Tag Archives: brush

New Information on Tree Invasion in Prairies

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One of the biggest challenges of prairie management today is the suppression of woody invaders.  Both native and non-native woody species can spread rapidly in prairie, making it difficult to maintain the open grassy habitat that most prairie species depend upon.

There has been extensive speculation about why shrubs and trees appear to be more aggressive and successful now than in the past.  Fire suppression has been a factor identified by many as a likely cause, but it’s clearly not the only factor because there are examples such as Konza Prairie in Kansas where shrubs have spread strongly under more than 20 years of regular fire application.

Fire can help suppress shrubs, but there are plenty of examples where frequent fire is not sufficient to stop their expansion.

Now, a new study from Konza Prairie may shed some light on at least some of the reasons behind the agressive expansion of shrubs in the Kansas Flint Hills and other mesic tallgrass prairies.  The research paper, written by Zak Ratajczak, Jesse Nippert and others, addresses both the initial survival of new woody plants and the subsequent spread by clonal species (such as dogwood and sumac, which spread by underground rhizomes).  It’s worth reading, and you can find a PDF here.

The question of why woody plants are able to establish more successfully in prairies now than they could several decades or more ago is still largely speculative.  Jesse Nippert explains his reasoning in an interview here.  Changing atmospheric conditions – especially higher nitrogen and carbon levels – are altering the competitive balance in grasslands to favor C3 plants over C4 plants.  Because shrubs like dogwoods (Cornus sp.) are C3 plants, higher levels of nitrogen and carbon in the atmosphere are likely giving them an advantage over C4 plants such as big bluestem and other warm-season native grasses that have historically had a competitive edge in tallgrass prairie.  This could explain why woody plants are surviving their seedling stage more now than they did in the past – but the idea still needs to be tested further.

However, while initial survival of shrub and tree seedlings is one important component of the issue, the research paper by Ratajczak et al. also addresses the subsequent spread of those shrubs – and they do so through field data collection.  They focused their work on the primary shrub species spreading at Konza Prairie – rough-leaved dogwood (Cornus drummundii).  What they found was that while most prairie plant species get the vast majority of their resources from the top foot or so of the soil profile, dogwood plants get almost half of their resources from below that level.  In other words, dogwoods are using resources – especially moisture – that most prairie plants aren’t taking advantage of.  (Yes, most prairie plants do have deep roots, but they typically reserve the use of those deeper roots for periods of drought and rely on their much more abundant shallow roots most of the time.)  Importantly, not only do “parent” stems of dogwood use deep soil water, new stems that are initiated by rhizomes (below-ground stems) do too – probably because they can pull water from their parents until they get their own deep roots established.

Taken together, the two ideas proposed by Ratzjcak, Nippert, and others provide an interesting hypothesis about how today’s shrub invasion may be taking place.  Higher levels of carbon and nitrogen in the atmosphere and/or soil provide a new competitive edge to colonizing woody plants.  That “fertilized” environment overrides the traditional advantage that warm-season grasses have over shrubs, which is that grasses are very good at monopolizing soil resources within the top foot or so of the soil profile.  Today, young woody plants are surviving long enough in that dry upper soil layer to extend their roots into deep moist soil – below where most other prairie plants mine resources.   Once those woody plants tap into that deep soil moisture, their survival is much more assured.  Woody plants that are clonal – such as rough-leaved dogwood and smooth sumac (Rhus glabra) – can then spread by rhizomes, continuing to take advantage of their ability to utilize the deep soil moisture their neighbors aren’t using.

Smooth sumac and flint hills prairie - Kansas. Konza prairie researchers have found that shrub invasion in upland prairies has much less aggressive than in lowlands. Is this because deep soil moisture is less abundant in uplands, reducing the competitive edge to those shrubs?

In addition to the carbon/nitrogen levels and deep soil moisture that both favor shrubs, anyone who has conducted prescribed fires in prairies containing large clones of dogwood or sumac knows that those shrub patches can inhibit the growth of grasses around their edges, reducing the amount of fuel for fires.  In other words, shrub patches can reduce nearby fire intensity – thus greatly reducing the effectiveness of one of the most important threats to their survival.  You really do have to admire their strategies, don’t you?

I think the hypotheses proposed by Ratajczak, Nippert, and their colleagues could explain a good portion of the puzzle.  Atmospheric conditions have certainly changed over recent decades, and that could explain why trees and shrubs have an easier time getting started in grasslands now.  However, the competition for deep soil moisture shouldn’t be much different now than it was historically.  We know there were at least some shrubs in historic prairies – why didn’t they grow into gigantic unstoppable clones?  What controlled their spread that isn’t doing so now?  Was the historic abundance of browsing animals high enough to control those clones?  Are the fewer browsers today simply overwhelmed by the increased number of new clones that are successfully establishing?  Are there other factors we’re not even considering yet?

There are plenty of questions left to answer, but it’s great that we’re moving in the right direction.  Besides the work of Ratazcjak, Nippert, and their colleagues, there are several other projects I’m aware of that are working to investigate the issue of woody invasion of prairies.  There are certainly plenty of us interested in their results!

Lessons from a Fenceline

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Last week, I spent some time collecting data and thinking about next year’s plans for managing our Platte River Prairies.  While I was out, I made an interesting observation along the boundary of one of our properties and that of the neighbor.

