Category Archives: Prairie Natural History

Primrose Explosion

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One of the more intriguing parts of land management is watching for (and trying to interpret) temporary explosions of abundance within various species.  Sometimes it’s pretty clear that the rapid increase in population had nothing to do with management decisions – e.g., the huge numbers of orange sulphur butterflies we saw last year – but other times it’s possible to make at least some tenous links to management.  For example, I’ve collected enough plant data to know that daisy fleabane (Erigeron strigosus) often flowers in abundance in the year following a burn in our patch-burn grazing system.  It appears to germinate during the year of the burn (and intensive grazing) and then flowers during the next season when another the cattle have shifted their grazing to the next burn patch.  Interestingly, though that fire/grazing disturbance certainly opens the door for germination of many plant species, only a few species actually track tightly with that burn/graze pattern.  Most others seem more strongly tied to weather conditions, though management appears important as well.  In other words, it’s hard to know which plant species will respond best when we do a particular management treatment.  (And that’s why it’s fun!)

In 2009, we burned two different patches of the same native sand prairie in the same year – one in the early spring and one in late July.  Cattle were on the site during the whole season and shifted their focus to each new burn patch as it greened up after its burn.  During the following year, it was interesting to see which plant species showed up most strongly in each of the two burn patches as the grazing shifted to yet another patch.  Deer vetch (an annual legume – Lotus unifoliolatus) was one that was particularly abundant in the summer burn patch in 2010 – though it also had a pretty good year elsewhere as well.  Anyway, I figured we’d seen the end of the big showy plant responses to that summer fire by the end of 2010, but now I think I was wrong.

Showy Evening Primrose (aka Four-Point Primrose or Oenothera rhombipetala) in sand prairie - Platte River Prairies, Nebraska.

 This week, that old summer burn area is just loaded with showy evening primrose (Oenothera rhombipetala), a biennial native wildflower that forms a rosette during its first season and blooms during its second.  I can’t be completely sure that it’s abundant because of the summer fire and grazing, but when I stood on a hill overlooking the area, the edges of the primrose explosion tracked right along the edges of the summer fire unit.  I’ve been trying to find out more about when this primrose germinates (fall or spring) to see if that makes sense with what I’m seeing, but haven’t found anything specific enough.  If it germinates in the spring, that would make sense, since the surrounding vegetation would have been weak in the spring of 2010 (because of the 2009 summer fire and subsequent grazing) and would have favored germination of plants like the primrose. 

Showy evening primrose is an easy species to notice, and it’s hard to miss it when it has these kinds of temporary population explosions.  Other plant species are less obvious, and I have to rely on my annual plant data collection to see patterns, but the primrose doesn’t hide its activity!  Makes me wonder how many insects and other species are responding strongly to our management but going unnoticed…

Sensitive Briar Explosion

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It’s been a cool and wet year spring and summer here.  Seed harvest is about 2 weeks behind schedule because plants are flowering later than in recent years, and we’re seeing unusual abundances (high and low) of some plant species.  I assume most of what we’re seeing is related to the weather, though it’s always hard to tell for sure.

One species I’ve been noticing over the last week or two is sensitive briar (aka cat’s claw or Mimosa quadrivalvus).  It actually doesn’t seem to be blooming too far off schedule, but what I’ve noticed is that it seems to be popping up everywhere!  In our Platte River Prairies, sensitive briar is a species that is found in scattered patches.  A small prairie might have one or two patches of it, usually in fairly specific habitat niches – on the slopes of alluvial sand ridges in our lowland tallgrass prairies, or on tops of hills in our upland sand prairie.  It also does well on some sandy roadsides.  It’s never widespread, but often forms pretty large patches (living room size or so) where it does grow.  On some of the steeper loess hills east of us I’ve seen much larger populations on steep hillsides.

The "koosh ball" flower of sensitive briar is pretty hard to miss - especially because it usually appears with many others on plants that can cover areas of 6 feet in diameter or more. It's a distinctive-looking plant. Besides the showy flowers, sensitive briar has abundant spines on its stems and compound leaves that fold up when touched.

I’m not really noticing more populations in our remnant prairies this year, though the existing populations are certainly full of flowers.  Our restored (reconstructed) prairies are where I’m really seeing a lot more plants.  I’ve seen sensitive briar for the first time ever in a couple of sites, and have seen many more plants than ever before in others.  These are obviously not first-year plants, because they’re large sprawling individuals with lots of flowers, so they’ve clearly been around for a while – unnoticed by me.  (To be fair to me, most of the prairies where I’m now seeing sensitive briar also have large populations of Illinois bundleflower, which looks very similar to sensitive briar when neither species is blooming.) 

Because the restored prairies are all of different ages, I’m pretty sure that I’m not seeing plants that are simply getting to the age where they begin flowering – I think something happened that made a lot of them flower for the first time (or made more of them flower simultaneously).  The question is whether this year’s weather is responsible, or whether something that happened last year or previously that set this up ahead of time!  Don’t you love prairie mysteries?

One thing is for sure, my kids will be happy to see the additional sensitive briar plants.  Whenever we’re hiking past a sensitive briar plant, we always have to stop and pet it so the kids can watch the leaves fold up.  It is a neat trick, but I’m beginning to wish I’d never shown it to them…  Besides having to stop at each sensitive briar plant, we also have to stop and check any plant that has any resemblance to sensitive briar (bundleflower, leadplant, Canada milkvetch, groundplum, etc.) to make sure its leaves don’t fold up too!

As an interesting aside, the spines on sensitive briar seem like they’d be well-designed to prevent aggressive grazing by large herbivores.  They’re aligned along the long sprawling branches so that they point back toward the center of the plant.  This means that if an animal picked up the stem and tried to strip the leaves by pulling away from the center of the plant, they’d be pulling right toward the teeth of those spines.  Under light stocking rates, sensitive briar is rarely grazed by cattle, but under heavier stocking rates they will eat it.  I’ve only gotten to watch the process once, and sure enough, the cow I watched picked up the stem and simply stripped all the leaves right off – against the spines – and didn’t seem to suffer any ill effects at all.  Those are tough lips!