowers also produce an abundance of pollen and nectar, and make it very accessible to pollinators and many other creatures by laying it out on a big open platter.  It’s rare to find a sunflower in full bloom that doesn’t have at least one little creature feeding on its nectar, pollen, or both.  Grazing animals can get a lot from sunflowers as well; the forage quality of sunflowers is very high, especially before they bloom.

During or after droughts, intensive grazing bouts, fires or other events that leave bare soil exposed, annual sunflowers thrive, and they can provide abundant resources at a time when many other plant species can’t.  We see this often in the Nebraska Sandhills, where plains sunflower (Helianthus petiolaris) turns the hills yellow during the summer after a spring fire or the year after a big drought.  Plains sunflower isn’t the only plant that flourishes under those conditions, but its presence in plant communities is a great example of the kind of built in redundancy that helps ensure there are plants for animals to eat, even when many normally-abundant prairie plants are scarce or weakened.

Nebraska’s perennial sunflowers span a wide range of habitats, from wet to dry and sunny to shady.  You can find a sunflower in just about any habitat type in Nebraska.  That’s another great example of built-in redundancy, and a reason for optimism about the future.  As climate change alters the growing conditions across much of Nebraska, it seems unlikely that any habitat will change so dramatically that it will become devoid of sunflowers.  Instead we’ll probably see changes in the relative abundance of each species from place to place.  In addition, remember that what we call a sunflower is a fairly arbitrary categorization; there are lots of other wildflowers that provide very similar resources/services, including plants like rosinweed (Silphium integrifolium), false sunflower (Heliopsis helianthoides), sneezeweed (Helenium autumnale), and many more.  Those sunflowerish plants also span a wide range of habitat preferences and growth strategies, making it likely that some of them will be blooming abundantly every year, no matter what drought, fire, or grazing conditions are thrown at them.

An illustration of the general habitat preferences of several perennial sunflowers found in Nebraska.  The variety among habitats used by these species makes it likely that some kind of perennial sunflower will persist in most locations, regardless of how climate and disturbance patterns change over time.

Milkweeds are another group of organisms that demonstrate the diversity and redundancy in prairie ecosystems.  There are 17 milkweed species here in Nebraska, along with several other related species (like dogbane) that produce the same kind of sticky white latex.  While that latex is toxic to most creatures, a number of invertebrates have figured out how to feed on milkweed plants without suffering harmful effects.  Many have actually turned the toxin into an advantage by ingesting the substance and making themselves toxic to potential predators.  The most famous of these critters, of course, is the monarch butterfly, which uses milkweeds as larval hosts.

A selection of milkweed species found in Nebraska, demonstrating the variety in flower colors and shapes among the group.

When you picture a monarch caterpillar on a milkweed plant, you probably envision a tall plant with a big pink flower.  In reality, monarchs can use many (maybe all?) milkweed species as larval hosts.  Because each species of milkweed has its own unique set of preferred habitat and growing conditions, the diversity of milkweed species in Nebraska should help monarchs find a place to lay eggs regardless of weather, disease outbreaks, or other events.

The spring of 2017 provided a compelling example of this.  In most years, monarchs overwintering in Mexico fly into the southern United States and lay eggs on milkweed plants there.  The subsequent generation than flies northward into Nebraska and other  nearby states.  For some reason, many monarchs broke from that pattern in 2017, and arrived in Nebraska much earlier than normal.  This caused a great deal of concern because the milkweed most commonly used for egg laying – common milkweed (Asclepias syriaca) wasn’t up yet, and just as it started emerging, a freeze knocked it back down.  Fortunately, common milkweed wasn’t the only option available to monarchs.  Whorled milkweed (Asclepias verticillata) is also fairly common, starts growing earlier in the year than common milkweed, and is more resistant to cold weather.  Monarchs seemed happy to lay their eggs on the skinny leaves of whorled milkweed, and those of us worried about monarchs breathed a sigh of relief.  Once again, diversity created redundancy, and monarchs found habitat for their babies, even though they arrived well ahead of schedule.

A monarch egg and caterpillar on whorled milkweed earlier this spring (April 27, 2017) in Nebraska.

A broader example of redundancy and resilience in prairies includes the interdependence between bees and plants.  If you’ve followed this blog for long, you’re surely aware that there are thousands of bee species in North America, and potentially 80-100 or more species in a single prairie.  Most of those bees can feed on the pollen and nectar from many kinds of wildflowers, though some are restricted by their size or tongue length from accessing certain species. Because most plants only bloom for a few weeks, and most bees need considerably longer than that to successfully raise a family, bees require more than one kind of wildflower near their nest.  In fact, in order to support a broad diversity of bee species, a prairie needs an equally diverse set of wildflower species.  That way, a bee can find sufficient food throughout the growing season, even if drought, grazing, or other events keep some plant species from blooming in a particular year.

On the flip side, most wildflowers rely on the diversity of bees and other pollinators to ensure successful pollination.  While some insect-pollinated plants are very selective about who they let in, most rely on the availability of many potential pollinators.  If some species of bees are suffering from a disease, or have a weather-related population crash, it’s awfully nice to know that there are other bees (along with butterflies, moths, wasps, and other insects) that will still be able to transfer pollen from one flower to another.  A diverse pollinator community relies on a diverse wildflower community, and vice versa.  Diversity, redundancy, and resilience.  No matter what happens, flowers make fruits and seeds – which, by the way, is pretty important all the various creatures that rely on those fruits and seeds for food.

Bees rely on plant diversity to ensure a consistent supply of pollen and nectar across the growing season. In this case, tall thistle, an important native wildflower, is supplying food to a bee in return for pollination services.

All of us have our favorite prairie species, whether we’re fans of flowers, butterflies, birds, or some other group of organisms.  It’s easy to focus our attention on those favorite species, and worry about whether they will survive all the challenges that face prairies today.  If we really care about prairies, however, we should probably focus more on (and celebrate) the richness of species that keep prairies humming along, no matter what gets thrown at them.  The variety of yellow-flowered sunflowerish plants, the broad array of latex-producing milkweed-like plants, the complexity of the plant-pollinator relationship, and countless other examples of diversity and redundancy help ensure the survival of prairies well into the future.  That resilience is why I remain optimistic about the future of prairies.