Cabbage Time in Carolina!

Skunk Cabbage (Symplocarpus foetidus) is one of our earliest, native bloomers. In mid-February, I observed dozens, possibly hundreds, of individuals pushing up through leaf litter, muck, and shallow water at a site near Warrenton, NC.

The stout emerging stalks are amazingly thick and leathery to the touch. They unfurl at various rates to form leafy "spathes".  I observed incredible variation in the appearance and coloration of these, ranging from light green to heavily striped, to mottled, to almost solid dark red.




Skunk Cabbage is a member of the Araceae family, which includes mostly tropical species. In our area its closest relatives include Golden Club & Jack-in-the-Pulpit.  Symplocarpus foetidus is the only species of the genus in North America; Western Skunk Cabbage belongs to a different genus. Symplocarpus also includes some disjunct members in eastern Asia. It has been suggested the Asian and North American species of Symplocarpus have been isolated from one another for over 6,000,000 years (1).

Symplocarpus foetidus spathe; few plants observed had
developed this perfect oval shape
Warren County, NC (Feb 2015)

Skunk Cabbage open spathe revealing enclosed spadix,
Note the amount of water inside the spathe

Skunk Cabbage is famous for the ability to produce heat during early season flowering, a process sometimes called "thermogenesis". Indeed, S. foetidus is one of the most precise thermoregulators known, and is apparently able to maintain temperatures within ~ 3.5 ° C range, while ambient temperatures range over 37.5 ° C (2). Exactly what value thermogenesis conveys to the plant, if any, is unclear. Some believe it is an evolutionary hold-over present in a few primitive plant families.

According to Seymour & Blaylock (3), "there is no doubt that warming advances development and permits early flowering, but the adaptive value of this is obscure".

I observed several plants with groups of what appeared to be fruit flies buzzing around. Some authors have suggested the plant's thermogenic qualities provide a more attractive site for early season insects like these. Grimaldi & Jaenike (4) found that Skunk Cabbage is "a major breeding site" for one species of fly that "looks enough like Drosophila so that the casual observer might be confused" (5); I certainly wouldn't have known the difference! Whether these insects pollinate the flowers on the spadix is unclear.

Symplocarpus foetidus "ripe" & flowering spadix

The condition and maturity of the spadix and presence or absence of flowers was quite variable during my visit.  The following series of images shows a bit of this, as does the image above (with the dark brown spadix). However, very few spathes were open enough to easily reveal the contents. Even fewer were flowering, like the examples shown to the left. Several appeared to be just past and others much more so, while at least one spadix (below) seemed to still bedeveloping.

S. foetidus immature spadix

This ripening spadix was
lying in an open spathe,
like an Easter Egg waiting to be gathered

Bright yellow stamens exposed, zooming into these seems to show pollen that has
fallen onto the spadix surface 

North Carolina has the southernmost ranging populations of Skunk Cabbage is eastern North America. The species creeps into a few northern mountain counties in eastern Tennessee and western NC. Further east, there are 3 or 4 reported counties in the NC piedmont.  The site for these images is in Warren County, close to the Fall Line.  Although often considered a piedmont county, it sure felt like I was in the coastal plain.  The habitat was a non-alluvial wetland ranging from moist to mucky soils. Tulip Poplar & Red Maple were probably the dominant trees, but Black Gum and an occasional Pond Pine were present, Cane & White Bay Magnolia were common as well. Most of the plants were found in and around the muckiest and wettest area near the streamhead.

Skunk Cabbage habitat (Warren County); Feb 2015
most plants occurred above the standing water where a faintly developed "Y"
can be seen, just downstream a dense understory of cane develops

Skunk Cabbage habitat, same site as previous (April 2014)

References:
(1) Jun Wen, R.K. Jansen, and K. Kilgore. 1996. Evolution of the Eastern Asian and Eastern North American disjunct genus Symplocarpus (Araceae): Insights from chloroplast DNA restriction site data. Biochemical Systematics & Ecology 24: 735-747.
(2) Seymour, R. S. 2004.  Dynamics and precision of thermoregulatory responses of eastern Skunk Cabbage. Plant, Cell,& Environment. 27: 1014-1022.
(3) R. S. Seymour & A.J. Blaylock. 1999. Switching off the heater: influence of ambient temperature on thermoregulation by eastern skunk cabbage (Symplocarpus foetidus). Journal of Experimental Botany 50: 1525-1532.
(4) D. Grimaldi & J. Jaenike. 1983. The Diptera Breeding on Skunk Cabbage, Symplocarpus foetidus (Araceae). Journal of the New York Entomological Society 91: 83-89.
(5) H. D. Stalker, 1945; On the Biology and Genetics of Scaptomyza graminum Fallen (Diptera: Drosophilidae).

