Trying to Figure Out What We Did Right

When converting crop land to restored prairie, it’s always hard to predict what you’re going to get.  Numerous examples prove that even when you control as many variables as possible – including soil conditions and the rate, timing, and technique of planting – no two seedings turn out alike.  Sometimes, you can use hindsight to explain what happened (weather conditions, herbicide carryover, etc.) but most of the time it’s clear that we just don’t understand much of what’s happening out there.

I’ve been analyzing some data from one particular restored prairie lately, and trying to puzzle out what’s going on.  In this case, the results are good – which is nice.  It’d be nicer, of course, if I could explain WHY things worked so well and then replicate whatever happened…

The Dahms 2000 prairie restoration has turned into one of the most aesthetically pleasing prairies we manage along the Platte. It has tremendous diversity and abundance of wildflowers. Most importantly, its plant diversity is still increasing fairly rapidly after twelve field seasons.

The prairie in question was seeded with a mixture of about 200 plant species onto 69 acres of disked cropland that had been in corn the previous season.  The seed was planted sporadically between December 1999 and April 2000.  Wetlands were added to the site by excavating down close to groundwater and recreating the kind of swale/ridge topography that is typical of nearby Platte River meadows.  Those wetlands and sandy spoil piles (ridges) were seeded with appropriate seed as well. 

All of the seed was broadcast onto the site – some by fertilizer spreader and some by hand (I was experimenting) and no harrowing or packing of the soil was done.  Unfortunately, this was the last year BEFORE I started keeping good records of the amount of seed from each plant species I included in the mixture, so I only have a list of the species we harvested seed from that year.  What I know is that my seeding rate per acre was about 15 gallons of grass seed (mostly big warm-season natives) that was harvested by combine from nearby prairies, and about 1/2 gallon of hand-harvested forbs, grasses, and sedges.  That’s roughly 12 bulk pounds of grass seed and 1/2 pound of forb (wildflower) seed per acre.  I have no idea what germination rates were that year, but it was a pretty light seeding rate compared to what many others around the country use.  Today, our typical mix is a little lighter on grass and includes about twice the forbs.

To cut to the results, this prairie has turned into our most diverse and showy restoration we’ve ever done.  You’d never know we’d used such a light seeding rate of forbs by looking at the site now – its appearance is dominated by big showy wildflowers.  By every measure I use to look at the plant communities of our restored prairies, it comes out high.  I’ve found 178 plant species in the site so far, which is excellent.  The mean Floristic Quality (combination of species number and “conservatism values”) is high, and still climbing rapidly.  It averages twelve plant species per square meter, which is higher than most other restored or remnant prairies in the area.  (Yes, I know that seems like a very low number to you eastern tallgrass prairie folks, but it’s good for out here.  Don’t rain on my parade, ok?)  Twelve years after it was planted, tall warm-season grass species are still not very dominant.  The species found at the highest frequency is big bluestem, and it was only in about 80% of 1m2  plots stratified across the site last June.  In short, it’s a beautiful prairie.  And I don’t know why.

I know most of you are ITCHING to see the actual data tables and graphs, but because there are a few who aren’t, I’m including them as a PDF file, which you see by clicking here.  The PDF also includes a cumulative list of plant species found in the restored prairie.

Flower species such as black-eyed Susan (foreground) and bee balm (pink flowers in the background) are still dominating the plant community in this photo from 2009 (the 10th growing season of this seeding). The lasting abundance of those species is, I think, tied to the lack of dominance by major grass species.

It’s particularly impressive that this seeding turned out so well, because the odds seemed stacked against it early on.  It was seeded right at the beginning of a 7 year drought.  The first several years were dominated (as usual) by weedy species and a few colonizing native species such as Canada wild rye and common evening primrose, but in this prairie those species remained dominant for several more years than is typical.  Once other plant species started breaking through, there were few legumes present – and we don’t typically have problems establishing legumes in our prairies.  Those legumes are still more scarce than in other nearby sites, but they’re increasing over time.  Finally, in about its eighth season, the site stopped looking like a weed patch and matured into something that most people would recognize as a prairie.

