Maryland Senna - A Savanna Species in North Carolina

Maryland Senna (Senna marilandica) is a plant I don't often think about in the dead of winter, and especially in moist, riparian forests, but I recently walked by a patch laden with pods near the Eno River.

Senna marilandica full of fruit approximately 100' from the Eno River
(January, 2015)

Plants are stout, herbaceous perennials, to 6' or so tall in our area. In late summer they produce some of the brightest blossoms around, both at the top and axils of the main stem.

Maryland Senna, full bloom
(Durham County, 8/19/2014)

An interesting evolutionary aspect of Maryland Senna is the presence of extrafloral nectaries (EFN) near the base of the compound-leaved petioles.  In general, these nectaries are sugar producing glands that offer nectar to ants, who in turn provide protection to the plant from herbivores.

Extrafloral nectaries (EFN)  are the dark tick-shaped objects shown above;
the one to the upper left is being visited by a black ant
(Granville County, NC 8/01/12)

Maryland Senna EFN with ant visitor (Durham County, 08/20/14)  

Brigitte Marazzi and co-authors, writing in the American Journal of Botany (2006), documented that Senna species with EFN have colonized a wider range of habitats and climates than species lacking EFN. They believe the "ant–plant protective mutualism" has a positive effect on plant fitness and may help to explain the greater species richness of the EFN bearing Senna, as well as the greater diversity of habitats they occupy.

Maryland Senna EFN with a different visitor (upper left)
Note the developing seed pod (right)
(Durham County, 08/20/14)  

Other than the odd occurrence of Maryland Senna near the Eno River (powerline cut along with Heliopsis helianthoides), I find it most often in open canopied, uplands associated with diabase soils. I sometimes refer to these as savannas (never having seen the plant in closed canopied forests on the same soils and geology). One of the sites for some of these images has been referred to as a "cedar glade."  In their study of Piedmont Prairie remnants, Davis and colleagues (Castanea 2002) indicate Senna marilandica has a "strong association with Piedmont Prairies" although they did not document it at any of the sampled prairie sites, only a power-line right of way. There are numerous references in other parts of eastern North America to this species being found in prairie-like habitats. I was lucky to see it, or closely related species, in a Bur Oak Savanna in northeastern Indiana, late this November (see image below), which is managed by prescribed burning. 

Maryland Senna in an open, diabase glade, Granville Co, NC
The uppermost stem is full of buds, near a developing compound leaf  

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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: The Partheniums

Dense Quinine patch in Durham County;
plants in the foreground are P. auriculatum, taller
stems in background are P. integrifolium

Wild Quinine (Parthenium integrifolium) and Glade Wild Quinine (Parthenium auriculatum) are frequent and sometimes common members of North Carolina Piedmont diabase woodlands. For unknown reasons, they may co-occur (as in the image to the left) or be found singly at a given site.

Both flower profusely in open, prairie-like sites, especially after burning although individuals may persist vegetatively for long periods under somewhat shaded conditions.

Wild Quinine is the wider ranging of the two, being found nearly throughout the eastern US (where often associated with high quality, prairie or savanna-like habitats). Glade Wild Quinine is the southern cousin ranging roughly from West Virginia to Alabama. The former may be found nearly across NC, while the latter is restricted to a handful of Piedmont counties.


     

Typical P. auriculatum
basal leaf

Typical P. integrifolium
basal leaf

              

                 

Both species have similar appearance with large, coarse, basal leaves.












P. integrifolium is much less hairy, often has a dark reddish mid-vein apparent on the larger, mostly basal leaves. It also tends to be a taller and larger plant when flowering (up to 3' or so).  P. auriculatum has a more petite, compact form, and longer blooming period. The toothier leaf margins help distinguish Glade Wild Quinine, and many plants show a sheen of white pubescence even from a distance while the hairs on Wild Quinine are much less developed and obvious.



The upper image (right) is Parthenum auriculatum, the lower is P. integrifolium; both in full flower (an insect has chewed into one on the lower plant.





           

Wild Quinine patch flowering after late spring
prescribed fire in Durham County
on exposed diabase outcrop

Large expanse of Parthenium integrifolium on a clayey, upland flat with deeper soils derived from diabase.
Site burned early Spring

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