Tag Archives: prairie

A Prairie Ecologist’s Perspective on Arbor Day

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23

Today is National Arbor Day – a holiday initiated by J. Sterling Morton right here in my home state of Nebraska.  The idea of Arbor Day is to encourage the planting of trees. However, as a prairie ecologist, I spend considerable effort trying to keep trees from taking over the prairies I manage and/or care about.

As a result, I have some mixed feelings about Arbor Day.

A cottonwood tree in a Platte River prairie at sunrise.

A cottonwood tree in a Platte River prairie at sunrise.  Beautiful, but not without some impacts on prairie ecology.

I don’t harbor any particular enmity toward trees themselves.  In fact, some of my favorite places in Nebraska have fantastic woodlands, including The Nature Conservancy’s Rulo Bluffs Preserve and Niobrara Valley Preserve.  I also really like the trees in my yard – especially the one that produces a big crop of pie cherries each year!

On the other hand, I don’t hesitate to use all necessary force to remove trees from grasslands.  Why?  There are numerous reasons.  Here are a few examples:

– The shade from trees changes the microclimate underneath them, suppressing the growth of many prairie plants and favoring others, including some nasty invasive species that can then spread into prairies.

– Trees completely change the habitat structure of a prairie, making it unsuitable for many wildlife species that rely on wide open habitats.  As trees and shrubs increase in density, prairie animals are forced out.

– Many grassland birds avoid nesting near woodlands, or even lone trees.  There are multiple reasons for this, but a big one is the abundance of predators that hang around in and under trees.  A line of trees along the edge of a prairie creates a wide “dead zone” within which very few prairie bird species will nest.  In landscapes where most prairies are already small and fragmented, the loss of that additional habitat can have serious consequences.

– Once trees and shrubs become established, they tend to promote the establishment of more.  Some spread by rhizomes (underground stems) and all of them are good perch sites for birds, which drop seeds out both ends onto the ground beneath the trees.  Once on the ground, shade from the trees reduces the vigor of prairie plants and helps woody seedlings thrive.  Dense tree and shrub patches can also become fire proof because their shade prevents grass growth beneath them – and it’s grass that carries fire.

Burning prairie can suppress the encroachment of trees, including eastern red cedars.

Burning prairie can suppress the encroachment of trees, including eastern red cedars.

Woody plants, including both trees and shrubs, have always been good at invading prairies but, historically, fires and dry climate helped keep them from becoming established across most of the grasslands of the Great Plains.  Today, fire suppression, landscape fragmentation, and even changing levels of carbon and nitrogen in the atmosphere are giving trees the upper hand.  As a result, my staff and I (ok, mostly my staff nowadays) spend an inordinate amount of time cutting trees down, burning grasslands to kill trees, and treating trees with herbicides.

Can you see why a holiday that promotes tree planting – especially in a grassland state like Nebraska – might make me a little nervous?  There’s absolutely nothing wrong with planting trees, but the kind of trees being planted and the location they’re planted in makes all the difference in the world.

If you live near a prairie, here are two important tips to consider as you prepare to celebrate National Arbor Day by planting a tree (or lots of trees).

1. Do some research on the tree species you plant.  Some species, such as Siberian elm, Russian olive, Autumn olive, and other non-native trees and shrubs can be very invasive in grasslands, quickly spreading by seed far from the parent tree.  A quick internet search using the name of the tree and the word “invasive” will tell you whether or not the tree species you’re considering is one that could cause problems.  However, even many native trees can spread into grasslands, so that leads us to…

2. Carefully consider the location of your proposed tree planting.  As mentioned earlier, even a single tree, let alone a row along the edge or (heaven forbid!) through the middle of a prairie can wreak havoc on grassland birds and other species.  Prairies and their plant and animal inhabitants thrive in wide open habitats; adding trees to those habitats can really mess things up.

By all means, plant trees in your yard, around your farmstead, or in a local park or school.  Trees provide shade, habitat, food, and aesthetic beauty to cities, towns, and acreages, and you should feel good about contributing toward those things.  However, as you celebrate Arbor Day, please don’t forget about prairies, the plants and wildlife that rely on them, and the hard-working prairie ecologists and land managers trying to conserve them.

So,

Happy Arbor Day!

(please celebrate wisely)

Tuning Into Fire Frequency

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16

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.