The Role of History In Today’s Prairie Management

Past performance is not necessarily indicative of future results.

I’m no expert in financial investing, but I’d like to retire someday, so I muddle along the best I can.  As I skim through various financial statements and investment newsletters, I often see some variation of the disclaimer above.  The concise statement emphasizes that while history is important, many factors change over time, and we shouldn’t simply assume that what happened previously should drive what we do now.

I was thinking about this statement and its implications while attending the North American Prairie Conference last month.  During presentations and hallway discussions, the topic of history came up frequently.  How often did prairies burn prior to European settlement?  Were bison only abundant in eastern tallgrass prairies after human populations crashed during the smallpox catastrophe?  What was the role of big native ungulates like elk in suppressing woody plants?

Fire

We have reasonably good data on the historic fire frequency in prairies around the U.S.  How should that information drive today’s prairie management?

Questions like those are fascinating to contemplate, and important to our understanding of how prairies have changed over time.  Which of us wouldn’t jump at the opportunity to step into a time machine and go see North American prairies in the 1400’s or other historic times?  Wouldn’t it be fantastic to somehow find and pore over hundreds of years of data on bison population numbers, plant species composition, elk feeding patterns, and lots of other grassland phenomena?  While, that kind historic data is very limited, mining what we do have is fascinating and instructive.

However, just as with stock market investments, we can’t just look to the past to guide what we should do in the future.  The business world has evolved over time.  Simply investing today in the same corporate stocks that were profitable 30 or 60 years ago wouldn’t make a lot of sense.  Instead, we need investment strategies that fit today’s world.  Many companies disappeared over time because their products became obsolete.  Those that are still around, like General Electric, Nokia, and IBM, reinvented themselves.  Why?  The business landscape changed and they changed with it.

The prairie landscape has changed too.  Row crop agriculture and other human developments have replaced grassland across huge swaths of our country, leaving many prairies relatively small and isolated.  Trees and shrubs have flourished in landscapes where they were once scarce, and woody encroachment into small prairies now comes from all directions.  Many new species of plants and animals have found their way into North America, and some have become very aggressive.  Significant amounts of nitrogen from industrial and agricultural sources now enter grasslands by both air and water, changing soil chemistry to favor some plants over others.  Finally, prairies have endured a century or two of impacts from factors such as fire suppression, livestock grazing, haying, and broadcast herbicide use.  Today’s remaining prairies don’t look or function as they did a century or two ago.

corn

Prairies today exist within landscapes that are dramatically different from what they looked like historically.  Row crop agriculture has replaced grassland across much of the Midwest and Great Plains, and trees, invasive species, and many other factors threaten the remaining patches of prairie.

Big changes to prairies and surrounding landscapes mean that land managers face equally big challenges as we try to sustain biological diversity and ecological function.  For most managers, invasive species suppression is our most time consuming and expensive task.  Because of that, we are always searching for new ideas, strategies, and technologies to help us be more effective and efficient.  The herbicides we use to kill invasive plants were not part of the prairie ecosystem a couple hundred years ago, but I can’t imagine trying to do our job without them.  Similarly, brush mowers and the tractors that pull them are certainly not historically accurate, but they are invaluable when creating firebreaks or mowing down large patches of encroaching brush.

Today, land managers’ decisions about when to burn a prairie should be based on the myriad management objectives we face rather than on what the historic average fire frequency might have been at that site.  In many prairies, managers struggle to weigh the benefits of frequent fire to control brush and other invasive species against the potential impacts of frequent fire on vulnerable insects, reptiles, and other species.  Looking at historic fire patterns can help us understand how prairies developed, but today’s fire patterns need to address current challenges and help us sustain our imperiled grasslands.

Similarly, studying the historical population abundance of bison or elk can teach us about how those species influenced prairie communities long ago, but decisions about grazing as a contemporary management strategy need to be made based on today’s objectives and needs.  I wrote last week about the introduction of bison into the Nachusa Grasslands in Illinois, and attempts to capture the impacts of bison grazing at that site.  I’m sure the staff at Nachusa have been in numerous discussions about what historic bison populations were like in what is now northern Illinois.  The decision to bring bison in, however, was not based on history, but rather on defined needs for habitat structure and plant community management.  Nachusa staff are hoping to see more diverse grassland bird communities, for example, and positive effects on a wide variety of mammals, reptiles, and invertebrates.  They also hope bison will help maintain high plant diversity.  In particular, they hope to increase the long-term survival of relatively short-lived plant species that often disappear over time in restored prairie.

bison

Bison and cattle grazing can be useful in meeting some prairie objectives, but is not appropriate for all sites.

