Tag Archives: prescribed fire

Blowing Against the Wind?

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As I mentioned last week, I recently spent a couple days helping our land manager, Nelson Winkel, pull garlic mustard at our Rulo Bluffs Preserve in southeast Nebraska.  The invasive species has just started to invade our property within the last several years.  We’ve heard stories from colleagues in other places about beautiful woodland plant communities turning into monocultures of garlic mustard within a relatively short time period.  We’d sure like to keep that from happening at our Preserve.

So, we pulled garlic mustard plants.  A lot of them.  On the first day, I figured we pulled at least 25,000 plants.  That’s a very conservative estimate.  The second day was longer, but we did more searching and less pulling.  This wasn’t the first trip to pull either, so we were just trying to get what was leftover from the previous efforts.

Nelson Winkel, showing off one patch's worth of pulled garlic mustard plants. The Nature Conservancy's Rulo Bluffs Preserve - Nebraska.

Nelson Winkel, showing off one patch’s worth of pulled garlic mustard plants. The Nature Conservancy’s Rulo Bluffs Preserve – Nebraska.

The Rulo Bluffs Preserve is 444 acres.  Hand-pulling weeds doesn’t seem like a very sustainable strategy for invasive species control at that scale.  In fact, it’s downright depressing because we pull more plants from more locations every year.  We’re clearly not winning.  So why bother?

It’s a good question, with several answers.  The first answer is that we’ve got some ideas for increasing our effectiveness.  Nelson and I talked as we worked about how we might put together a small army of volunteers to come help us pull each spring.  The big challenges are that the site is far from population centers (more than two hours from Lincoln and Omaha), has difficult terrain to hike in, and garlic mustard doesn’t bloom at exactly the same time each year, so we’d have to schedule work days on fairly short notice.  On the other hand, I think there are people who’d be glad to help, and it is a beautiful place to work in the spring time – lots of warblers and other birds above, and plenty of woodland wildflowers below.

In addition to finding more people to help hand pull, we hope to decrease the number of plants we need to pull in the bigger, more established, patches by doing some herbicide work in the late winter.  Garlic mustard is a winter annual or biennial which germinates in one season, overwinters as a rosette (a few leaves, low to the ground), and then flowers in the late spring of the next year.  Our colleagues in more eastern states have been dealing with garlic mustard longer than we have, and have had luck spraying the rosettes with Glyphosate herbicide on warm February days.  Spraying in the winter works well because there are very few other woodland plants that are green (and thus susceptible to Glyphosate) in February.  They don’t usually spray in the early winter because many rosettes die on their own over the winter, and by waiting until February, they can focus only on those most likely to bloom in the coming year.  Nelson was marking the bigger patches we found with a GPS unit so he can find them next winter and try the spraying technique.

Small patches of garlic mustard such as this one might eventually be eliminated by hand-pulling.

Small patches of garlic mustard such as this one might eventually be eliminated by hand-pulling – especially if we find and treat them every year.  Larger patches are much more problematic.

The second reason we’re still trying to suppress garlic mustard is that I hope we can buy some time until better control options become available.  There has been some work to develop a biocontrol technique (using insects from the native range of garlic mustard), for example, and if something like that turns out to be effective, I want to be sure we still have some woodland left to save.  Unfortunately, I’m hearing that biocontrol development has stalled at the moment.  Apparently, in at least some places, people are seeing garlic mustard populations decline steeply on their own – as if the plants are outcompeting themselves and self-thinning.  That could be great news, but only if the native plant community rebounds as the garlic mustard declines, and I haven’t been able to find anyone who can tell me whether or not that’s the case.  I sure hope it is, but I’d feel better if the biocontrol folks kept forging ahead on the development of that control option anyway.  Regardless, I’m holding out hope that either garlic mustard will turn out to be a temporary nuisance (seems unlikely?) or that biocontrol or better control options will be developed in the next several years.  I could be naive, but at least it gives us something positive to think about while we’re pulling up thousands of garlic mustard plants…

While we look for better control options, we’re also trying to change the playing field for plant competition at Rulo Bluffs and give garlic mustard less of an advantage.  With considerable help from Kent Pfeiffer of Northern Prairies Land Trust, and funding from the Nebraska Game and Parks Commission and others, we’ve been trying to ramp up our stewardship work during the last several years.  We’ve not done as much burning as we’d like to, but are earnestly trying to change that.  Last fall, a contractor did some “hack-and-squirt” herbicide treatment to kill many of the smaller understory trees that are shading out the herbaceous plants on the ground.  We’ve also been doing mechanical shredding of brush on ridgetops to help the grassland, savanna, and open woodland plants there.  All of this work is aimed at getting more light to the ground, which should stimulate increased oak regneration and a stronger, more diverse, herbaceous community on the woodland floor.  In addition, we hope that increased light will put shade-loving garlic mustard at a disadvantage, at least in some parts of the woodland.  From talking with others around the country, they’ve seen mixed results from similar work.  I guess since we want that light on the ground anyway, we’re going to forge ahead – and hope we don’t make things worse.

Jack-in-the-pulpit is one of many woodland flower species that help make the Rulo Bluffs Preserve unique and valuable.

Jack-in-the-pulpit is one of many woodland flower species that help make the Rulo Bluffs Preserve unique and valuable.

Finally, we’re pulling garlic mustard because the Rulo Bluffs Preserve is worth the effort.  It’s one of the few remaining high-quality oak woodlands in Nebraska, and hosts a wide diversity of plant and animal species – many living at the edge of their geographic range.  In addition to lots of mayapples, jack-in-the-pulpit plants, and woodland phlox, we also found two orchid species blooming last week – the showy orchid and the yellow lady’s slipper orchid.  We walked around beneath eastern deciduous tree species such as chinkapin oak, black oak, and Ohio buckeye.  Several animal species at the preserve, including zebra swallowtails, timber rattlesnakes, and southern flying squirrels, are only found on the very eastern edge of Nebraska.  While some of those species are common to the east of us, it is probably important to protect their genetic diversity by maintaining populations across their entire range.  That should allow the species to better adapt and survive in changing conditions over time.

Genetic and biological diversity aside, the Rulo Bluffs Preserve is also important because it’s a beautiful place.  We need to keep some aesthetically-pleasing natural areas around for people to enjoy.  Despite our aching backs, Nelson and I had a great time exploring the preserve last week, marveling at warblers, flowers, velvet mites, and other wonders.  It’s possible that we’ll invest a tremendous amount of time and money into stewardship and restoration at the Rulo Bluffs Preserve over the next several years and still lose out to garlic mustard.  There are plenty of examples of that happening elsewhere.  I guess we’re not ready to concede the battle, however – there’s too much at stake.

Tuning Into Fire Frequency

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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.

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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.