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.