Here in Nebraska, The Nature Conservancy uses both cattle and bison to achieve prairie management objectives.  Grazing strategies are designed with specific objectives in mind, and we collect as much data as we can to evaluate the impacts of grazing on plant and animal communities.  Grazing helps us suppress the vigor of both non-native invasive grasses and aggressive native grasses and foster a more diverse plant community.  Plant species that would otherwise be outcompeted by dominant grasses can usually maintain strong populations under various combinations of intensive grazing and long rest periods.  Both cattle and bison can also help us create a wide variety of habitat conditions, including large areas of both short/sparse and tall/rank vegetation and other areas where patches of short and tall vegetation are intermixed.

Just as with fire, mowing, and herbicide use, the value of grazing as a prairie management tool needs to be evaluated not by its historic role in local grasslands but on its potential utility today.  In many prairies, grazing is not feasible or does not fit with management objectives.  For example, grazing is unlikely to make sense in small isolated prairies where wildlife/insect diversity is limited more by habitat quantity than habitat structure, and where plant composition objectives can be met through other means.  At larger sites, however, grazing may allow managers to provide more habitat variety and/or manipulate plant competition in positive ways.  Regardless, decisions about whether or not to graze should be based upon how grazing might help address current management challenges, not upon historic populations of bison or elk.

Prairie management is complicated and we have a lot left to learn.  We can’t afford to be overly conservative or rely too much on what happened long ago.  Imagination and experimentation are crucial components of adaptation, and we desperately need to keep adapting to new challenges if prairies are going to survive.  Companies like General Electric, Nokia and IBM rightly celebrate their history, but they also have to innovate and evolve to keep up with the changing landscape.  Prairie managers need to innovate and evolve to keep up with changing landscapes too.  Let’s learn what we can from the past but keep looking for new ideas and tactics so we can keep prairies healthy and vibrant well into the future.

After all, prairie conservation is worth the investment, right?

Tuning Into Fire Frequency

HOW OFTEN SHOULD PRAIRIES BE BURNED?

It’s a question prairie ecologists and managers have been wrestling with for many years.  Unfortunately, research on the impacts of fire management is somewhat limited and often contradictory.  Much of the best research has come from Konza Prairie in the flint hills of eastern Kansas, but many have rightly pointed out that translating flint hills research to other prairies – especially eastern tallgrass prairies – can be tricky.

Prescribed is an important tool for prairie management, but how often should it be employed?

Prescribed is an important tool for prairie management, but how often should it be employed?

At Konza and other western tallgrass prairie sites, frequent application of fire (in the absence of grazing) tends to increase the dominance of grasses, and decrease the abundance and diversity of wildflowers.   However, prairie ecologists and managers working in eastern tallgrass prairies (particularly in Wisconsin and Illinois) point to numerous prairies that have been frequently burned for decades with no apparent loss of plant diversity.  Those experts make strong arguments against applying western experience with frequent fire to eastern prairies.  Unfortunately, the discussion has suffered from a scarcity of published long-term data from eastern prairies to help evaluate impacts of fire management there.

Just last month, an excellent research paper by Marlin Bowles and Michael Jones helped fill that void.  In 2001, Bowles inventoried the plant communities of 34 prairies around Chicago, Illinois – ranging from dry to wet-mesic sites – and compared those data to similar inventories conducted twenty five years earlier.  The similarity in sampling methods between the two efforts allowed Bowles and Jones to look at how fire frequency affected changes in plant species composition over a significant period of time.  In short, they found that a high fire frequency had a positive correlation with plant diversity.

Using data from a series of 0.25m2 plots, Bowles and Jones analyzed changes in the average number of plant species (species richness) between the 1976 and 2001 data sets.  Frequent burning increased species richness overall, but had a particularly positive impact on summer wildflower richness.  Spring wildflowers, warm-season grasses, and legumes didn’t necessarily increase in species richness with higher fire frequency, but strongly decreased in richness within prairies that were not burned very often.  The authors speculated that the greatest impact of burning on plant species richness was likely the removal of detritus (previous years’ vegetation), which can greatly reduce the amount of light available to growing plants and also change microclimatic conditions and nutrient availability.  Because eastern tallgrass prairies receive more rainfall than do western prairies, they produce more plant biomass each year.  Bowles and Jones pointed to that increased biomass production as a probable reason that frequent fires have such a strong positive impact on plant diversity in eastern prairies.

In the eastern tallgrass prairies studied by Bowles and Jones, summer wildflower diversity increased under frequent burning.

In the eastern tallgrass prairies studied by Bowles and Jones, summer wildflower diversity increased under frequent burning. (Photo from Taberville Prairie – Missouri)

The contrast in response to frequent fire between western and eastern tallgrass prairies is intriguing, and it’s great to have published long-term data to help quantify it.  It may be that increased vegetative growth due to higher rainfall in the east is largely responsible for the difference, but surely the story is more complicated than that.  It will probably take quite a bit more research across the entire east-west continuum of tallgrass prairies before we really understand what’s going on.

