Recently I’ve been thinking about plants that stay green during the freezing temperatures of our winters. What function does that green serve in the plants’ annual cycle?
As a naturalist I have become accustomed to thinking that there is a reason for every structure and behavior we observe in the natural world. And if what we observe seems nonsensical to us, it just means that we need to observe more carefully, and gather further information about the creature or habitat we are learning about, in order to make sense of what we are seeing.
But evergreen — what’s up with that? Green means plants have active chlorophyll and that means photosynthesis is going on. It’s a complex process that is temperature-dependent for the chemical reactions that produce carbohydrates from water, carbon dioxide and micronutrients in the presence of sunlight.
Maintaining chlorophyll takes metabolic energy, which is at a premium in low temperatures. For many years I progressed just this far in my thinking and stopped there. But two years ago, at the Northeast Natural History Conference in Springfield, I heard an amazing talk by Jack Tessier, a professor of environmental science and forestry at SUNY Delhi in central New York. He studied and got really fascinated with evergreen ferns.
I’m sure you’ve seen them in our local forests: Christmas fern, marginal wood fern, rock polypody and intermediate wood fern, among others. They are gorgeous and green all summer. And then, come autumn, they remain a shining, forest green.
Most of our other ferns have died above ground and shriveled into brown. The deciduous trees’ leaves have turned and dropped and, still, these ferns remain green and healthy-looking. When the snows of winter arrive, the fronds, still green, flatten to the ground. It turns out they have a special hinge that becomes flexible with moisture in the autumn, allowing the fronds to “relax” onto the ground on top of the autumn leaf layer.
Why do they remain on top of the fallen leaves? Do they photosynthesize under the snow? Wouldn’t it be too cold? In spring the winter snows melt and the still healthy, vigorous greens re-emerge.
Tessier studied the photosynthetic activity of these species and discovered that they are very active during this window between snowmelt and leaf-out of the tree canopy. In fact, he found that they carry out an amazing 43 percent of their annual photosynthesis during just these 10 weeks of the year.
But Tessier had a deeper question: What biochemical mechanisms work together to support the ferns’ annual cycle? Other researchers had found that the old, still-green fronds of Christmas fern lose nitrogen and phosphorus, effectively transferring it into the newly emerging fronds, thus assisting in their growth. But no one had studied whether the carbon compounds produced during this window of photosynthetic opportunity help support the newly emerging fern fronds. So, Tessier set out to discover whether this happens.
He created mini-photosynthesis chambers in the field surrounding intermediate wood fern plants. He then pumped radioisotopes of carbon dioxide into the chambers, labeling the fronds, which were actively manufacturing hydrocarbons, with these isotopes. Tessier discovered that the newly emerging spring fronds contained significant amounts of the isotopes, even before they had started photosynthesizing on their own. So the early spring photosynthesis of the old evergreen fronds was contributing directly to the growth of the new emerging fern fronds.
This was what we theorized was happening, but Tessier had found convincing evidence to document it.
It turns out that this strategy of maintaining photosynthesis through the winter is widespread in our evergreen plants — dormant during the coldest temperatures of winter but ready to jump into action when the temperatures rise.
Plants that have adopted this approach have to face a number of challenges. Protecting the sensitive photosynthetic pathways from too much solar radiation during the winter, when these pathways are not active, is one of the challenges. And maintaining them all winter in order for them to be up and ready when temperatures begin to warm in spring is no easy task.
In the eastern hemlock, one of our common evergreen trees, the ability to photosynthesizes changes dramatically from no carbon metabolism at 23 degrees to active photosynthesis at 35 degrees. This is not surprising since these values bracket the freezing temperature of water.
So, in my wanderings I keep striving to pay attention to what I can observe even in our common plants, to ask questions, and then to pursue finding out answers to my questions. Nature is endlessly fascinating and engaging and provides us with clues to the complex functioning of our natural systems — those systems that maintain planetary life in all its complexity and mutuality.
Ted Watt is an educator/naturalist at the Hitchcock Center for the Environment. Data sources for this essay are a 2000 study of eastern hemlock at Harvard Forest by Julian Hadley, published in the journal Arctic, Antarctic and Alpine Research (vol. 32, pp. 368-374) and Jack Tessier and Matthew Bornn’s 2007 study of intermediate wood fern in the American Journal of Botany (vol. 94, pp. 25-28).
Earth Matters, written by staff and associates of the Hitchcock Center for the Environment at 525 South Pleasant St., Amherst, appears every other week in the Daily Hampshire Gazette. For more information, call 413-256-6006, or write to us.