As I’ve discussed in other blog posts, I’m still struggling to define success in our overall prairie restoration efforts, but at the scale of individual seedings, there are a couple things I look for.  First, I want to see a good diversity of plant species, and I want to see that diversity sustain itself over time.  Second, I don’t want to see invasive species increasing at the expense of that overall plant diversity, even as the prairie is exposed to disturbances such as drought, fire, and grazing.  So far, this restored prairie passes those tests with flying colors.  We’re moving toward implementing some measures of invertebrate use as well, but aren’t there yet.  Initial data and observations, however, show higher butterfly abundance and diversity in this site than in other nearby restored prairies – for whatever that’s worth.

The prairie has been managed with some periodic fire and grazing, which should be helping to suppress dominant grasses. However, this site has gotten much less of that kind of management than nearby restored prairies, and those other prairies have stronger populations of major grasses, so management can't explain the whole phenomenon. In this photo, cattle are grazing in the burned portion of this site - within a patch-burn grazing system. The grasses are primarily grazed short, helping to showcase the abundance of the forbs.

So why did this restoration turn out so well?  I really have no idea.  It caught a couple nice rains during its first spring, but the rest of the summer was awfully dry.  The overall seeding rate for forbs was considerably lower than we use now, but I don’t know how much seed we had of individual species.  I wish I understood why it has taken the big grasses so long to fill in, but I don’t.  I think the delayed grass dominance probably plays a role in encouraging the abundance and diversity of wildflowers at the site, but I don’t know how to replicate it.  The soils at the site are a little sandier than some of our other sites, but we’ve worked on sandier soils and had very quick grass establishment, so it seems unlikely that the sand is the key.

Besides its aesthetic appeal, the prairie is also a great seed harvest site because of its wildflower abundance. Nanette Whitten (left) and Mardell Jasnowski (right) are harvesting seeds in this photo.

The vast majority of our prairie restorations turn out pretty well, but this one is extraordinary, and I can’t explain it.  Was it something about our technique?  Something about the weather or soil conditions?  I know I should probably just be happy with the results, but I want to know WHY! 

Success is sure frustrating.

A Measure of Ecological Resilience in a Restored (Reconstructed) Prairie

Back in May of this year, I wrote about ecological resilience in prairies.  In Part 2 of that double post, I gave an example of a 1995 prairie seeding and talked about how it appears to be maintaining its plant community integrity – through wet and dry years, fire, and grazing.  Since that time, I’ve collected and analyzed more data from that same prairie seeding, and wanted to flesh out that earlier story.

With regard to prairie restoration, my objective is to use high-diversity seedings to expand and reconnect fragmented prairies and thus increase the viability of prairie species and communities.  Because of that, I don’t measure success by whether a prairie seeding looks like any particular remnant prairie.  Instead, I’m trying to establish as many native plant species as I can, let them sort themselves into communities that are adapted to today’s conditions, and use management techniques such as fire and grazing to maintain that plant diversity.  I’m assuming that by providing that plant diversity, I’m also providing the habitat needed by the animals in adjacent remnant prairies, and that those animals will move into – and through – prairie seedings, thus increasing animal population size and viability.  I’m beginning to test those assumptions, and will be ramping up that effort during the next several years.

In the meantime, I’ve been tracking the plant communities within our prairie seedings to look at how many plant species establish and maintain themselves.  More importantly, I’m tracking the long-term trajectory of those plant communities using plotwise floristic quality analysis (you can read more about that technique here).  If the prairie seedings are ecologically resilient, one measure of that resilience should be that populations of individual plant species, and overall species diversity, are stable over time – even through stress.   The 1995 seeding, for which I’m presenting data here, is located in our Platte River Prairies, south of Wood River, Nebraska, and it has certainly undergone stress.  Since it was hand-planted in 1995 by the Prairie Plains Resource Institute with approximately 120-150 plant species, it has seen both very wet years and a long severe drought (7 years), and has been managed with patch-burn grazing since 2002.  Over the years, I’ve accumulated a total plant species list of 164 species for the 45 acre seeding, which I’m very pleased with.  However, the real question is whether or not the seeding will be able to maintain its ecological integrity over time.  Below is a series of photos and graphs that tell that story – at least the story up to this point.

