It’s awfully frustrating when I fail to solve a puzzle – especially when all the information I need is right in front of me. As an ecologist, I’m supposed to be good at this sort of thing. Ecologists, after all, study the interactions between plants, animals, and their environments. Why it’s taken me so long to figure out why annual sunflowers are so abundant in some places/years and not in others is beyond me.
But I think I’ve got it now.
Annual sunflower, aka garden sunflower (Helianthus annuus) is a very large plant with conspicuous blooms. While they’re considered to be weeds by most farmers, they are native wildflowers and important food sources for insect and wildlife species.
A few years ago, I was looking through some data I’d collected on the responses of individual plant species to our patch-burn grazing work. Over several years of studying several of our prairies, I was seeing a very clear difference between a patch of prairie that had just been burned, and was being intensively grazed, and the same patch in the following year – after the intensive grazing had shifted to a new burned patch. At the 1 square meter scale, I was seeing 20-30% more plant species in the year following the burn/graze period. This made sense to me because I knew that intensive grazing suppressed the vigor of dominant grasses, and opened up space for other plants to encroach upon those grasses’ territories while those grasses were recovering. Now I was looking more closely at the data to see which species were most responsible for that increase in plant species density following the fire/grazing events.
Many (but not all) of the plants moving into the space left open by weakened grasses were short-lived plants – not a surprise, since those plant species are strongly represented in the seed bank and well-suited to respond quickly to disturbances. However, one short-lived species that didn’t increase in the year following fire/grazing was the annual sunflower. In fact, the number of sunflowers actually went down (a lot) in the year after burning/grazing. This was confusing, because in our prairie restoration (reconstruction) work, annual sunflowers are very quick to establish in our first-year plantings. They excel at germinating and growing in bare soil conditions. Why wouldn’t they also take advantage of the open space left after intensive grazing?
Last year, annual sunflowers (the big-leaved plants) were abundant across the burned/grazed portion of this 2002 prairie restoration. In the several-acre-sized exclosure that was also burned, the sunflowers were quite a bit less abundant – and appeared to be small in size.
In struggling to figure out why the sunflowers weren’t responding to grazing the way I had expected, I made a classic blunder. I was paying so much attention to the pattern I’d expected to see, I didn’t step back and ask myself what other factors the sunflowers might be responding to. It wasn’t until last week that I finally figured out what was going on, and boy do I feel dumb.
I was out in the field last week, doing my annual plant community monitoring (listing the plant species found inside 1m sampling plots). I started in a portion of a restored prairie we’d burned/grazed the previous year. As I was collecting data, I was reminded how abundant annual sunflowers had been in that same portion of prairie the year before – not difficult, since I was stumbling through last year’s stems as I walked between plots. However, though I was seeing lots of stems from the previous year, I wasn’t seeing a single annual sunflower growing there this year. Odd…
About an hour later, I reached the portion of the prairie we burned THIS spring, and BINGO – I started finding annual sunflowers. Oh look, a pattern!
To be sure I was seeing what I thought I was seeing, I went back to my data from previous years, and sure enough – annual sunflowers were always much most abundant in areas of prairie burned in the spring of the year I was collecting data. It wasn’t the grazing that controlled sunflower establishment – it was the burning.
…Which makes perfect sense since annual sunflowers like bare ground…
Splitting the sampling plots by the burned and unburned portion of this prairie shows a very strong pattern of occurrence in the recently burned portions.
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Data from a different year (2008) and a different site show the same pattern.
The annual sunflower is only one of hundreds of plant species in our prairies. I’ll never be able to understand what drives every one of those other plant species, but at least I’m making progress on the sunflower.
I just wish it hadn’t taken me so long…
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The Prairie Ecologist 
That’s a neat association. I’ve often wondered what drives their abundance in cultivated fields, and whether it is more important that the soil be disturbed in the fall to plow in tillage or in the spring during planting here in Texas.
The only place I abundantly see annual sunflower in the Chicago region is when driving through or riding the train through the dune areas on the South side of Lake Michigan. Most of these areas are not burned. Therefore, I think the droughty conditions created by the well drained sand is what allows the annual sunflowers to proliferate. The lack of rain during a critical part of the growing season would likely result in an abundance of Annual Sunflower the following year.
Speaking of the quadrant technique and plant succession in general, this would be a good time to remember Nebraskan Frederic Clements, especially his 1905 “Research Methods in Ecology” and his 1916 “Plant Succession” works. I have nominated Frederic and his wife, ecologist Edith Schwartz Clements, for admission to the Nebraska Hall of Fame and would welcome comments to joberg@aol.com. See http://www.nebraskahistory.org/admin/hall_of_fame/nominees.htm. Public hearings are in September.
Is it possible that the sunflowers are autopathic? Where the stems remain the chemicals released by decomposition could inhibit germination, whereas when the stems are burned there is no decomposition so no autotoxicity.
I think that’s an excellent hypothesis.