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Embracing Unpredictability in Prairie Management

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Much of what determines the outcome of prairie management treatments is out of our control.  Sure, we can decide when to burn a prairie or set the timing and stocking rate for grazing treatments, but cascades of interactions between countless factors such as weather, insect population cycles, and many others can easily overwhelm any decisions we make.  Over the holiday season, I found time to analyze some plant community data for one of our Platte River Prairies.  The results provide a thought-provoking look at the unpredictable ways prairies respond to management.

Cattle

The results of grazing treatments are always a little unpredictable because while they tend to follow certain patterns, cattle can still make their own decisions about what to graze and what not to.  However, variability in management results goes much beyond just the way cattle choose to act in a certain year.  Burning, haying, and other management treatments can also have very different results from year to year.  High-diversity restored prairie – The Nature Conservancy’s Platte River Prairies, Nebraska.

The following data are from a prairie that was managed with variant of patch-burn grazing, in which a different portion of the prairie was burned each year (in the spring) and cattle had access to both burned and unburned portions of the prairie during most of the growing season.  As typically happens, cattle grazed the most recently burned patch pretty heavily and lightly grazed the remainder of the pasture.  Details about the fire and grazing treatments can be found at the end of this post.

If you look at the two graphs below, you can see that there was a significant increase in the average number of plant species (species richness) per square meter between 2012 and 2013 in the portion of prairie that was burned in 2012.  There was no significant change in species density in the unburned areas of the prairie – which were also grazed, but much less intensively.  Data were collected during June in both years.

Two graphs

Two graphs showing the average number (Mean N) of plant species per square meter in one of our Platte River Prairies.  The left graph shows an increase from about 9 spp/m to 11 spp/m in a patch of prairie burned in 2012, and the right graph shows data from the remainder of the prairie, which was not burned in either 2012 or 2013 and had about the same species richness/meter in both years.  The entire site was grazed in both years, with the 2012 burned patch getting much more intensive grazing than the unburned areas.  The error bars show 95% confidence intervals.

We see this pattern of higher species richness after fire/grazing treatments over and over in our prairies.  My explanation of the phenomenon is two-fold, and was covered in more detail in a previous post.  First, the burn exposes soil (and seeds) to light, allowing relatively high germination rates of seeds waiting for an opportunity.  Second, the concentrated heavy grazing in those burned areas weakens the dominant grasses, prolonging the light exposure of the soil/seeds and also reducing the root masses of those grazed grasses, thereby opening up space belowground for new plants to gain a foothold.  Those new plants start both from seed germination and from the rhizomes and buds of existing perennial plants.

The color-coded table below shows some species-level data from the same site that help explain the above graphs.  The numbers indicate the frequency of occurrence (the percentage of sample plots each species was found in) each year.  Some plant species appear to have responded positively or negatively to the burn and subsequent grazing intensity, while others seem unaffected by those management treatments.

Table 1

Plant species frequency (% of occurrence within sample plots) between 2012 and 2013 in a portion of the prairie that was burned in 2012 and the remainder of the prairie that was not burned.  The entire prairie was exposed to livestock grazing in both years.  An * indicates species with particularly high abundance, for which frequency is shown at the 1/10 m scale.  The remainder of the data are from 1  m square plots.

So far, so easy.  Fire and grazing are having positive effects on the vigor and occurrence of some species, negative effects on others, and no effect on still others.  Cumulatively, the effects add up to an increase in the number of plant species per square meter in the year following the burn.  Simple and clean.  The next two graphs (below) seem to fit the same pattern, showing similar increases in small-scale species richness in burned patches back in 2007 and 2008, respectively.

Two other graphs

Two more graphs, showing results from burned patches from 2007 and 2008, respectively, and an increase in the number of plant species per square meter in the year following each burn.  Again, cattle grazing was much more intensive in these burned patches than in the unburned remainder of the prairie each year.  Error bars indicate 95% confidence intervals.

Here’s where it gets interesting…  The next table (below) shows how individual plant species responded within the three burn patches (2007, 2008, and 2012) between the year of the burn and the subsequent year.  Of the 29 most abundant species, only THREE showed the same response to fire/grazing in each of the three years, and another two were consistent in that they didn’t appear to respond at all to management in any of the three years.  The remaining 24 species were inconsistent in their responses – some wildly so.  Look, for example at indiangrass (Sorghastrum nutans), rough dropseed – aka tall dropseed – (Sporobolus compositus), and blue prairie violet (Viola pratincola, aka Viola sororia).  Each of those three species showed a strong increase in frequency of occurrence in the burn patch one year, but a strong decrease in another.

