Prairie Flora in North Carolina - All Hands on Dock (Silphium terebinthinaceum)

Prairie Dock (Silphium terebinthinaceum), a.k.a Prairie Rosinweed, is among the most conspicuous herbaceous perennials of tallgrass prairies of the midwestern United States (1).  The same is true for certain remnant, prairie-like habitats in parts of the NC Piedmont as well. Part of its conspicuousness, no doubt, comes from the distinctive & large basal leaves that may reach 12" in length (see image below).

Flowering stalks bolt upwards in late July & early August, topped by ball-like buds. Stalks may reach 6-10 feet in height before opening into bright yellow flowers around mid-August.

Silphium terebinthinaceum basal leaves in Durham Co, NC

The epithet, "terebinthinaceum" translates to "with turpentine" a reference to the presence of rosins which give many species in the genus their common name. I assume the rosin content would be toxic or inhibitory to grazing animals, but rumor has it that the "rosin" was once chewed by pioneers and that the plant is actually favored by some grazing animals. Curtis (2) apparently observed a "virgin prairie" when it was first put to use as a livestock pasture. He indicated that the horses and cattle sought out prairie docks and compass plants "like hidden candy at a child's birthday party."  There are suggestions that bison also feed upon Prairie Dock, but I have found no definitive references to this, possibly because their ranges no longer overlap.

Prairie dock develops a significant taproot (see image at Reference # 3), unlike Silphium astericus with which it commonly co-occurs on many Piedmont, North Carolina sites. Dhillion & Friese (4) found that Silphium terebinthinaceum had high levels of mychorrhizal activity (a symbiotic association between the roots and specialized fungi that often confer advantages to the plant in the form of drought resistance, nutrient uptake, etc.).

Very little seems to be known about seed dispersal or pollination, but there are strong suggestions that both are extremely distance limited. Coupled with habitat fragmentation, the result is extreme isolation between remaining populations.

A review of the S. terebinthinaceum range map (see also #5) indicates that North Carolina populations are the easternmost in the nation, significantly disjunct from the heart of the main population centers in Illinois and Missouri (2). What is it doing here in the Piedmont of North Carolina?
 
                         

The NC Natural Heritage Program records Prairie Dock occurrences in Cabarrus, Davie, Durham, Granville, Mecklenburg, Stanly, Union, & Wake counties. Populations in Durham & Granville occur on rich clayey soils apparently weathered from diabase rocks, often with other species with prairie affinities. Until such sites are managed to maintain, or recreate, open conditions Prairie Dock appears to decline and persist only on road edges and powerlines.

Prairie Dock rosettes persisting along roadway in Durham (2006);
lack of mowing allowed dense saplings to encroach & suppress
 most individuals present

Same site as previous; mowed during growing season (May 2007)

Same site as previous; note numerous Prairie Dock rosettes (Sept 2007)

For long-term habitat improvement & maintenance prescribed fire is the most desirable technique and we have applied it with spectacular results at several Piedmont sites.  In most cases, our spring fires move relatively rapidly, exposing individual plants to direct heating for a short amount of time.  Hahn & Orock (6) found that Prairie Dock seeds are able to maintain viability even after 15 minutes of exposure to 150 degree C!

I concur with Schramm (6) "one would think that by now, fire would be universally accepted and vigorously applied in all restoration and management efforts, but fire is being used to conservatively." In addition, he says, "most remnants ... currently need regular burning to regain their original quality", a statement I also agree with in the Piedmont where prairie associated flora remains.

A grove of Prairie Dock flowering (Aug 2014) at site in Durham Co, NC
(same site as shown in preceding sequence)

REFERENCES:
(1) Gleason H.A. and Cronquist A. 1991. Manual of Vascular Plants of Northeastern United States and
Adjacent Canada. 2nd ed. The New York Botanical Garden, The Bronx, New York.
(2) Curtis, J.T. 1959. The Vegetation of Wisconsin. UW Press.
(3) Robertson, K. R. 2007 image.  see - http://www.inhs.uiuc.edu/~kenr/prairiephotos/silptere.root1.jpg
(4) Dhillion & Friese; http://images.library.wisc.edu/EcoNatRes/EFacs/NAPC/NAPC13/reference/econatres.napc13.sdhillion.pdf)
(5) http://plants.usda.gov/core/profile?symbol=SITE
(6) Hahn P.G. & Orock, J.L. 2014. Effects of Temperature on Seed Viabaility of Six Ozark Glade Herb Species and Eastern Red Cedar (Juniperus virginiana). American MIdland Naturalist 171: 147-152
(7) Schramm, no date; http://images.library.wisc.edu/EcoNatRes/EFacs/NAPC/NAPC12/reference/econatres.napc12.pschramm2.pdf

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Prairie Flora in North Carolina: Eastern Gamagrass

Tall culms of Eastern Gamagrass in a "Piedmont Prairie" in central NC
with Daniel Overcash & Louis Suther

Eastern Gamagrass (Tripsacum dactyloides) is one of North Carolina's tallest and most robust native grasses.  A clump forming, warm season, perennial, with flowering culms that can reach 7-8' in height, the species responds well to fire and thrives in open sunny, yet somewhat moist sites.