In the meantime, it’s important to recognize differences in the way eastern and western tallgrass prairies respond to fire management, but it’s also important to not overly generalize those results. Every prairie will still respond individually to management (based on many factors, including soil type, presence/abundance of invasive species, etc.,) and it’s important not to implement any management regime without evaluating and adjusting over time.  In addition, east vs. west is only one of many ways to characterize differences between prairies and their responses to management.  Northern and southern prairies, for example, also respond very differently to management – due in part to a stronger cool-season grass component in northern prairies, including several invasive grasses which are really not a factor in the ecology of southern prairies.  (Southern prairies have their own set of invasive species as well.)

Prairies like this one in the flint hills of Kansas respond differently to fire and other management treatments than do prairies further east.  However, other variables (latitude, soil type, topography, land use history, and more) all influence management responses as well.

Prairies like this one in the flint hills of Kansas respond differently to fire and other management treatments than do prairies further east. However, other variables (latitude, soil type, topography, land use history, and more) all influence management responses as well.

.

BUT BE CAREFUL…

While I thought the paper by Bowles and Jones was very well done, two thoughts occurred to me as I read it.  I will deal with each only briefly now, but will flesh them out more in future posts.  I think both are important to consider before entering into a management regime dominated by frequent burning.

First, Bowles and Jones emphasized the importance of frequent fire as a “stabilizing force” in tallgrass prairie plant communities.  In other words, they inferred that good management should result in a plant community that changes little from year to year.  They called these stable plant communities “late-successional,” a term that I have a difficult time applying to prairies, which require frequent disturbances to keep from becoming woodlands.  Regardless of terminology, however, the question of whether or not prairies should have stable plant communities is an interesting one.  I’ve argued in the past that healthy prairie plant communities should look different each year (see, for example, my post on “Calendar Prairies”.)  However, most of my experience comes from more western prairies, so I have an admitted bias.  Still, it worries me to have a management regime that always favors the same species year after year, because other species are – by default – being perennially managed against.  Reducing the overall pool of species in a prairie seems potentially risky, but I don’t know how serious that risk might be.

These black-eyed susans are blooming in profusion the year after a Platte River Prairie was grazed.  An abundance of short-lived opportunistic species such as this one might lead to a prairie being characterized by som as "mid-successional'.

These black-eyed susans are blooming in profusion the year after a Platte River Prairie was grazed. An abundance of short-lived opportunistic species such as this one might lead to a prairie being characterized by som as “mid-successional”.  Whether or not that characterization is apt or useful is an interesting question, and may depend upon where a prairie is located (or may just depend upon the background of the ecologist thinking about it!)

Second (but related to the first), arguments for frequent fire tend to focus primarily on plant diversity rather than the overall diversity of the prairie community, including both vertebrate and invertebrate animals – not to mention fungi, bacteria, and other organisms.  Fire can have serious negative implications for some of those other residents, especially when small isolated prairies are burned in their entirety, leaving no unburned refuges for vulnerable species.  Insects that overwinter in the stems of plants, for example, are particularly vulnerable to spring fires.  The dramatic change to habitat structure wrought by fire can also have big impacts on vertebrates (as well as invertebrates) that require thatchy cover for survival.   As I mentioned above, reducing the pool of species in a prairie (plant, animal, or otherwise) may have serious implications for the overall health of the prairie – especially in fragmented landscapes where species are unlikely to recolonize areas from which they are eliminated.

I generally find prairie skinks such as this one in prairies with a certain amount of thatch.  I'm not sure how this and other species would do in prairies that were burned in their entirety on a frequent basis.

I generally find prairie skinks such as this one in prairies with a certain amount of thatch. I’m not sure how this or other thatch-dependent species would do in frequently burned prairies.

I tend to favor prairie management that provides multiple habitat types and growing conditions each year, and shifts the locations of those around the prairie from year to year.  That kind of mixed and dynamic management should help ensure that animal species can always find a place to live within a prairie, and that every plant species will have positive growing conditions at least every few years.  However, I’m making some big assumptions about the importance of that philosophy, and I’m certainly not advocating that every prairie should be managed that way – especially very small prairies for which subdivision of management may not even be feasible.

UPSHOT

Saying that prairies are incredibly complex and difficult to understand is an understatement.  I think our prairie management should account for that complexity.  Good managers carefully evaluate the responses of their prairies to management, and adjust accordingly.  The Bowles and Jones study helps us better understand the way prairies respond to management, but also highlights the danger of simply applying what works in some prairies to others.  Their paper focuses on differences between east and west, but regardless of geographic location, soil type, size, or degree of isolation, every prairie needs (and deserves!) management that is custom tailored.