Table 2

Plant species frequency (% of occurrence within sample plots) between the year of burn and the subsequent year in prairie patches burned in 2007, 2008, and 2012.  The entire prairie was grazed in each of the years, with higher grazing intensity in the more recently burned patches.  An * indicates species for which the frequency is shown at the 1/10 m scale – the remainder of the data are from 1 m square plots.

So what are we to make of all this?  How can we manage prairies effectively if the impacts of very similar management treatments are so different from year to year?

As far as the first question, it’s clear that other variables – particularly weather – interact strongly with management treatments to influence the response of plant species.  Every species has a different optimum range of temperature and moisture conditions, so the timing and intensity of rains and warm/cool spells affects the competitive balance between species differently each season.  Many other factors and interactions are also in play.  Year-to-year changes in population size of various seed-eating species can influence the seed bank, for example. In addition, the success of a particular plant species in one year catalyzes or suppresses other species (plant, insect, mammal, etc.) in the following year.  All of those interactions are signs of healthy prairies, but make it very difficult to predict how those prairies will respond to our management.

As to the question of how we’re supposed to manage prairies in light of all this variability, there are at least two ways to respond.  The first is to throw our hands up in disgust because we can’t seem to predict results well enough to make useful decisions.  The second option is to relax, embrace the variability, and roll with it.

prairies

Healthy prairies have a great diversity of plant and animal species, but also a great diversity of responses to year-to-year stresses imposed by both managers and the environment.  Tallgrass prairie in Richardson County, Nebraska.

Our human nature makes us want (need?) control over our environment.  In agriculture, landscape architecture, engineering and many other fields, the objective is to manipulate the world to our benefit – often by reducing the variables and controlling the remaining ones.  However, when managing ecosystems in order to conserve the diversity of species and their functional roles, that approach simply doesn’t fit.  Instead,we need to think differently about both short-term objectives and long-term goals.

To help me deal with unpredictability, I tend to consider prairie management objectives in terms of ecological resilience, rather than in terms of desired responses from particular species.  (See this earlier post on ecological resilience for background.)  I think of a prairie as a ball in a bowl.  The bowl represents a range of conditions, within which the prairie (the ball) maintains its integrity as a prairie – even if it rolls around quite a bit.  Some disturbances, such as plowing or broadcast herbicide can quickly push a prairie out of its bowl and into another state, from which it is tremendously difficult to recover.  Other disturbances, such as drought, tree encroachment, or intensive grazing may push the ball toward the edge, but as long as those disturbances don’t continue for too many years in a row, the ball doesn’t leave the bowl.

My management objective is not to prevent the metaphorical ball from moving, or to push it toward some optimal part of the bowl.  Rather, I have two big objectives:

1.)   Make sure the ball can move freely around the bowl in response to disturbances, but nudge it back from the edge when necessary.  I celebrate the fact that our Platte River Prairies change in appearance from year to year.  That tells me they are rolling around that bowl as they need to do.  In order to prevent an exit from the bowl, I make sure that we’re suppressing tree encroachment before a prairie becomes a woodland, and that we’re avoiding the kind of chronic intensive grazing that can push some prairie species to local extinction.  During extended droughts, we modify our management to avoid imposing too much additional stress on prairie communities.

2.)  Keep the bowl as large as possible by maintaining a high diversity of prairie species, along with the functional roles each contributes.  That diversity of function allows the prairie to absorb stress and respond to adversity without leaving the bowl.  Maintaining that diversity means manipulating the field of competition between prairie species so that every species can win now and then (by reproducing) and maintain itself within the community.  We create an annual patchwork of various fire, grazing and rest treatments and shift the location of those patches from year to year to provide for the needs of as many plant and animal species as possible.

Prairies are dynamic, and thank goodness for that.  Their ability to flex and respond to various stresses is a key characteristic that has allowed them to survive for thousands of years.  As prairie managers, we need to facilitate that flexibility instead of becoming frustrated with our inability to predict how prairies will respond to our management.  After all, our job is to keep prairies healthy, not turn them into something they’re not just to help ourselves feel more in control.