Crater the bay to restore it?

Carolina Bay wetland (1998);
the egg shaped, dark mass near the center of the image

Carolina Bays are enigmatic, isolated wetlands scattered across parts of the Atlantic Coastal Plain; enigmatic due, in part, to their characteristic northwest-southeast orientation, and ovoid shape, not to mention speculation as to their origin.

Whatever their ontogeny, most have been destroyed by ditching, draining, and other land-uses.  Many appear only as faint impressions on aerial images.





One reasonably intact example of a Carolina Bay is the object of our conservation & restoration efforts in Sampson Co, NC (shown on the image above). This bay persisted amidst a maze of logging roads and uplands managed intensively for timber production, unlike an adjacent example which was repeatedly logged and shows few remnants of its original wetland character. I have no explanation why this particular bay was spared. However, remnant vegetation persists, displaying characteristics of "cypress savanna".

Cypress Savanna (Sampson Co, NC)
with open canopy, multi-aged, scattered Pond Cypress (Taxodium ascendens),
Loblolly pines are also evident, inc; browned & dying trees in the background

The dominant trees throughout the wetland would likely have been Pond Cypress (Taxodium ascendens). Like the image above, there would have been a relatively open canopy, with few Loblolly pines (Pinus taeda), above a dense, herbaceous dominated ground layer. However, most of the bay is now closed canopied and dominated by Loblolly which invaded the wetland due to a combination of periodic droughts and very infrequent fire. 

Pond Cypress (left), loblolly pine (right)

Pines grow alongside cypress (see left), sometimes capitalizing on what appear to be raised mounds created by the Cypress. Because Cypress is both slower growing and more shade intolerant than loblolly, the predictable result is that pines will eventually replace the cypress.  They also absorb and transpire more water, drawing down the water table and further increasing their competitive advantage. 




Therefore, a primary restoration issue is removal of the pines. 

Given the sensitivity of the site, conventional logging was not an option, and felling individual trees was deemed too labor intensive. After extensive consideration and experimentation, we decided to bring fire back to the bay and implemented a prescribed burn in the middle of November.  

Loblolly pine overtopping pond cypress

Fire weather forecasting called for northeast winds @ 9 mph, 55 degrees, 44% humidity. Keetch-Byram drought index (KBDI) was high (over 500), and fuel moisture across all size classes was less than 20%. The fire carried well across the entire site, as expected.  Flame lengths stayed generally low although there was significant variation as measured by scorch heights afterwards. Active fire burned out quickly and was easily contained.  Dry fuel conditions allowed fire to smolder pockets of surface organic buildup consisting mainly of intertwined pine roots.  

Post Fire Conditions in Carolina Bay after November fire

Top: small pocket burned several inches deeper than surrounding, 

Middle: Red Maple roots exposed, heavily injured & toppled

Bottom: consistent surface organic reduction, some root exposure 

As a result, a number of trees throughout the bay succumbed. A few, like the Red Maple (Acer rubrum) shown above fell over and died immediately, while most trees either survived or died more slowly. The following chart displays the survival of the three most common trees, Loblolly Pine ("Pinus"), Cypress ("Taxodium"), and Maple ("Acer") across 5 diameter size ranges.  For a given species the colored lines within each size class show the number of stems before fire ("Pre") and the number surviving after ("Post"). 

These data show relatively heavy stem losses of Pines (blue lines) across all size classes, while mortality of Cypress (red lines) was concentrated in the two smaller size classes. Taken together, this shows an overall shift away from the strongly pine-dominated vegetation of the Carolina Bay before fire to an equal mixture of pine & cypress after fire.  The lone fire was not sufficient to restore cypress savanna but it produced significant progress.

Growing season conditions following November fire;
Pond Cypress tree stands amidst several dead loblolly pines,
and above a living red maple