Tag Archives: plant diversity

Konza Prairie Trip Part 3 – Questions About Frequent Prairie Burning

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A few weeks ago, I wrote about our trip to the Konza Prairie Biological Station in eastern Kansas.  On that trip, we learned about research results showing that frequent spring fires (one or two year frequency) can prevent encroachment of tallgrass prairie by trees and shrubs.  Less frequent fire allows shrubs, especially dogwood and sumac, to invade.  Pretty simple – we should be burning tallgrass prairie at least every two years, right?

Hang on just a minute, Sparky.

As you might expect, there is more to the story.  It turns out that the frequent spring fire (with no grazing) regimes at Konza has other impacts.  One example is that frequent fire favors grasses over forbs and decreases plant diversity over time.  Prairies that are burned every year or ever two years develop a grass-dominated plant community in which many forb species are difficult to find.  So, frequent fire is bad for plant communities…  Right?

Annually-burned tallgrass prairie at Konza Prairie, in the Flint Hills of eastern Kansas.

Annually-burned tallgrass prairie at Konza Prairie, in the Flint Hills of eastern Kansas.  August 2014.

But in other tallgrass prairies, especially in more eastern prairies such as those in Illinois, researchers are seeing very different impacts of frequent fire.  A paper by Marlin Bowles and Michael Jones, for example, found that frequent fire “stabilizes” plant communities in the Chicago, Illinois area, and that plant diversity was positively correlated with fire frequency.  In fact, their results are almost the exact opposite of what was seen at Konza.  In the Illinois study, frequent fire decreased the dominance of warm-season grasses and increased the diversity of summer wildflowers.

Frequently-burned prairie at the University of Wisconsin-Madison Arboretum. Clearly, frequent fire is not incompatible with plant diversity...

Frequently-burned prairie at the University of Wisconsin-Madison Arboretum. Clearly, frequent fire is not incompatible with plant diversity…

What the heck is going on??

If we could answer that question, we’d be miles ahead of where we are now in terms of understanding prairie ecology.  I can come up with lots of potential reasons for the difference between frequent fire application in Illinois versus Kansas, but I can’t confirm or deny any of them – we simply don’t know.  As I think about why prairie plant communities might become more dominated by grasses over time, I wonder about factors such as small scale plant diversity, soil productivity, and the soil microbe community – all of which may correlated with each other.

Eastern remnant (unplowed) prairies tend to have a higher density of plant species (e.g., plant species per square meter) than western prairies. High species density could potentially help suppress grass dominance because of the variety of competition strategies each of those plant species employs to take and defend territory.  Those strategies include allelopathy (releasing chemicals that retard growth of nearby plants), rhizomes (underground stems that can connect even fairly distant stems of the same plant together), root density/depth, and many others.  It seems logical that communities with that kind of well-established complex competitive environment would present a major challenge for any species (grasses or otherwise) trying to become dominant.  Maintaining consistent growing conditions by burning or haying annually might facilitate stability within those plant communities because it essentially maintains a stalemate; no plant species is ever given a window of opportunity to gain advantage over its neighbors.  Or this could all be balderdash – I’m just throwing out ideas here.

Does the rich soil organic matter content of eastern tallgrass prairies help suppress grass dominance?  It’s probably the major reason for the higher plant species density in those prairies, so maybe.  On the other hand, research at Konza and elsewhere in more western prairies shows that adding nitrogen to prairies (increasing productivity) increases grass dominance and lowers plant diversity.  Hmm.

Soil microbial communities surely have a very important influence on plant diversity, but (as discussed in a previous post) we don’t know much about them yet.  I’m optimistic that our understanding of soil microbes will grow tremendously during the next few decades, but the complexity of that world might mean that it will take many more decades before we start to get a grasp on it.  In the meantime, we can use it as a convenient scapegoat.  If something you don’t like is happening to your prairie, it’s probably a soil microbe problem…

Those of you who read this blog frequently may be surprised that I haven’t mentioned insects or other animals yet.  Let’s talk about them now, shall we?

It’s very important to remember that even if frequent burning seems to maintain high plant diversity in (some) prairies, a prairie is much more than just plants.  The use of frequent fire tends to create fairly homogenous habitat conditions across a prairie.  Regrowth rates are similar across the whole burned area, so vegetation height and density is relatively uniform.  Dead material, including both litter and standing dead vegetation, is scarce.  Because habitat diversity is limited, so is the diversity of creatures that rely on that habitat.  In taxonomic groups ranging from grasshoppers and spiders to mice and birds, research shows that habitat diversity is positively correlated with species diversity.  Creating habitat heterogeneity – through grazing, patchy mowing, or other means can help facilitate a more diverse animal community.  Burning in a less regimented way can help too, especially if that means splitting a prairie into multiple management units and burning only a subset of those each year.

Habitat

Heterogeneous habitat structure, including tall, short, and mixed-height vegetation – like that shown here – can help maintain diverse invertebrate and wildlife communities.  Grazing is one great way to manipulate vegetation structure, but isn’t feasible at all sites.  Grazed prairie at The Nature Conservancy’s Platte River Prairies, Nebraska.

In small isolated prairies, it’s also critically important not to burn (or mow) the entire prairie at the same time.  With limited opportunities for species to recolonize from other prairies, a fire that kills all of the individuals of a particular species (e.g., an insect that overwinters in the stems of grasses or in the litter along the ground) can mean the end of that species’ existence in that prairie.  Maintaining a floristically diverse prairie without a full complement of invertebrates doesn’t seem like success to me.  On the other hand, I also appreciate the difficulties associated with managing small prairies.

To wrap this up, I think there are two really important points to make about fire frequency in prairie management.  First, there are some big questions about why frequent fire seems to maintain high plant diversity in some prairies but encourages grass dominance in others.  Figuring out the answers to those questions may be one of the more important keys to prairie conservation success.  Second, even if frequent fire maintains high plant diversity and repels invasion by shrubs and trees, it still might not be the best choice for a prairie management strategy.  There is much more to a prairie than its plants, and even if you don’t much care for invertebrates, birds, reptiles, or mammals (and how could you not?) those species – especially the invertebrates – are strongly tied to the long-term viability of the plant community, so it’s probably not good to ignore them.  To be clear, I’m not saying people who use frequent fire are evil destroyers of animals.  I’m just raising a flag of caution and pointing out some potential tradeoffs.

Prescribed burning is an important management tool, but its impacts on prairie communities can be complicated.

Prescribed burning is a valuable management tool, but its impacts on prairie communities can be complicated.  It’s important for prairie managers to recognize and account for both the negative and positive impacts of fire.

Those of us who work with prairies are used to the seemingly overwhelming complexity of grassland communities and the way those communities respond to management.  In fact, for many of us, it’s a big reason we love prairies as much as we do.  While we still have more questions than answers about effective prairie management, we have enough information to go forward with.  Most importantly, prairies are pretty tough, so excepting drastic measures such as broadcast spraying with 2,4-D (and maybe burning an entire tiny prairie), we have the latitude to try out lots of ideas and see what works.  We’ll learn as we go.

In the meantime, it’d be great if all you researchers out there would get cracking on the issue of disparate effects of frequent fire in eastern versus western prairies.  It should only take a few decades to figure it out…  Right?

 

Conservation Grazing in Iowa

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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.