About two years ago, we replaced a section of fence that separated our prairie from the prairie to the west.  The fenceline had become overgrown with shrubs – something that is great for some species of birds but really hard on a barbed-wire fence.  We ended up treating the shrubs with herbicide, pulling out the fence, and shredding the remains of the shrubs before re-building the fence.

This fenceline was cleared of shrubs two years ago so we could re-build the fence. The photo shows the vegetation that came in after the shrubs were removed (left side of the fence). Note the lack of flowering plants in our property to the right.

The above photo shows that fenceline as it looks now.  In place of the shrubs, there is now a stand of thick prairie vegetation, including grasses, sedges, and forbs.  Ok, there’s also some smooth brome and even the beginnings of a stand of Phragmites (the thick-leaved grass in the photo).  But the interesting thing to me is the species that are blooming right now.  If you look at our prairie – which is shown on the right side of the photo – you don’t see any flowers blooming.  That’s not a photography trick.  There really are very few plants blooming right now in that prairie.  Yet in the fence row, Maximilian sunflower and Canada goldenrod are abundant and showy, and there are a few other flower species blooming as well.

So what’s the deal?  Here’s what I think is happening.  Our prairie had a long history of annual summer haying – as far as we can tell.  I don’t think it’s been sprayed with 2,4-D, though I’m not 100% sure, because it has a pretty good abundance of spring and early summer flowers, including pale poppymallow, blue-eyed grass, violets, prairie clovers, Illinois bundleflower, and others.  However, I think many years of annual haying likely eliminated late-blooming flowers from our prairie.  If perennial plants aren’t allowed to bloom and complete their annual life cycle they don’t produce seeds.  Perhaps more importantly, annual defoliation while the plants are still actively growing removes the plants’ ability to produce food for their root systems that were, until they were defoliated, gathering enough resources to support tall vigorous plants.  If wildflowers are like grasses (don’t know if there is research that has tested this) that sudden loss of aboveground vegetation causes an abandonment of large segments of roots – because there is no food coming in to maintain them.  If those plants are defoliated at the same growing stage every year without a break, it’s easy to see how they would eventually die without replacing themselves.

The Maximilian sunflower and Canada goldenrod plants growing in the fenceline are not rare plants, or even plants that are particularly difficult to maintain in most prairies.  But they’re almost non-existent in our prairie.  We’ve begun to overseed some late-season-flowering plants into the prairie, and species such as Maximilian sunflower, stiff sunflower, wild bergamot,  and a few others are slowly establishing – and seem to be surviving well under our patch-burn grazing management.  I hope we can bring them back so that late-summer pollinators (for example) can have something to eat other than what they can find in the fenceline!

A closer view of some of the plant species growing in the unhayed/ungrazed portion of the prairie along the fenceline. The prairie to the left in this photo is our neighbor's prairie - which is annually hayed.

So, if the wildflower species that are missing from our historically-hayed prairie are common in the fenceline where they weren’t being mowed, burned, or grazed, does that mean we should manage (or not manage) our prairie by perennially idling it?  No, I don’t think so.  First, remember that although those flowers are blooming in the fenceline now, they weren’t two years ago because they were covered up by tall vigorous shrubs.  Prairies managed without fire – or something that sets back trees and shrubs – tend to grow quickly to woody vegetation, changing their very nature pretty dramatically.  In addition, though I haven’t looked carefully, I would guess that the fenceline “prairie” is missing many of the early season plants that are so abundant in our prairie because there is nothing that removes each successive year’s thatch along the fence to allow those small plants to get sunshine.  I think one lesson of these plants along the fenceline is that it’s important to provide a periodic rest period for plant species – especially when a site is being managed with the same treatment every year.

One could, I suppose, make the argument that we should manage some portions of the prairie for late-season plants and other portions for early-season plants, and everything will be fine.   Unfortunately, by doing that, we would be reducing the plant diversity – and ecological resilience – of those individual portions by not facilitating the full range of biological diversity and ecological function.  As a result, the entire prairie would be weakened.  This is, by the way, why I disagree with biologists who promote leaving portions of prairies permanently unburned in order to protect insect species that are vulnerable to prescribed fire.  I completely agree that small isolated prairies should not be completely burned (or hayed) in any one year, helping to ensure that refuges are left behind for those species that might be harmed by the management treatment.  Shifting the location of management treatments from year to year should allow species to find appropriate habitat, especially if (as in small isolated prairies) the distance between one management treatment and another is short.  If that’s insufficient, it might be that the real issue is not management, but size and isolation of the prairie – something that can only be solved by strategic prairie restoration of surrounding land parcels.  Reducing the ecological resilience of small prairies by managing them for a small suite of species is certainly not a good long-term solution for those species or the larger community.  (Baby with the bathwater…)

There is one more observation that’s too important not to mention.  The plants that are blooming in these photos somehow survived being covered by dense shrubs for at least a couple decades, and also survived the foliar herbicide application that removed those shrubs.  Plants are tough.  Granted, the wildflower species we’re talking about are species that have a reputation for toughness anyway, but I still think their survival is worthy of note.

I’m not completely sure I’m right about why those flowers are in the fenceline and not my prairie, but even if I’m wrong, just going through the thought process is valuable!  If nothing else, it gives me a hypothesis I can test (and sufficient fodder for an entire blog post).  Isn’t it amazing what we can learn just by looking around us?