Management Details

For those of you who might be interested, here is some additional information about the management treatments applied to the prairie in the above example.  The pasture the burns were conducted in is often grazed in conjunction with adjacent prairies, so the total grazed area varies in size from year to year.  In each year, the overall management was patch-burn grazing, in which cattle had access to the entire grazing area, but did most of their grazing in the most recently burned patch.  The stocking rate shown (AUMs/ac) is calculated for the entire pasture and entire season.

2007 Burn (North 1/2 burned on Apr 16, South 1/2 on May 2)

240 acres grazed overall in 2007.  27 pairs from 4/1 – 10/31 (Approx. 1 AUMs/ac)

 

2008 Burn  (Burned on March 18)

171 acres grazed overall in 2008.  27 pairs from 4/1-5/20, 15 pairs till 9/15 (Approx. 0.7 AUMs/ac)

 

2012 Burn (Burned on March 30)

300 acres grazed overall in 2012.  75 pairs 4/20-Jun 1, 50 pairs till 10/1  (Approx. 1.3 AUMs/ac)

Why There Is No Cookbook for Restoring and Managing Prairies

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From Anne Stine, Hubbard Fellow:

My big goal for this fellowship is to learn how to make a prairie from scratch.  I also want to know enough about prairie restoration/management that I can evaluate a prairie’s condition and then prescribe treatments to fix it.  For these first few months with The Nature Conservancy, and especially at the Grassland Restoration Network workshop (July 16-18, 2013 in Columbia, MO), I’ve been asking questions about the problems and solutions common to prairie restorations.  My naïve desire is to develop some sort of prairie restoration cookbook.  When I asked Chris why this didn’t exist, he laughed and said “If that were possible my book would’ve been a lot shorter.”  I built my flowchart anyway.

This blog post will be a bit different- I’m going to share the “Patch-Burn Grazing Flowchart” I developed.  Then Chris will respond and explain why the cookbook method doesn’t work.

(Click on the flowchart to see it as a larger image)

PBGbigfcResponse from Chris:

I give Anne credit – it’s clear she’s paying attention and learning a lot during the first couple months of her Fellowship experience.  Her flow chart includes very appropriate treatments for issues that pop up in prairies, and it’s a nice guide to some of those options.  However, prairie restoration and management is a more complex and dynamic process than can be easily captured in a flowchart (or even in a book).  That complexity can seem daunting to some, but is really what makes prairies fun and interesting to work with.  The trick is to accept the complexity and roll with it.

I've been working to rejuvenate our family's prairie south of Aurora, Nebraska for well over a decade. It's getting there, but it's been anything but a straightforward process. Every year brings new challenges and surprises.

I’ve been working to rejuvenate our family’s prairie south of Aurora, Nebraska for well over a decade. It’s getting there, but it’s been anything but a straightforward process. Every year brings new challenges and surprises, and we continue to tweak our management strategies.

When I wrote my book on managing prairies, I purposefully stayed away from prescribing any particular management regime (or recipe), and instead tried to provide some background on how prairies work and some guiding principles for managing them.  You can find a partial compilation of those ideas by going to PrairieNebraska.org and clicking on the “Prairie Management” or “Prairie Restoration” tabs at the top of the page.  There are lots of reasons I didn’t prescribe particular management recipes.  Here are a few of them:

1. Every prairie has its own unique species composition (plants, insects, animals, fungi) and that composition drives the way it responds to weather and management.  In some ways, prairie management is like parenting – each prairie (and child) has its own personality and needs to be treated in ways that match that personality.  The best parenting books are the ones that suggest general philosophies and offer tips to try in various situations.  Anyone who has been a parent knows that there is no cookbook for how to do it well.

2. Every year is different.  Last year was the driest on record for our Platte River Prairies.  This spring was very cool and wet, followed by a hot dry July, followed by a cool and wet August (so far).  Prairies respond very differently to fire, grazing, seeding, herbicide treatments, and other techniques due to weather conditions.  Countless times, we’ve applied a treatment to part of a prairie and were excited to see how it worked.  The next year, we applied the treatment in exactly the same way and things would turn out very differently.  We try to tailor our management and restoration to the weather, but we know we’ll be surprised by how things turn out.  Those surprises are what I look forward to most each year.

During the drought of 2012 this sandhill prairie was burned and grazed pretty intensively. By late summer, it was looking pretty tough.

