Why are there stripes in my prairie restoration?

The photo below was taken in September 2008.  It shows a long stripe passing through a restored prairie at the end of its 7th growing season.  The site was seeded with a drop spreader in November of 2001 (seed was broadcast on top  of soybean stubble shortly after harvest).  There are at least half a dozen of these stripes running through the prairie.  They vary slighly in width from a 1/2 meter to about 2 meters.

One of the stripes in the prairie restoration, delineated by the light colored seed heads of Canada wildrye inside the stripe and the reddish-colored big bluestem in the surrounding prairie. The site was planted in November 2001, and this photograph was taken in September 2008.

I’m confident that the stripes are a result of the way we seeded the site.  The drop spreaders we use (photo below) are named because they simply drop the seed straight out of the bottom of the spreader.  We load them up with seed and pull them behind ATVs, and try to overlap our passes slightly so we don’t create long skinny gaps that don’t get any seed.  In this case, I’m pretty sure we were a little sloppy and that these stripes are a result of not overlapping our passes very well – they line up exactly with the direction we were driving.  Because we were seeding on soybean stubble, it was hard to see the tire tracks from the previous pass, but we followed the rows as we planted so we’d at least go in a straight line.

The spreader we seeded the restoration with was similar to this one except it was a different brand and was slightly wider. There is an agitator inside and holes in the bottom, and the seed simply drops onto the ground as the spreader is pulled along.

As interesting as the visual effect is, what’s really intriguing to me is the exercise of trying to figure out why the stripes are still so obvious after 7 growing seasons (actually 9 growing seasons, because the stripes were still obvious in the fall of 2010).  We seeded this prairie with a light seeding rate (around 4lbs PLS per acre) but by its fourth growing season it was already dominated by perennial native plants.  Even if the strips we missed with the seeding didn’t get prairie seed that first year, they should have been getting seed rain from native plants (like big bluestem, which is all around them) by the 4th season.  Two or three seasons later, you’d expect that plants like big bluestem would be filling in those stripes – but it’s apparently not happening.

The stripes seem to be dominated primarily by Canada wildrye, with a smattering of other species like Canada goldenrod, annual sunflowers, and other annual weedy plants.  I’m assuming the wildrye colonized from plants that established adjacent to the stripes and subsequently dropped seed.  I can understand that.  But why didn’t other species colonize the same way?  Did the wildrye come in first and fill all of the open root spaces below ground, preventing other plants from colonizing?  Surely not.  The presence of annual plants in the stripes shows that there is space available for new colonizers.  In addition, wildrye was dominant in the rest of the prairie during the 3rd and 4th growing seasons but soon gave way to big bluestem and other longer-lived plants.  Why hasn’t that happened in the stripes?

It sure seems like there should be plenty of seed falling into the stripes from plants along the edges, but those seeds either aren’t falling, aren’t germinating, or aren’t surviving seedlinghood.  It’s like the stripes are in a sort of suspended animation – they developed into a 3rd season restored prairie and just stopped.

I’ve seen this kind of striping in some of our other prairies as well.  The most apparent is in a prairie that was seeded by Boy Scouts in 1997.  The Scouts walked back and forth across 1 acre squares we’d flagged out for them and threw seed as they walked.  That particular site apparently had a lot of Canada goldenrod in the soil when we planted prairie there because goldenrod established well across the whole prairie even though we put very little (if any) seed in our mix.  Most of that goldenrod has diminished and has given way to native grasses and other wildflowers, but there are stripes of goldenrod that have persisted – and they line up exactly with what must have been strips that the Scouts missed as they walked back and forth.  The stripes are starting to become less stark now (14 seasons after planting) but are still visible.

I’m very interested to hear from others who have seen this phenomenon.  I’d be even more interested to hear good rational explanations of why it happens.  I’m guessing it could be related to the difficulty people have experienced establishing diverse prairie restorations in old fields that have been idle for a couple of growing seasons.  I’ve always assumed that in the old field situation, a year or two of weed domination of the plant community resulted in such a high density of weed seeds in the soil that prairie plants couldn’t compete during early seedling establishment.  Maybe there’s something similar happening in the restored prairie stripes?  I’m really not sure.  I’m not worried by what’s happening (I like the heterogeneity and there aren’t any nasty weeds in the stripes).  I just don’t understand why it’s happening.

Help would be appreciated!  Thanks.

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About Chris Helzer

Chris Helzer is an ecologist and Eastern Nebraska Program Director for The Nature Conservancy. He supervises the management and restoration of approximately 4,000 acres of land in central and eastern Nebraska - primarily along the central Platte River. Chris is also the author of "The Ecology and Management of Prairies in the Central United States", published by the University of Iowa Press.
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11 Responses to Why are there stripes in my prairie restoration?

