Surprisingly, subspecies with different growth forms can be within a few feet of one another

Hasley Meadow, in the Maroon Bells-Snowmass Wilderness near Aspen, produces an impressive wildflower display that I have enjoyed for two summers in a row. Experienced friends urged me not to miss the view from Hasley Pass, above the meadow, at an elevation of 12,144 feet.  

So, despite growing weariness, I headed up the hill. When I reached the pass, the view of Hasley Basin and Snowmass Peak was exhilarating. After enjoying and capturing the view on camera, I saw a wildflower species that I had never noticed before. Stems reaching about a foot above the ground were topped with white plumes fluttering in the breeze. I found myself in a large population of mountain dryads, Dryas octopetala.

A dense rosette held thick and leathery leaves, one quarter inch to almost two inches long. The leaves were glabrous (shiny and smooth) on top but pubescent (white with hair) underneath. Each leaf rolled under at the edges and had rounded teeth along the margins. Earlier in the summer, the flowers were one to two inches in diameter, with eight white petals—hence the species name octopetala. Yellow anthers and stigmas packed the center of the flower.

Mountain dryad has a circumboreal distribution, meaning that it encircles the globe at high latitudes. It extends from Siberia through Asia to Scandinavia, northern UK, Iceland, Greenland, northern Canada and Alaska. It reaches into middle latitudes in alpine tundra in the Cascade Range, Rocky Mountains, Himalayas, Caucasus, Alps, Pyrenees and Apennines.

Several adaptations allow mountain dryads to occupy cold, windy sites with poor soils. They are low-growing, which minimizes wind damage. They have deep roots that anchor them and spread laterally, creating an expanding clone.

The flowers have eight white petals, which form a bowl that reduces wind and reflects light and heat to the pistils. In addition, the flowers are heliotropic, meaning they turn so that they can collect heat from the sun all day.

As seeds mature, pistils elongate and elaborate plumes that will catch the wind and disperse the seeds. Roots have nodules with nitrogen-fixing bacteria that deposit nitrogen in the soil, enhancing growth in nutrient-poor soils. Finally, they can live 100 years, increasing the probability of successful reproduction in hostile environments with high variability and low predictability.

Evolutionary biologists have been working with mountain dryads because dryads are able to adapt to contrasting environments and sustain that adaptation even when the different habitats are just a few feet apart. A well-documented example by James McGraw and Janis Antonovics started out as an analysis of the differences between growth forms in adjacent, contrasting habitats.

One habitat is rocky scree slopes, also called talus slopes, that are usually steep hillsides of broken rock. The other habitat is snowbed sites, where wind-blown snow accumulates to such depths that plant growth starts substantially later, effectively reducing the length of the growing season.

The simplest way to distinguish the growth forms is with leaf characters: leaf size, pubescence vs. glabrous and evergreen vs. deciduous. When the growth forms were raised in growth chambers to make their habitats identical, the growth forms retained much of the difference seen in the field.

This experiment demonstrated that the differences were predominantly determined by genetic differences and justified naming the growth forms as subspecies. D. octopetala ssp. octopetala grows on limestone scree slopes and has pubescent, deciduous leaves that are 5- 15 millimeters. D. octopetala ssp. alaskensis grows in snowbeds and has glabrous, evergreen leaves that are 15-50 millimeters.

The stunning point here is that these subspecies with different growth forms can be within a few feet of one another. The seeds easily disperse much farther than that, so seeds are all mixed up by the wind, but the subspecies survive and reproduce in different habitats.

By transplanting plants within and between these environments, they found very strong natural selection for survival of D. octopetala spp. octopetala in scree slopes and, conversely, strong selection for survival of D. octopetala alaskensis in snowbeds.

The Dryas genus is so strongly associated with cold places that geologists use it to identify historic temperature trend anomalies. As the last Ice Age glaciers were withdrawing and temperatures were rising, two abrupt and unexplained reversals in temperature trends occurred about 13,800 and 12,000 years ago.

Colder temperatures caused arctic tundra species to reappear in places where forests had been growing. Ecologists studying historic plant communities saw resurgences of Dryas fossils, so these periods were named the Older Dryas and the Younger Dryas.

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