Signs of Spring 7: Skunk Cabbage on the Nature Trail

skunk_cabbage_cropped2.jpgLast Wednesday I went for a noon-time walk down to the Campus Nature Trail. I had two “signs of spring” goals for this walk: I wanted to check for arriving flocks of robins (they often cluster on the wet soil of the soccer field next to the Nature Trail to feast on earthworms), and I wanted to see if the skunk cabbage flowers had come up down in the small wetland alongside the stream.  (Image: D. Sillman)

There were no robins on the soccer field. Maybe they are lingering in their more sheltered and fruit-filled habitats down in Pittsburgh and the close-in suburbs. So, I walked past the quiet soccer field and stepped into the dense woods of the Nature Trail.

The week before the trail had been buried under several inches of snow, but our recent (but short-lived) warming spell had done a good job melting it. Even well shaded spots under the black pines were snow-free. There was an even, brown, ground litter cover all through the woods that was broken only by scattered green fronds of evergreen wood fern and Christmas fern. I made my way down the Oak Trail and Spice Bush Trail all the way to the bottom of the ravine. The stream was low but flowing steadily. I splashed across it (water-proof boots!) and walked downstream in the soft, mucky soil of the far bank for about 30 yards. There I spotted the red and white striped metal stakes that we had put out several years ago to mark the skunk cabbage plot. Looking very carefully, I saw seven, striped, reddish-purple skunk cabbage spathes (the cone of tough, modified leaves that encase the flower structures of the spadix) standing up about two inches above the surface of the cold, wet soil. 

Skunk cabbage is able to push up through frozen soil and snow and maintain its tissue integrity even in very cold air temperatures because of its ability to metabolically generate heat. The spadix is the center of this remarkable catabolic event. Enzymes in the spadix tissues use atmospheric oxygen to rapidly breakdown stored root polysaccharides with the subsequent generation of heat. During the two weeks or so from spathe emergence to spadix flowering, a skunk cabbage will use as much oxygen as a comparably sized mammal in order to keep this heat generation system running. Further, the spadix is able to control the activity of these enzymes and can keep its intra-spathe temperature optimally constant for flower maturation and pollination. These internal temperatures may be up to 20 degrees C warmer than the surrounding environment! The skunk cabbage is a “warm-blooded” plant!

The warm micro-environment inside the spathe along with the foul odors produced by the spadix flower (odors reminiscent of rotting meat or decaying carcasses….the obvious source of the plant’s common name and also its scientific species designation (“foetidus”)!) attract early spring flies and hymenopterans. These flying insects crawl into the opening of the spathe and move across the flower parts of the spadix. The flower parts develop sequentially with the lower female (ovule producing) parts maturing first and the upper male (pollen producing) parts maturing later after the ovules have been pollinated. This sequencing prevents self-pollination of a given individual and helps to sustain the genetic diversity of the population.

The flower forms a rounded fruit head after pollination, and then the surrounding spathe withers away exposing the 2 inch or so diameter ball which is often the same reddish to brownish purple color of the spathe.  This is also the time when the large green leaves of the plant unfurl around the central fruit head. In late summer the fruit heads crumble and deposit their berry-like fruits on the ground around the fading leaves of the plant. These berries may decompose in the wet soil or be eaten (by a number of species including wood ducks, ring-necked pheasants, ruffed grouse, and bobwhite quail). Only a very small number will germinate to form new plants. The rhizomes and roots of an established skunk cabbage plant can persist for many years, possibly even a century or more, so new plants are are not annually needed in abundance.

The leaves and stems of the skunk cabbage are quite fragile and lack substantial woody, supportive tissues. These leaves begin to decompose on the standing plant through the summer and are so fragile that they make very little persisting leaf litter around the central rhizome. The leaves when crushed emit the same foul odor as the flowers. These leaves are also rich in calcium oxalate crystals which are irritating and may even be toxic if ingested. Both the odors and the presence of the calcium oxalate crystals greatly inhibit any grazing on the skunk cabbage leaves. Some animals, though, (like black bears, Canadian geese, and ring-necked pheasants) eat these leaves especially when they are young and just unfurling in the spring..

I put my hand on one of the spathes and imagined that I could feel the heat of the plant. I looked up the steep sides of the ravine and upstream and downstream, along the run of the creek. Everything is still hanging on in winter. There are no bird songs, no buzzing insects, no green except the scattered ferns. The striped spathes of the skunk cabbage have this place and the season all to themselves.

If you would like to read more about skunk cabbage, check out its species page on our Virtual Nature Trail.

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