First With a Flower

Natural History, March, 2000 by Peter J. Marchand

Some spring bloomers get a head start beneath the snow and ice.

IN THE FIELD

My first glimpse of a snow buttercup flowering beneath a thin pane of ice was not unlike my first experience of watching a monarch butterfly emerge from its cocoon. It seemed a marvel of metamorphosis. From under the granular crust of a subalpine snowfield sprang forth life as tender and fresh as a butterfly's newly unfolded wings. I had often seen the hardy crocuses of town and country gardens poking their cheerful colors through the fresh white powder of a spring snowstorm, but these buttercups had reached their flowering stage entirely beneath the waning winter snowpack, finally absorbing sufficient sunlight to reradiate heat and melt a hole through the ice grains around them.

The vivid yellow buttercup's ability to grow beneath snow is not unique. Seeds germinate, leaves unfurl, flower buds swell as numerous spring wildflowers--among them the familiar trout lilies, coneflowers, spring beauties, and pasqueflowers--begin preparations for emergence long before the snow cover recedes. Digging into the midwinter snow, I have often found plants that were browned and dead-looking in late autumn already greening at their leaf bases or sprouting new shoots.

Growth of any sort, whether it involves the elongation of tiny root hairs or the unfolding of new leaves, requires energy. With the temperature under the snowpack hovering at about 32 [degrees] F in late winter and with carbon dioxide levels often elevated (mostly the result of root respiration and decomposition of organic matter during winter), the environment beneath the snow is not inhospitable to plant growth. And there is some evidence to suggest that the wildflowers of early spring can support their growth partly through photosynthesis beneath the snow. The plants' phytochromes--pigment molecules that are sensitive to slight changes in day length and that control a plant's growth timetable--may respond to dim light coming through the snow. But while the ability to photosynthesize under snow has been demonstrated in a few evergreen plants, including aquatic plants locked beneath ice, evidence of the process in other plants is difficult to come by. In some early spring flowers, the machinery for capturing radiant energy, although mostly assembled by late winter (even the chlorophyll for photosynthesis may be produced in response to light penetrating the snow), probably isn't fully activated until immediately after the plant is released from the snowpack.

In the absence of photosynthesis, energy for growth under the snow must come entirely from reserve carbohydrates. Many spring wildflowers have relatively large below-ground storage depots--in the form of tubers and rhizomes--that are stoked with fuel at the end of the previous growing season. As growth resumes under the snow, the weight of these storage organs declines steadily, in direct proportion to the increase in size of new shoots and flower buds. If the arrival of spring is timely, photosynthesis will start before these reserves are depleted.

The earliest--and arguably the most resourceful--of the spring flowers is the skunk cabbage, ubiquitous in the wet places of North America and Eurasia. Its flowering stalk emerges from the soil (and snow) before its leaves do--and often while air temperatures are still below freezing. So high is the cellular respiration of the skunk cabbage at this time of year that the plant (except for one western North American species) generates heat--one of the few in the world to do so. When the air temperature is below freezing, the plant may be as much as 30 [degrees] warmer than its surroundings, and the flower stalk is able to melt its way through frozen ground and ice.

But aren't these early bloomers too precocious for insect pollinators? In fact, a number of insects are available to do the job, although some of the plants have the capacity to self-pollinate as well. It's not uncommon, for example, to find flies and solitary bees foraging as the snow recedes. And when a flower is the only show in town, there's a strong likelihood of its attracting attention and being cross-pollinated, even with few insect species around. As for the skunk cabbage, it leaves less to chance, having evolved an additional lure: a malodorous scent whose dispersal seems to be facilitated by the heat-producing flower. This dung or carrion mimicry attracts flesh flies, rove beetles, and even mosquitoes, all of which have been observed with pollen on them. (Some biologists believe that the heat produced by skunk cabbage flowers may also attract pollinators by providing a warm basking site during cool weather.)

Still, one other question begs for an answer: Why all this effort to be first with a flower? Reduced competition for pollinators may be one advantage of early flowering, but for many of these spring ephemerals, time is the most pressing issue. Most occupy habitats where the growing season is greatly compressed, either because of the short interval between the final spring and the first autumn frosts or because the closure of the overhead tree canopy in early spring smothers them in deep shade. In the race to reproduce, these plants can't wait for the snow to melt.