Stonecrop climbs mountains as glaciers shrink

By Jeff Mitton

I visited a stand of bristlecone pines at tree line on Mount Evans several summers ago, in early July.

Bristlecone and limber pines at tree line evoke reverence, for some survive for more than thousand years, enduring bitter cold and thrashing winds.

But I came away with appreciation for something else -- an interaction that has lasted for many thousands of years.

Beneath the bristlecones the lanceleaf stonecrops, Sedum lanceolatum, were ready to bloom.

Stonecrop is a perennial herb with erect stems 2 to 8 inches tall. The leaves are succulent, or water storing, and are nearly round in cross section and tapering to a blunt end. The leaves and stems are smooth and vary in color from green to maroon.

Flower buds were swollen and bright yellow with maroon piping at the seams where petal margins were fused. When the flowers open, they have five petals and 10 stamens and the average inflorescence contains five to 10 flowers. All structures in the flower are bright yellow.

Locally, stonecrop is found on dry, rocky sites from the Great Plains to the alpine tundra and it grows naturally from Alaska to New Mexico and from western Nebraska to California. This broad geographic range, centered on the Rocky Mountains, placed stonecrop in the path of advancing glaciers in the numerous glacial cycles in the last million years. Imagine the slow-motion scattering of stonecrops as glaciers grew and descended.

Throughout the summer, Rocky Mountain parnassians, Parnassius smintheus, are often seen visiting stonecrop.

Parnassians are predominantly white with short gray bars on the leading edges of the forewings and gray, furry bodies. The upper side of wings has small red circles fringed with brown. Parnassian caterpillars are specialists on stonecrop, so adults lay eggs on or near the only food their offspring will eat.

University of Colorado biologists Eric DeChaine and Andy Martin studied stonecrop and parnassians to infer, from their current patterns of genetic variation, whether they had been tightly allied through their evolutionary history.

They estimated that stonecrop populations existed at least 400,000 years ago and perhaps more than 1 million years, while the parnassian populations have a shorter history, perhaps 100,000 to 135,000 years.

Clearly, this pair of species has had the opportunity to move up and down mountains several to many times, driven by glacial expansions and retractions.

As glaciers shrank, stonecrops would colonize higher and higher ground, until they were restricted to small populations on the highest peaks. Parnassians would be obliged to follow, unless they could switch host plants, and we have no indication that they can or do.

When glaciers advanced, the stonecrop and parnassians would move to lower ground, and this usually meant that they would be able to occupy much greater geographic areas. As populations merged at lower elevations, they would have much greater opportunity to exchange genes than when they were isolated on high peaks.

So glacial cycles would alternately restrict the species in small, isolated populations, where they had the opportunity to adapt to their local habitats, and then merge populations and reshuffle their genes.

Although minor details of inferred biogeographical history differ between stonecrop and parnassians, they share their most salient features of geographic variation. Both species show a strong pattern of genetic difference across the Laramie Basin, the relatively low ground between the Wind River Range in Wyoming and the Medicine Bow Range in Colorado.

Far back in history, both stonecrop and parnassians were able to cross this low land, but in more recent times, this has been an impenetrable barrier to gene flow.

Jeff Mitton (mitton@colorado.edu) is a professor in the Department of Ecology and Evolutionary Biology at the University of Colorado. This column originally appeared in the Camera.

April 29, 2011