When I started working for The Nature Conservancy in 1997, many of our prairies along the Platte River were unplowed but in fairly poor condition. Chronic overgrazing and broadcast herbicide use prior to the Conservancy’s ownership had greatly reduced plant diversity and allowed invasive grasses to gain dominance. It became quickly apparent that simply managing those sites with fire and/or moderate grazing was enough to reduce the dominance of the invasive grasses, but not enough to bring back many missing wildflower species.
Because we were actively harvesting large amounts of local wildflower seed to restore crop land to prairie, we started experimentally broadcasting seed onto our degraded prairies as well. After much trial and error, we came up with a technique that was effective and fit into our broader management strategies. Essentially, we broadcast seed onto recently-burned prairies and then used grazing to reduce competition from grasses. In the early days, progress was slow and hard to see, but over time the grass-dominated grasslands started to gain more and more color.
Seeing more wildflowers as we walked through those prairies made us feel good, of course, but there’s a big difference between feeling good about a technique and knowing it is providing actual benefits. We had data showing long-term persistence of the new plants that emerged after our seeding – in other words, our management was sustaining higher plant diversity once we got it established. However, we also wanted to know whether or not those more colorful prairies were actually providing better habitat for pollinators or wildlife.
Over the last two years, we’ve collected data on flower abundance across both our Platte River Prairies and Niobrara Valley Preserve. We walked belt transects (divided into multiple segments) each month and counted every flowering stem within a meter on either side of us. Those data give us the ability to see how plant diversity and abundance changed through the seasons, something that is obviously important for pollinator populations. The data also let us look at how various management treatments and systems affect flower availability in our prairies.
As an additional step, Mike Arduser (former Missouri Department of Conservation ecologist) looked at our Platte River data and categorized our flower species in terms of how they are used by bees. While that is a messy process and there are various ways to approach it, we have (among other things) a list of the flower species that are used by a moderate to high number of bees. For the sake of this post, I’m calling that a measure of “quality”, but of course there’s a lot more to the value of a flower to pollinators than the number of bees using it.
Regardless, all of that description is just background for the main point of this post, which is: we can use those data to see whether or not our overseeding work is meaningful to bees! Even better, the answer seems to be yes – with a few interesting twists.
The three graphs below show different parts of the same prairie/pasture (also featured in the three above photos), which is being managed with patch-burn grazing. The west patch was burned in 2016, the middle in 2017, and the east in 2018. Following each burn, that area was grazed intensively by cattle for a full growing season. Grazing intensity generally decreases with time after a burn. Thus, in 2018, the west patch (burned in 2016) was grazed least intensively and the east patch (burned in 2018) was grazed hardest.
The orange bars show flowering stem numbers for species that are used by a moderate to high number of bees. The blue bars show only those species that were there before we did our overseeding work. In other words, looking at the difference between the orange and blue bars lets us see how much our overseeding has added to the availability of important flowers for bees.
We confidently say now that our overseeding work has increased the number of available flowers for bees. The increase varies somewhat by season and location, but there is an especially strong and consistent difference in early September. Much of the increased flower abundance during that period comes from one species – Maximilian sunflower (Helianthus maximiliani). That species should be a major component in our prairies but was missing from this one because of past management. In addition to being an important resource for bees, Maximilian sunflower is also listed by the Xerces Society as an important nectar plant for monarchs, which migrate through Nebraska in September. Because of that, it seems likely that our overseeding has increased the value of our degraded prairies to migratory monarchs as well.
Another story that comes out clearly in the above graphs is the lack of bee-friendly flowers during the early part of the growing season. This is not restricted to our degraded prairies, and it’s not unusual for this part of the country. Many of the flower species used most by bees in May and June don’t typically grow in large patches, but rather as widely-scattered individuals or small patches (which means we miss most of them with our narrow belt transects). We’ve done a lot of brainstorming, but haven’t identified many bee-friendly flower species we would focus on if we wanted to really boost blossom numbers during the late spring. The best option we’ve come up with is prairie ragwort (Senecio plattensis), a short-lived plant that can become temporarily abundant under some conditions – including after intensive grazing – but not at all sites. Regardless, these data help highlight that continuing issue and provide a reminder to keep up our discussions.
Collecting monthly data on flower abundance/diversity was a pilot project for us to see whether or not we could gain valuable information from that technique. I’d say the effort was very rewarding, and for much more than just confirmation that our overseeding work was worthwhile. Hopefully, we can look at some different sites in the coming years and continue to learn about how our work is affecting pollinator resources. I would encourage others to try something similar, even if you just use one or two short transects. The data have really helped us evaluate our sites in a new way.
To be clear, the degraded prairies we have overseeded along the Platte River have not regained their full wildflower diversity. We’ve put some species back into the community, but the history of broadcast herbicide use and overgrazing is still evident. We may never get those prairies back to full strength, but we feel good about the progress we’ve made.
…And now we have data to back that up!