Mountain-avens -- Dryas — Plant in Place

In rock-strewn fellfields beyond the tree line, plants tend to grow in compact cushions or sprawl along the ground. Cold temperatures, high wind, and thin soil make it difficult to attain height. The soil is regularly subjected to freeze and thaw, heaving any ill-anchored plants out of the ground. Huddling low and with deep roots, plants endure. Yet paradoxically most of these plants cannot survive in less devastating conditions because they are not well equipped to compete with other plants for resources. What they lack in competitive ability, they make up for with sheer obstinacy, happily growing where it seems nothing should. And so it is with one type of plant you are likely to see ranging through high elevation scree and vast Arctic tundra, mountain-avens.

Mountain-avens (Dryas) are low-growing, long-lived evergreen shrubs in the rose family (Rosaceae). They are named after dryads, wood nymphs of Greek mythology that lived in oak trees, because the lobed leaves of eightpetal mountain-avens (Dryas octopetala), the most common and widespread of mountain-avens species, appear somewhat like the leaves of certain oak species. Their leaves bear the hallmarks of a hardscrabble existence: thick, waxy and small with hairs covering the underside. Leaves like these work to limit water loss when soil moisture is low and physical damage from cold and desiccating winds. Above the entwined leaf-covered branches, flowers are held singly on stalks with petals spread open and white in the case of eightpetal mountain-avens and entireleaf mountain-avens (Dryas integrifolia), while yellow, nodding, and only partially opened for Drummond’s mountain-avens (Dryas drummondii). All three species produce a tuft of feathery seeds tightly twisted at first but unfurling over time before taking to the air. Beautiful and functional, their feathery achenes make use of the wind for seed dispersal. This is useful where animals are relatively scarce and seeds can be carried airborne long distances over featureless land.

The three species listed above are morphologically distinct and clearly differentiated but beyond that, the taxonomy of the genus Dryas is somewhat unresolved. Numerous subspecies and variations identified within the three species are now thought by many scientists to be species of their own. Though these are less readily distinguished, they do manifest differences that are apparent to a practiced eye and reflect their minutely distinct genomes. To actually work out the genetic differences in these plants is certainly hard. More difficult though is to decide and agree on what constitutes a unique species as opposed to a variation within a species. This is a task well suited to a certain type of person and thankfully we can leave it to taxonomist to figure all that out. The main take away message for me is how interesting it is that the more we advance our science, the more convoluted our system for organizing life becomes. Where exactly is the line between something being variation within a species and a unique species of its own? The complexity of life is revealed ever more as we make discoveries and find new questions to ask. Establishing the true nature of relationships between living things and trying to fit life into a logical human construct, something that can be written down on a piece of paper, is difficult and maybe even absurd. But in the end it is worthwhile because we must work to better understand this world for which we have become caretakers. This is our ultimate job and one which we have yet to show we are qualified to perform.

However the individual components may be divided, when taken together as a genus Dryas has a range typical of many Arctic plants. With a circumpolar distribution from Alaska east through Canada, Greenland, the British Isles, Scandinavia, Russia, and Japan with populations dotted along the Arctic islands and extending southward along mountain ranges through the western United States, Central Europe, and the Caucasus, mountain-avens cover a lot of territory. Although that is a lot of land, the growing conditions are remarkably similar and mountain-avens showcase certain adaptations that make them well suited for life across this range.

For Dryas octopetala and Dryas integrifolia, one interesting adaptation is found in the shape of their petals. They possess parabolic flowers, a trait sharee with other Arctic plants, and a brilliant feature in a land where summers are short but intense. Similar to a satellite dish in form and function, mountain-avens flowers catch solar radiation all along their highly reflective white petals. Because these petals are inwardly curved in a near perfect parabola, they reflect the light toward sexual organs that project from the center. This concentrated solar energy works to raise the temperature at the center of the flower a few degrees above ambient air temperature. That’s not the only trick these plants have. They also exhibit heliotropism, physical movement in response to sunlight. For populations above the Arctic Circle, flowers bend endlessly through the summer following the sun along its unbroken circuit of the horizon. Facing directly toward the sun throughout the day allows the elevated temperature within the flower to remain consistent. An Inuit name for mountain-avens is isurramuat, which refers to this remarkable movement of the flowers.

