Dealing With a Pervasive Invasive – Kentucky Bluegrass in Prairies

Many of the prairies we manage have pretty degraded plant communities, characterized by low plant diversity and dominance by a few grass species – including the invasive Kentucky bluegrass (Poa pratensis).  Our primary objective for these prairies is to increase plant diversity, which, in turn, bolsters ecological resilience and improves habitat quality for a wide range of prairie species.  Because bluegrass is so pervasive in our prairies, we’ve had to modify our objectives and strategies from those we use to address most other invasive species.

Kentucky bluegrass can stifle plant diversity by crowding out other plants. In many prairies degraded by years of overgrazing and/or broadcast herbicide use, bluegrass is now the dominant plant species and few other plants can compete with it.

When attacking invasive plant species, a common strategy is to contain, and (hopefully) shrink, patches of invasive plants in order to protect plant diversity in non-invaded areas.  In the case of Kentucky bluegrass, however, we have to take a different approach because the species already spans the entire prairie.  Kentucky bluegrass acts like a thick blanket of interwoven stems, roots, and rhizomes – smothering most other plant species beneath it.  Because our goal is to increase plant diversity, we want to make that blanket thin and porous enough that a wide variety of other plant species can grow up through it.   

An illustration of a floristically diverse prairie (left) with several large patches of an invasive plant.  When dealing with many invasive species, we can focus on reducing the size of infestations in order to restore a more diverse plant community.

   

Kentucky bluegrass acts as a thick blanket that smothers most other plant species.  In a case like this, we need to manage the prairie in a way that increases the mesh size of that blanket and allows a more diverse plant community to poke through.

Our primary strategy for suppressing Kentucky bluegrass is the periodic application of prescribed fire and grazing.  We can weaken bluegrass by burning prairies when bluegrass is just starting to flower and/or by grazing prairies harder in the spring than in the summer.  We mix those treatments with rest periods within a patch-burn grazing regime.  The result has been a steady increase in plant diversity in most of our degraded prairies.

If we were fighting a different invasive species, we might expect that if plant diversity was increasing, the amount of territory occupied by the invasive species would be decreasing.  With Kentucky bluegrass, however, our bluegrass blanket is getting thinner, but still covers the whole prairie – something that shows up clearly in data I’ve been collecting over the last decade.  Through the use of nested sampling plots of 1m2, 1/10m2, and 1/100m2, I’ve been tracking plant diversity and floristic quality through time, along with changes in the frequency of various plant species (the percentage of plots in which they occur).  Over the last 8-10 years, as plant diversity within 1m2 plots has increased, the frequency of Kentucky bluegrass has stayed about the same.   Even at smaller plot sizes, which are more sensitive to changes in the frequency of very abundant species, bluegrass is still in nearly every plot.

We also have a number of restored (reconstructed) prairies in and around our remnant prairies.  Within restored prairies, plant diversity is in pretty good shape, but Kentucky bluegrass is rapidly invading.  In one particular prairie, Kentucky bluegrass is now in almost 90% of 1m2 plots and more than 50% of 1/100m2 plots.  That sounds bad, but as bluegrass becomes more abundant, plant diversity – and the frequency of other prairie plant species – is actually holding steady.  There are a few small areas in which bluegrass appears to be forming near monocultures, but for the most part, it looks like bluegrass is just filling in around the other plants instead of actually displacing them.  My guess is that some soil conditions provide such ideal growing conditions for bluegrass, it’s going to be king of those areas no matter what we do.  Elsewhere, however, I think our management is preventing it from becoming dominant.

Here’s the take home lesson for me:  When trying to manage for plant diversity in the face of a pervasive species such as Kentucky bluegrass, it’s more important to track plant diversity than to worry about how much territory bluegrass occupies. 

Just consider the data from our prairies… if I concentrated only on how much Kentucky bluegrass is in our prairies, it would look like we’re failing miserably in our management attempts.  We’ve got just as much bluegrass as we ever did in our degraded remnant (unplowed) prairies, and we’re quickly losing ground in some of our restored prairies.  However, my data also shows that plant diversity is increasing in remnant prairies and holding steady in restored prairies.  Since the ultimate goal is to have a diverse plant community, that’s success! 

Click here to see a PDF showing some of the data I mentioned in this post.

The Right Metaphor for Prairie Restoration

Prairie restoration can be a powerful tool for grassland conservation, but we’re not taking advantage of its full potential.  Too often, we think and talk about prairie restoration (aka prairie reconstruction) in the wrong way.  Instead of trying to restore an ecosystem, we try to reproduce history.