Flowering Tripsacum dactyloides with pistillate (below)
and staminate flowers (above)

Tripsacum is monoceious, producing both male (staminate) and female (pistillate) flowers on an individual inflorescence.

Tripsacum is perhaps one of the most ecologically and agriculturally important native genera from North America. Widespread throughout the eastern US, Mexico, & parts of S. America, Tripsacum was a key ancestor of modern day corn (1) and could eventually contribute to agricultural sustainability efforts focused on perennial rather than annual crops.  At least one cultivar has been developed between Gamagrass and Zea mays that may be a step in that direction (2)

Tripsacum patch expanding in old field at Manassas Battlefield, VA

It produces significant live, leafy biomass with high live - dead ratio and is highly palatable for many grazers. Because of this excellent forage quality, Gamagrass is a classic "decreaser" in range management terminology; when present, grazers actively and preferentially seek it out and cause it to decline under sustained pressure.

Tripsacum dactyloides cupule with persistent pistillate flowers

In mid-summer, large seeds develop and ripen progressively from the top, eventually disarticulating when ripe (see image, right). Due to their size, they have little chance of dispersing far from the mother plant. Under intense grazing pressure or declines due to other factors, it is easy to understand why Gamagrass would be slow to recolonize. Heavy seed predation by rodents may also be a limiting factor in spread (3).

Individual seeds are enclosed in a hardened case sometimes called a "cupule". When intact, this structure can inhibit germination of the enclosed seeds (3)

Tripsacum dactyloides cupules on mineral soil 

Large clumps of Gama tend to develop a characteristic "hollow" in the middle of an almost circular ring of leaves.  As the clump develops upright stems, a tent of sorts forms over the nearly bare middle, providing nesting habitat for a number of ground nesting birds associated with prairies (4). Many such birds are among the most declining in the Piedmont region.

Tripsacum clump developing concentric ring,
 beneficial to some ground nesting wildlife

Herb Amyx, Joan Schnier, John Thomas, Jon Stucky
observing a riverside patch of Tripsacum

As suggested by its relatively wide natural range, Gamagrass is adaptable to a host of conditions. In part, this is due to extensive aerenchyma  root tissue (essentially air passages) that allow it to thrive in wet or even infrequently flooded situations (see image, left).

In addition, the roots are able to penetrate restrictive soils (including claypans) and survive droughts and other limitations that other species can not (5).






Consequently, Eastern Gamagrass can be an important member of a number of natural community types. These include certain blackland prairies in Texas (5), and coastal prairies in Louisiana and Texas (6). I have also observed Gama as locally abundant or dominant in longleaf pine woodlands in eastern Texas and other remnant, prairie-like communities in southeast Texas. In North Carolina, Tripsacum is an important member of what may be North Carolina's only remaining prairie, and is becoming locally abundant in several frequently burned areas elsewhere in the Piedmont providing clues to other habitats it may have occurred in.

Gamagrass clumps in an open woodland in which prescribed fire has been used twice
Durham Co, NC

Although Tripsacum dactyloides appears to be indicative of prairie-like habitats across North Carolina, and perhaps through its natural range, I hesitate to suggest "introducing" it to natural areas without first applying regular prescribed fire (and having the means to continue to do so), obtaining open habitat structure, removing invasive plants, and having a pretty good indication it may have been present historically.  Having said that, not every "restoration" of such habitats needs Gamagrass or likely ever had it. I prefer allowing even like sites to have their own unique floristic assemblages.

References:
(1) Eubanks, M. 2001.  The origin of maize: evidence of Tripsacum ancestory. Plant Breeders Reviews
(2) http://www.google.com/patents/US20140123357
(3) Anderson, R.C.  1985. Aspects of the germination ecology and biomass production of Eastern Gamagrass (Tripsacum dactyloides). Botanical Gazette 146.
(4) http://www.houstonaudubon.org/default.aspx?act=newsletter.aspx&category=Natives &newsletterid=1730)
(5) see http://link.springer.com/article/10.1023/A:1004256100631#page-1
(6) http://images.library.wisc.edu/EcoNatRes/EFacs/NAPC/NAPC04/reference/econatres.napc04.ocollins
(7) Diamond, D. D. & F. E. Smeins. 1984. Remnant Grassland Vegetation and Ecological Affinities of the Upper Coastal Prairie of Texas. Southwestern Naturalist 29.

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