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Lingering question: Do variations in scent correspond to variations in color?

Spring weather in Boulder is difficult to predict because it seems that it is not a time of gradual warming but rather alternating days of summer and winter.

Meyers Gulch, with an elevation of 7,350 feet in the Boulder County Open Space and Mountain Parks, is a fine place to see the first flowers of spring in a highly variable environment. Notable among the earliest blooming flowers are pasque flower and mountain ball cactus, Pediocactus simpsonii.

Scanning through my photos of blooming mountain ball cactus (hereafter MBC) I was surprised to find one photo taken in March and most photos taken in late April—in many years MBC present flowers while it is snowing. Another testimony to its hardiness is that it occurs at the highest elevations of any cactus in North America. It is found from 4,600 to 11,500 feet in elevation, and it grows in places known to have winter temperatures that plunge far below zero degrees Fahrenheit.

MBC has a wide geographic distribution, and it grows in a diverse set of plant communities. It is native to Arizona, Colorado, Idaho, Montana, Nevada, New Mexico, South Dakota and Utah, where it is found in prairie grasslands, piñon-juniper woodlands, sagebrush and coniferous forests.

Among all these places and habitats, MBC exhibits such a bewildering amount of variation that biologists have described 13 subspecies, most of which do not withstand critical scrutiny—only three subspecies are commonly recognized.

One of the examples of extreme variation was described by Dr. Elzada Clover, known for describing new species of cacti during the first botanical expedition through the Grand Canyon in 1938. A few years later, she reported that MBC might be two species based on growth form. The cacti growing on Monarch Pass in Colorado use somatic growth to form densely packed mounds of 25 to 30 balls, each with a diameter of 1 to 2 inches. Just down the road, around Gunnison, MBC grow as singletons reaching 6 inches in diameter.

Floral color variation caught my eye. The flowers that I have encountered in the Front Range are rich magenta, but in other localities the flowers can be white, pink, yellow, or yellow-green. Colorado National Monument has white flowers, Malta, Idaho, has bright yellow flowers.

Mountain ball cactus grow as single balls or mounds of balls produced by vegetative reproduction. (Photos: Jeff Mitton)

Katherine Darrow, in Wild About Wildflowers, reports lovely pink flowers near Crested Butte, and Al Schneider notes white, pink and yellow flowers in his website Southwest  Colorado Wildflowers.

I was unable to find any publications describing geographic or environmental patterns for these flower colors. Surely different colors thrive in xeric  piñon-juniper woodlands at low elevations with long growing seasons versus moist coniferous forests with short growing seasons high in the mountains.

Some evolutionary biologists would propose that this is neutral genetic variation, meaning that the alternate flower colors have no consequences that influence any component of life history, such as growth, development, reproduction or survival, that would influence variation in reproductive success. That is, natural selection does not influence flower color and it plays no role in adaptation.

I had found an account that described bright yellow MBC in Malta, Idaho, and it mentioned that they have a lemon fragrance. In Wild about Wildflowers, I read Katherine Darrow's description of pink flowers with a rose fragrance. This covariation of floral colors and fragrance suggests that the key to the floral variation is in the discipline of pollination biology, and it reminded me of important and relevant work

Candace Galen has worked systematically to examine the consequences of floral scent variation in sky pilot, Polemonium viscosum at Pennsylvania Mountain Natural Area west of Fairplay. Sky pilots have variation in floral fragrance and size of the blooms, and these characters covary to adapt sky pilots to their heterogeneous environments.

Individual plants can produce either a sweet fragrance from large blooms or a skunky fragrance from relatively small blooms. Sweet fragrances attract predominantly bumble bee queens, which are nicely accommodated by large blooms. Small flies, seeking rotting flesh or feces, are drawn to skunky fragrances from small blooms. Flies are more reliable pollinators in the low temperatures at high elevations, so sky pilots emitting skunky smells increase with elevation.

This fruitful research program shows that floral fragrance and size can favor different pollinators, and these relationships can vary with elevation, gender of the pollinator and drought versus normal conditions. I see some similarities between sky pilots and MBC.

Will MBC be found to have floral scent and color variation adapting cacti to a wide variety of pollinators and plant communities throughout a large geographic range? Perhaps a first step would be to determine the degree to which floral fragrance differs with flower color.

Will pink (rose fragrance) and yellow (lemon scent) flowers always have the same fragrance? Will pollinators vary among the flower colors? Will the other colors—magenta, white and yellow-green—have their own distinct fragrances? I deeply regret never sniffing the magenta flowers in the Front Range.

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