Primrose Explosion

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…