This is what the 1995 seeding looked like in its 5th growing season. Species such as prairie clovers, perennial sunflowers, and other "matrix prairie plants" were abundant. Management to this point in time consisted of a couple of prescribed fires.


During the drought years between 1999 and 2006, there were times that the combination of intensive grazing and drought really stressed the plant community. This July photo shows warm-season grasses that have gone dormant, but also shows plant species such as rosinweed and prairie clover that were still green and growing - and largely ungrazed.


Within our patch-burn grazing system, a new portion of the prairie is burned each year, and intensive grazing tracks those burned areas across the site. Once a new patch is burned, the previous burn patch begins to recover from intensive grazing. This photo shows a burned patch the year after it was burned. The combination of drought and grazing made it look like a young prairie seeding again because of the abundance of short-lived weedy plants that were able to take advantage of the weakened grasses.


A burned patch in June, showing grazing impacts focused mainly on grasses, leaving many forbs ungrazed.


Under a light to moderate stocking rate, cattle display their selectivity (choosing to graze grass over forbs) - resulting in a very patchy prairie with short grasses and tall wildflowers.


In unburned patches, very little grazing occurs - providing rest for the plant community. This photo was actually taken this week, in a portion of the prairie seeding we fenced out this year to provide complete rest from grazing. We're beginning to include some periods of complete cattle exclusion into our patch-burn systems to ensure that no plant species is grazed every year. This growing season was very wet, so the rested prairie grew very tall - even though it had been grazed fairly hard in 2010.


This graph shows the mean floristic quality of the prairie seeding between 2002 and 2011. These data are collected from approximately 100 1m plots each year. Floristic quality is calculated within each 1m plot and averaged across the site. Error bars indicate 95% confidence intervals. Mean floristic quality has remained stable during the entire sampling period.


While the mean floristic quality of the prairie has remained stable, the frequency of some individual plant species (% of plots the species occurs in) has varied from year to year. This graph shows frequency (from top to bottom of the legend) of marestail, foxtails, annual brome, black medick, and curly dock - all opportunistic (weedy) species that would be expected to act in just this way.


More variability in the frequency of plant species between years. This graph shows (from top to bottom) stiff goldenrod, Canada goldenrod, heath aster, western ragweed, daisy fleabane, and hoary vervain. Interestingly, the species don't seem to track with each other - indicating that each is driven by its own unique set of factors.


In contrast to the two earlier graphs, these data show that perennial native grass species have relatively consistent frequencies between years - even though they were subjected to periodic years of severe drought/fire/grazing. From top to bottom, this graph shows data for big bluestem, Canada wildrye, indiangrass, little bluestem, prairie cordgrass, and switchgrass.


Similar to the perennial native grasses, many long-lived prairie wildflowers are also maintaining stable frequencies between years. Species that were common in 2002 are still common now, and species that were uncommon remain the same. Though I'm only showing a subset of species in these graphs, I've not seen any plant species disappear from this prairie.


Like others who restore prairies, I’m still experimenting with techniques for both establishing and maintaining diverse prairie plant communities.  However, data like these help me feel more comfortable that I’m being relatively successful to this point – and I see similar patterns in other seedings we’ve done.  I’m also more and more impressed with the toughness of prairies and prairie plants.  I tried to include photos that showed the kinds of variable stresses this prairie has endured during its 17 growing seasons.  Watching this and other prairies survive what they’ve survived helps keep me from worrying so much about whether the coming year will be dry or wet, or whether we’ve got the right number of cattle in the prairie each year. 

As I said earlier, there is still much to learn about how animals (vertebrates and invertebrates) are using our seedings, but that is a separate avenue of exploration.  Building resilient plant communities around and between those remnants is the first step to better prairie viability. 

So far, so good.