During the drought of 2012 this sandhill prairie was burned and grazed pretty intensively. By late summer, it was looking pretty tough.  The same burn timing and grazing stocking rate in a wet year would have resulted in a very different impact.

After adjusting our management plans to account for last year's drought, the prairie was grazed briefly this spring and - thanks to some good spring rains - looked lush and green by early June.

After adjusting our management plans to account for last year’s drought and grazing, the same sandhill prairie shown above was grazed briefly this spring and – thanks to some good spring rains – looked lush and green by early June.

3. Prairie restoration is not very predictable either.  We have developed and tested seeding rates, seeding methods, site preparation, and other techniques that seem to work well at our particular sites, but those same techniques wouldn’t necessarily work somewhere else.  One of the big pieces of advice shared each year at Grassland Restoration Network workshops is that when starting a large restoration project, the best plan is to spend several years experimenting with various techniques on small portions of the overall restoration site to figure out what works best at that particular location.  Once you figure out what seems to work best, start planting larger and larger areas each year.

However, even when the exact same techniques are applied, results can still vary from year to year.  Jeb Barzen and Richard Beilfuss did a great experiment at the International Crane Foundation in the early 1990’s in which they seeded 1 acre a year for five years, using the same seed mix and the same techniques.  Even though all the seedings were in the same crop field, each turned out very differently from each other.  The same thing happens everywhere.  Differences are partially tied to the rainfall and other weather that occurs in the early stages of the seeding, but there are many more factors that are difficult to understand or control.  This isn’t a bad thing, it just means that you have to relax your expectations a bit, and embrace the idea of variability.  Why would you want to create multiple prairie plantings that look exactly the same as each other anyway?

4. Invasive species are always a major challenge, and (you’ll not be surprised at this) have to be handled in unique ways depending upon the species and the site.  Every invasive species has its own growth and reproductive strategies, so an approach to controlling one won’t work well on others.  There are general approaches to controlling each species that have been tested and can be useful, but those approaches will work differently from year to year and from site to site.  One of most important aspects of invasive species control is prioritization, something Anne’s flowchart covers pretty well, and more information on that can be found here.

5. Finally, one of my guiding principles for prairie management is that diverse prairies require diverse management.  Doing the same thing every year means always favoring the same group of species – and, by default, managing against another group.  Eventually, that kind of repetitive management can reduce overall species diversity by eliminating plants or animals that can’t thrive under that management.  It’s good to mix things up to allow all the species in a prairie to have a good productive year now and then.

If a prairie is large enough, splitting it into multiple management units each year can help ensure that animals and insects can always find what they need for habitat (it’s more difficult to do that in very small prairies).  However, it’s also important to avoid simply splitting a prairie into the same three or four pieces and rotating management between them in a repetitive pattern – even those patterns can restrict species diversity over time.

Many prairie insects and animals have fairly specific habitat needs. Grasshoppers, for example, tend to thrive best in prairies with patchy vegetation structure that allows them to move easily back and forth between shade and sun. Splitting prairie into multiple management units each year can help provide the kind of habitat variety needed to maintain high species diversity.

Many prairie insects and animals have fairly specific habitat needs. Grasshoppers, for example, tend to thrive best in areas with patchy vegetation structure that allows them to move easily back and forth between shade and sun. Splitting prairie into multiple management units each year can help provide a variety of habitat conditions and maintain high species diversity.

During the next couple of months, Anne and Eliza will be part of our annual management planning process here in the Platte River Prairies.  Each fall, we go around to each of our prairies and go through a basic evaluation process.  How does the prairie look this year?  What were the impacts of weather and management this year?  What challenges, including invasive species, are we facing?  What kinds of management have occurred over the last several years?  What does the monitoring data from the last couple of years tell us about how past management has been working (sometimes we have hard data, but we always have field notes and other observations to consider).

We walk around, look at maps, and talk about ideas.  Then we sketch out a plan for the next season based on all of those factors.  We try to make sure it’s different from what we’ve done over the last year or two, but that it addresses the challenges the prairie is facing.  Most importantly, we make sure that we’re learning from and adapting to what we’ve tried in the past and the ways the prairie has responded.

I suppose I could capture that process in a flowchart.  It would look something like this:

Helzer Flowchart

That’s probably not exactly what Anne was hoping for, is it?