Are you interested in how people use plants for food, fuel, medicine, or other purposes? The study of plant use and associated human history and culture is called Ethnobotany, and it’s a topic of interest to me and my students. A previous 99wetlands post featured a few wetland plants eaten by people. And on a recent visit to Otter Creek Marsh Wildlife Management Area in Tama County, I thought again about plant use by humans…and by wildlife, too. In this post, let’s talk about wetland plants as a construction material.
In particular, marshes have a lot of grass-like (graminoid) plants growing up out of the water (i.e., emergent vegetation): examples at a place like Otter Creek often include Cattails (Typha), River bulrush (Bolboschoenus fluviatilis), Softstem Bulrush (Schoenoplectus tabernaemontani), true grasses, and so on.
Think about the challenges those plants face when growing here. Obviously, freezing in winter destroys tissues, so a die-back of everything above the mud is unavoidable. During the growing season, stems must be able to “go with the flow,” bending with the moving water, even including powerful wave action during windy conditions. Yet the photosynthetic stems and leaves must remain upright, out of the water, maximizing sunlight capture. In certain species, those aboveground parts connect to horizontal stems that are spreading through the mud, as well as to the roots. Those tissues in the anoxic (oxygen-free) mud have metabolism of course, and so must have oxygen delivered to them. So the emergent stems and leaves must access oxygen in the air up above, and deliver it down to the belowground tissues.
The leaf structure of Cattail shows solutions to these challenges. Even in winter, the leaves are remarkably sturdy, sticking up out of the ice (and they will persist through the next growing season). The leaves are more-or-less a cylinder down below, presenting less resistance to water movement; yet higher up, they spread out flat to act as efficient “solar panels.” In cross-section the leaf looks a bit like an airplane wing: hollow (saves on materials, and less resistance to wind/water forces) but reinforced with thin internal walls as supports; a spongy material fills in the voids. Even better, that internal structure forms a “snorkel,” allowing air (and oxygen) from from above to diffuse down to the plant parts in the mud.
That system allows cattails to grow, but the story doesn’t end there: those plant tissues prove useful to animals, as well. I saw several Muskrat (Ondatra zibethicus) mounds in the marsh, made of graminoids and Smartweeds (Polygonum) and other plants piled up, incorporating a little mud as a binding agent. The plant tissues are flexible and durable and even somewhat waterproof. They compress a bit, but not too much—great for mounding up. One mound was taller than I am, and formed an igloo-shape. It got me to wondering what it was like inside where the animals were overwintering. For example, does the mounded plant materials keep the muskrats warm??
To help answer that question, and thereby better understand the characteristics of the plant as a building material, I set up a little experiment back at the lab. I compared the temperature in the center of paper cups suspended above a heat source (60W incandescent light bulb). One cup was filled with dried, packed graminoid tissues; one cup contained fiberglass insulation (the material used to insulate the walls and attic of a house), and the Control cup remained empty (well, I suppose it contained air!). I monitored the temperature in each cup using both electronic and glass thermometers, observing the temperature change every couple minutes until it stabilized (about 30 minutes).
I expected a difference in “r-value,” with the fiberglass-filled cup changing temperature most slowly, the plant cup a bit more quickly, and the empty cup changing temperature most rapidly. I assumed that since each cup was right above an identical (very hot!!) bulb, they would all reach the same elevated temperature soon enough. But…that’s not what happened, actually.
Instead, the temperature changed at about the same rate in each cup—maybe the insulation layer was too thin or the heat-transfer surface too narrow to be important in blocking the temperature change? However, the materials made a BIG difference in the final temperature achieved: the control cup increased temperature by 15 degrees Celsius (59 degrees Fahrenheit), the fiberglass changed 25 C/77 F, and the plant material really heated up—increasing by 32 C/90 F!! It’s been a while since I studied physics, but I suspect this is due to the different thermal mass of the materials. Plant tissues can absorb and store more heat energy than can fiberglass; and air itself has little heat-storage capacity. So, the plant material heats up.
I wonder what all that means for the muskrats? Being “warm-blooded” animals, I suppose they huddle together down in that mound, sharing body heat. A mass of warm animals in a massive, heat-trapping home makes sense during a cold Iowa winter. They remain active throughout the winter, so their fur must be pretty good insulation, too…no wonder the fur is valuable to humans!
So, Muskrats find the plants to be a useful building material…but, so do humans! An interpretative display at the marsh describes traditional use of graminoids by the Meskwaki, a nearby settlement of Native Americans/First Peoples. The plants were woven into mats and incorporated into dwellings known as wickiups. We encountered this ethnobotany in a previous 99wetlands visit, to an interpretative nature center in Linn County. I’d love to learn more about traditional uses of plants (and other aspects of history & culture)…if you like, please leave a comment to recommend a reading or a place to visit, etc.
I think it would be fun to visit this site again, perhaps during a warmer season. Maybe I’ll watch the birds with the scope provided at the interpretative display. Till then, I’ll hunker down in my own cozy family dwelling, with an occasional wander out to the marsh. Please come back and read about my further wetland wanderings.