The handful of added degrees that these two attributes combine to create might not seem like much, but large gain is made from small steps. Their warmth makes these flowers a desirable place for insects to rest. While warming up in the flowers, they inadvertently pollinate; a great example of mutual gain in a demanding landscape. With pollination taken care of, the elevated temperature now improves seed production. Seeds grow larger in size and greater in number than would be possible without the added heat. Seed maturation is also accelerated by the heat, maybe the most important factor of all in a place where winter can appear early and without warning. The faster an Arctic plant can get from pollination to seed dispersal the better. Possessing small but ingenious adaptions that take full advantage of summer, isurramuat are deeply in sync with the seasonal rhythm of their home.

That home on the habitable edge is not just in the high Arctic tundra and alpine, but also disturbed land in more temperate regions. Mountain-avens are pioneer species on recently disturbed land in temperate regions where they often take root on scoured rock near the face of retreating low elevation glaciers. As pioneers, they don’t require much to get started. Just bit of organic matter accumulated in a crack is enough for a wind carried mountain-avens seed. Once rooted, the sprawling nature of this shrub allows it to cover ground quickly taking root again wherever it finds a suitable spot. Further organic matter accumulates under the secure hold of their dense branches and over time the soil is built up enough for other species to germinate. Mountain-avens also improve the soil as they help build and retain it. They are known to associate with the soil bacteria Frankia to create small root nodules which work, through a complex chemical reaction, to take nitrogen from the atmosphere and fix it into the soil. All terrestrial plants require a mix of different soil nutrients for healthy growth but nothing is more important than adequate nitrogen. New soils in harsh environments are invariably nutrient deficient. It’s no coincidence that many pioneer plant species, mountain-avens included, have the ability to fix nitrogen, thus improving the soil for themselves and future plants. Mountain-avens that colonize rock uncovered by the retreat of low elevation glaciers are true pioneers and can expect to be overrun by other plants in relatively short order. But throughout the Arctic tundra and on exposed mountain tops, their home is more secure. In places where harsh conditions are constant, they don’t have to worry about being out competed and dislodged by other plants. Here, they are both pioneer and climax species, and in these windswept places individual mountain-avens can live to be 100 years old with populations covering vast areas. Where they are found in Arctic tundra, they are one of the most prolific and important inhabitants comprising a large percentage of the overall biomass and providing much needed browse for migrating caribou.

Mountain-avens have a tangential connection to paleoclimatology. Given their nature as quickly establishing pioneers and indicators of a tundra plant community, climate scientists have been able to use evidence of ancient Dryas populations to delineate the southward extent of tundra habitat. Knowing where ancient tundra met ancient forest helps reveal past global climate trends. Since the massive ice sheets of the last glacial maximum began to retreat about 20,000 years ago, the Earth has experienced a general warming trend. However, there have been short periods of cooling since that time. Two of the most drastic and prolonged cooling periods are named for mountain-avens: the Older Dryas and Young Dryas. Cooling during the Older Dryas occurred about 14,000 years ago and lasted for 200 years while the Younger Dryas period was much longer, lasting around 1,000 years and ending about 11,500 years ago. These periods are named for Dryas species because the first evidence for their existence was the discovery of mountain-avens leaves and pollen in the ancient sediment deposits of lakes and bogs in Scandinavia. These sediment layers were sandwiched between layers with evidence of trees. The retreat of the ice sheets and general warming until the Older Dryas allowed forests to advance northward and overtake what had been tundra. During both cooling periods tundra pushed south again as the forests died off with the cold. At the end of both cooling periods, the warming temperature allowed forests to more north once again. The back and forth between these two distinct plant communities, evidenced by these sediment layers, made it clear that large scale climate fluctuations took place during those periods.

Understanding ancient climate trends can provide important insight for us as we attempt to manage our current crisis. As our accelerated warming trend progresses mountain-avens populations will be forced farther north and higher up, perhaps testing the edge of land.