Friday, March 25, 2011

Big Blue: A Grass for the Times?

Last fall I had an email from a botanist friend: What’s going on with big bluestem? After he pointed it out, I noticed that, sure enough, this species, one of the dominant grasses in the tallgrass prairie (as in the source of the original Kansas sods that once housed the homesteaders), was bustin’ out all over the foothills!
Above: Fall color in "turkeyfoot," Big Bluestem grass (Andropogon gerardii), looking across Section 16 and Boulder's Jewell Mountain Open Space. (You can even see the turkey feet in this photo!)

Normally, Big Bluestem does occur on the slopes of our mountain front in sporadic patches, but has been expanding in recent years. In 2010, this warm-season grass had an extraordinary year. Its abundance first drew attention at Lookout Mountain, on the slopes of Windy Saddle Park and adjacent areas, where patches of last fall's russet color could still be spotted in January, even from a distance, as long as the ground was free of snow. Big bluestem and its cousins in the grass family provide great winter color in the landscape.
[This post is a revised version of an article that appeared in the March edition of the Plan Jeffco newsletter, and is therefore oriented toward local open space parks.]

Tallgrass prairie, whether here or in the East, is a tattered remnant of what it was in its glory days. We’re lucky to have sizable patches in Jefferson County; efforts to protect some have been underway for more than 20 years. It’s been years since we talked tallgrass prairie here at FF, but we have continued to monitor developments in the Rocky Flats area, where our best examples of this rare ecosystem survive, including the Ranson/ Edwards property and Jewell Mountain, where Boulder conveniently protected several hundred acres. Patches of big bluestem on the foothills slopes are fine, but limited, reminders of the more extensive remnant prairie that once rolled out across the county but now have retreated to a fringe along the mountain front. Most of what you see west of Highway 93 and north of Hwy 72 is tallgrass prairie.

Big Bluestem (grass)
Andropogon gerardii
  • Warm-season sod-forming grass.
  • Height: 4-5 feet in Colorado; to 8 ft in prairie states.
  • Range: All but the five westernmost of the lower 48 states, as well as the central provinces of Canada.
  • Dominant and characteristic grass of the tallgrass prairie ecosystem.
  • Flowering heads are 4-5 inches long, with 3 (sometimes more) spreading branches, suggesting its other common name: turkeyfoot
  • Links: At USDA Plants database; at Illinois Wildflowers.

Don't Confuse Big Bluestem with:
Smooth Brome (Bromus inermis; Bromopsis inermis) is a ubiquitous pasture grass that was introduced in the late 1800s for livestock forage and erosion control. A sod-former, it tends to grow in extensive monocultures, unlike the patch effect of bluestem, although it is a similarly robust grass that turns color in the fall (more brownish than reddish, however). Smooth brome has leaves on the stem, not just at the base, and lacks the characteristic "turkey foot" seedhead. It is invasive in prairies, and some states consider it a weed. According to the USDA, Smooth Brome now occurs in every state except Hawaii, Alabama, and Florida.

Big Bluestem in Jeffco’s Landscape
Look for these foothills patches primarily on unforested south- and east-facing slopes: on Mt. Morrison, most central-county slopes visible west of Highway 93, and on the north slopes of our canyons, from Bear Creek to Golden Gate. In Mt. Galbraith Park, you can see the colorful auburn patches across the canyon opposite as you make your way up the trail. Driving west on I-70, look for it to the north as you enter the foothills, on the slopes in Matthews-Winters Park.

In the southwest part of Red Rocks Park, bluestem occurs on more prairie-like level sites, because that area has been long protected and contains isolated spots where the Rocky Flats alluvium persists on lands that would otherwise have been cultivated or developed. In fall, Big Bluestem blends perfectly with the outcrops of the Fountain Formation that define Red Rocks.

Lack of development, and a somewhat inhospitable climate, have also protected the Rocky Flats area. The extremely cobbly soils, beloved of gravel companies, hampered agricultural attempts on much of the area, and enhanced the available moisture content for the growth of these taller species. These soils are considered among the oldest in Colorado, in part because the area is unplowed, with some estimates placing their age at 2 million years. But Big Bluestem’s visibility in the landscape of any given year reflects how we’re doing on overall precipitation and temperature.

The Water Year and Other Influences
Based on the 30-yr average precipitation, our wettest month of the year should be May. When it is, that’s about perfect for big bluestem and other warm-season grasses (including blue grama, sideoats grama, little bluestem, and switchgrass), which are just getting started as the weather warms toward summer. By June, the cool-season grasses (such as the common lawn grasses, including Kentucky bluegrass, which take advantage of early season moisture to start growth) will be ready to flower and set seed, but you’ll still have to hunt around on the ground to find evidence that big bluestem is even alive. Large clumps of hairy, velvety blue-green leaves will be your hint.

In July, those spreading clumps send up tall flowering shoots, which produce the “turkey-foot” seedheads by August. When cool-season grasses are giving it up and their seed has scattered, Big Bluestem ripens into the terra cotta masses that reveal its presence even from a distance.

This seasonal habit represents an entirely different metabolism (called “C4”) in these warm-season plants, which use carbon along different pathways than many familiar plants. According to some researchers, this may give them an advantage under conditions of elevated carbon dioxide (think global climate change), especially during drought.

Interestingly, although it seemed, until February snows, that it’s been droughty for several months, 2010 was a good moisture year during its first half, giving Big Bluestem the start it needed. In fact, rainfall in this part of the county has been at or above normal every month from April 2009 through last June. (Later records have not yet been posted online; I’d guess July stayed high as well.) Average temperatures have also hovered a degree or two above normal since late in 2008. Both conditions have supported growth of Big Bluestem, a species that prefers it a little more moist and warm than is the rule in Colorado. If we go too dry and warm, we’re apt to see more of its cousin, little bluestem, but as long as it finds a moist spot, Big Bluestem will remain a part of the county’s natural prospect.

Further Reading
Many more references—bison grazing effects, mycorhizal associations, fire, etc.—can be found via Google Scholar; here are a few to get you started. If you've been following our Indian Gulch fire, some of the articles on fire may be of interest; parts of Mt. Galbraith Park, mentioned above, burned. Note that most articles are in Jstor, and only abstracts are available to those of us without university connections.

Biomass production and species composition change in a tallgrass prairie ecosystem after long-term exposure to elevated atmospheric CO2
Clenton E. Owensby, Jay. M. Ham, Alan. K. Knapp, Lisa. M. Auen, 2001, Wiley Online Library.

Photosynthetic and water relations responses to elevated CO 2 in the C 4 grass Andropogon gerardii
AK Knapp, EP Hamerlynck, CE Owensby, International Journal of Plant Sciences, V 154,#4, pp 459-466, 1993.
In absence of water stress, such systems don’t respond to elevated C, but in water-limited envts, levels of leaf phi, leaf-level ps rates, and stomatal conductance are all likely to be affected by elevated atmos CO2, with the net result being inc in biomass productn.

Growth and Gas Exchange of Andropogon gerardii as Influenced by Burning
Tony J. Svejcar and James A. Browning, Journal of Range Management, Vol. 41, No. 3 (May, 1988), pp. 239-244

Long-Term Effects of Annual Burning at Different Dates in Ungrazed Kansas Tallgrass Prairie
Gene Towne and Clenton Owensby, Journal of Range Management, Vol. 37, No. 5 (Sep., 1984), pp. 392-397

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