  1. Stephen Winter says:

    Yeah, stuff like this is what makes restoration work so much fun – it’s interesting and keeps us thinking. Anyhoo, I lean towards the idea that the dominant perennial grasses reproduce primary by vegetative means and that sexual reproduction (i.e., by seeds germinating and seedlings getting established) is a relatively rare event that requires exceptional circumstances, such as spreading seed on a recently-harvested soybean field where competition with already-existing plants is minimal to non-existent.

    If that’s the case, the next question would be – how long does it take for vegetative reproduction (i.e., rhizome spread) to fill in those stripes?

    • Chris Helzer says:

      Yes, but WHY would the bluestem seed not be able to germinate. Competition? I suppose so. I know the seed is good because we’ve harvested bluestem seed from the same site.

      You’re right – it does make restoration work fun.

      • Bob Den Hartog says:

        Chris – Do you think that grazing could be used to blur the stripes? Since it looks like the stripes are dominiated by cool season grasses, would grazing in the early season make sense not only to set back the cool season grasses but also to provide enough disturbance to allow for alternative seedling establishment?

        • Chris Helzer says:

          Bob – yes, I think that would do exactly as you suggest. In fact, we’ve been doing more grazing there, so we’ll see if it makes a difference. One issue is that the wildrye is not as favored by the cattle as the bluestem so it’s hard to get the impact on wildrye while favoring bluestem. As you say, there’s a little season difference there, but the wildrye grows back well in June if we pull cattle off too soon, and if we leave the cattle in, they tend to favor bluestem and ignore wildrye anyway!

          As I said in the post, though, I’m not really upset by the stripes – just intrigued by them!

          Chris

  2. Steve Clubine says:

    Dead furrows. Are the stipes abut 100 feet apart and parallel? Close you eyes and walk at right angles to them. Your feet will feel a slight depression when you cross them, maybe only a 2-4 inches. You can see them when you burn, hay or intensively graze (e.g. pbg) the area in which one lies.

    Dead furrows were made when you finished plowing our a headland. With the plow throwing the 6-8 inch slabs away, when you finished a headland you had a 6-8 inch trough. During subsequent tillage (disking, harrowing, etc.) you began on the dead furrow and tried to fill it in but it remained until you began the next season’s plowing. Then you found the dead furrow, and went down each side throwing the soil slabs into the furrow. The next furrow would end up about 100 feet away. Dead furrows moved back and forth across a field as long as it was plowed until cultivation ceased and the last dead furrows remained. The topsoil is shallower there so the plant community is different. I’ve found them in pastures that haven’t plowed for maybe 100 years.

    Steve

    • Chris Helzer says:

      That’s an interesting idea. I’ve never seen that in the sandy soils along the Platte (would this be limited to heavier soils that would form “slabs” better than sandy soil?

      Now, I have seen that the old pivot tracks (we filled them prior to seeding) look different than the rest of the site – and that would fit the dead furrow idea pretty well – at least as far as having a different soil profile and influence on the plant community.

      I’ll be sure to check the elevation of the stripes and see what I find!

      – Chris

  3. David says:

    Chris,

    We see the same things here. What I find interesting is how one can still see the drill lines, outlined by burned clumps of warm-season grass, in a planted prairie after a burn. I routinely see this in prairies that I know are 8 or more years old, as well as younger ones. My best guess as to why this happens is that grass seedlings do not survive through the summer due to canopy closure of the plants that first establish. I would suspect that if you mowed a few sections of your strips repeatedly (allow more light during the growing season) the plant composition would change much faster.

    Also interesting is how prairie can invade neighboring cool-season grass sod without any disturbance (fire, herbicide or mowing). I have seen this happen on several sites. But then again I know of sites where this does not happen. I sure wish I knew the secrets as to why sites vary because it would help our interseeding efforts.

    David

  4. I believe this is evidence that new seedling establishment in prairies is episodic and almost never happens without disturbance and elimination of competition. Overseeding of forbs into established grass-dominated prairie sod was only successful when this was mowed repeatedly throughout a growing season. The window for new individuals to be added to a prairie seems to be very narrow.
    In addition to considering the seed rain and seedling mortality as possible factors, remember seed mortality through predation on the soil surface, which may exceed 90%, then further sources of mortality (pathogenic fungi, bacteria) in the soil.

    • Chris Helzer says:

      I tend to agree about the episodic establishment and need for disturbance. That matches what I see in Nebraska. On the other hand, I wonder about overseeding efforts that do work when people just walk around and drop seeds – then come back several years later and find new plants there (without any mowing or other management). Maybe the disturbances can also include smaller-scale disturbances caused by burrowing animals, insects, etc? I don’t know. Thanks for the comment. – Chris

  5. Kole says:

    I bet your stripes largely disappeared this year with the heat and drought as deep rooted big bluestem seedlings pounced on the opportunity?

    • Chris Helzer says:

      Actually, I’m not sure they changed much. It seems to me that the big winners this summer so far have been deep rooted forbs – not so much grasses. I appreciate the reminder about this, though – I’ll have to go take a closer look and see if I can tell what’s going on in there.

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