Nelson Winkel, land manager for The Nature Conservancy’s Platte River Prairies, harvests grass seed using a pull-behind seed stripper.

I was in Washington D.C. a couple weeks ago and visited Ford’s theater, where President Lincoln was assassinated in 1865.  After the death of the president, the building went through drastic changes, including being completely gutted after a partial collapse of the interior.  By the time the decision was made to restore the building for use as a historic site, the National Park Service basically had to start from scratch.  Regardless, through painstaking research and a lot of hard work, the theater was rebuilt to closely resemble Ford’s theater of 1865.

The rebuilding of Ford’s theater is a decent metaphor for much of the early prairie restoration (or reconstruction) work dating back to the 1930’s in North America – as well for some of the restoration work that continues today.   In the case of prairie restoration, someone identifies a tract of land that used to be prairie but has been converted into something completely different (usually cropland), and tries their best to restore what was there before it was converted.  Just as in the restoration of Ford’s theater, the prairie restoration process requires lot of research and hard work to identify, find, and reassemble what had been there before.

Unfortunately, the Ford’s theater approach has turned out to be a poor fit for prairie restoration.  Prairies aren’t buildings that have specific architectural plans and well-defined pieces that can be collected and assembled to create a pre-defined end product.  Prairies are dynamic ecosystems that are constantly changing and evolving, and their components include organisms that interact with each other in complex ways.  Trying to recreate a prairie that looks and functions just as it used to – especially on a small isolated tract of land – is nearly impossible.

Reseeded prairie at The Nature Conservancy’s Kankakee Sands Restoration Project in Indiana. If the plant community today looks different than it did before it was farmed, is that really a failure of the restoration project?

That doesn’t mean small scale prairie restoration is a bad idea.  I think reestablishing vegetation that is similar to what was at a site many years ago can have tremendous historic and educational value, and can also provide important habitat for many grassland species.  Where this kind of prairie restoration falls flat is when we expect too much from it.  It’s really easy to find glaring differences between the restored prairie and what we know or think used to be there – soil characteristics are different, insect and wildlife species are missing, plant species are too common or too rare, etc.  These “failures” have led some people in conservation and academia to become disillusioned with the whole concept of prairie restoration.

In reality, prairie restoration has proven to be very successful, and is a tremendous tool for grassland conservation.  We just need to find and apply a better metaphor.

A Better Metaphor for Ecological Restoration

Unlike efforts to restore old buildings, prairie restoration projects should not be aimed at recreating something exactly as it existed long ago.  Instead, effective prairie restoration should be like rebuilding a city after large portions of it are destroyed in a major disaster.  When reconstructing a metropolitan area, replicating individual structures is much less important than restoring the processes the inhabitants of the city rely on.  The people living and working in a city depend upon the restoration of power, transportation, communication, and other similar functions.  Those people don’t care whether roads, power lines, or communication towers are put back exactly as they were before – they just want to be able to get the supplies and information they need, and to travel around so they can to do their jobs and survive.  Restoration success is not measured by how much the rebuilt areas resemble the preexisting areas, but by whether or not the city and its citizens can survive and thrive again.

Similarly, restoration of fragmented prairie landscapes should not be an attempt to recreate history.  It should be an attempt to rebuild the viability of the species – and, more importantly, the processes – that make the prairie ecosystem function and thrive.  Success shouldn’t be measured at the scale of individual restoration projects, but at the scale of the resultant complex of remnant and restored prairies.  Are habitat patches sufficiently large that area-sensitive birds can nest successfully?  Are insects and animals able to travel through that prairie complex to forage, mate, and disperse?  Are ecological processes like seed dispersal and pollination occurring between the various patches of habitat?  When a species’ population is wiped out in one part of the prairie because of a fire, disease, or other factor, is it able to recolonize from nearby areas?

Pollination is an example of an important process that drives prairie function. Increasing the size and/or connectivity of prairies by restoring areas around and between prairie fragments can enhance the viability of pollination and other processes.

At first glance, choosing the appropriate metaphor for prairie restoration may seem insignificant compared to other challenges we face in grassland conservation.  However, if we’re going to successfully restore the viability of fragmented prairies, we can’t afford to waste time and effort worrying about whether or not we’ve matched pre-European settlement condition, or any other historical benchmark.  Instead, we need to focus on patching the essential systems back together.

After all, we’re not building for the past, we’re building for the future.

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Read more on this subject…

– An earlier blog post about using prairie restoration as a landscape scale conservation tool.

– A prairie restoration project case study, with ideas about how to measure its success.

– Some recent early attempts we’ve made to measure restoration success by looking at the responses of bees and ants.

– A post about the importance and definition of ecological resilience in prairies.