Oct. 15, 2012

Frosted Flowers: The Complexity of Climate Change in the Rocky Mountains

by Kara Rogers

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As a warm July sun arcs high over an alpine meadow in the Rocky Mountains, glacier lilies nod gently in the wind, their little yellow petals radiant under the cloudless sky. Like many other species of subalpine and alpine wildflowers, each year, as the winter snowpack recedes, the glacier lily comes to life, sprouting leaves and flowers as soon as conditions are favorable and taking advantage of every moment of the short growing season.

But since the late 20th century, as snowpack in the Rocky Mountains has declined and as snowmelt has occurred earlier and faster than in centuries past (effects attributed to climate change) some species of subalpine and alpine wildflowers, including the glacier lily (Erythronium grandiflorum), have been enticed into blooming slightly earlier in the year than they might otherwise. Unfortunately, however, this head start on flowering is not as advantageous as it might seem.

Indeed, despite the recent warming trend, the last hard frost of the year still consistently strikes in mid-June in the Rocky Mountains. Thus, rather than extending flowers’ time for reproduction and promoting an increase in wildflower abundance, early growth renders most plants susceptible to frost damage, which ultimately undermines their successful reproduction.

The impact of climate change and frost damage on flowering has been studied in several species of Rocky Mountain wildflowers, notably Aspen fleabane (Erigeron speciosus), subalpine larkspur (Delphinium barbeyi), and Aspen sunflower (Helianthella quinquenervis). Research on these species has shown that in years of mild snowfall and warm spring temperatures, the plants frequently experience a reduction in flowering abundance due to the loss of buds from frost exposure.
 
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The effects of frost can be significant. For the Aspen sunflower, for example, each year between 1999 and 2006 almost 74 percent of plants were found to suffer from bud frosting. In contrast, between 1992 and 1998, about 36 percent of plants were frosted. The likelihood of frost damage for this flower was associated with the previous winter’s snowpack, with greater snowpack linked to lower bud mortality from frost. The Aspen sunflower, however, is a long-lived species, surviving about 40 years, which suggests that its local extinction likely would come only after many consecutive mild winters. Whether such sustained climate change could affect the Rocky Mountains depends largely on climate patterns driven by variations such as the Pacific Decadal Oscillation and the El-Nino Southern Oscillation, which can affect snowfall.

Scientists have also found that the timing of flowering for Rocky Mountain wildflowers has undergone additional shifts. Data from the mid-1970s, for example, reveals that at that time plants produced flowers throughout most of the summer season. More recently, however, plants have progressed generally toward early and late flowering peaks, with a reduction in flower abundance in mid-summer.

The changes in the timing of flowering and in the abundance of mountain wildflowers have raised concerns about what these effects might mean for populations of pollinators, herbivores, and other animals that depend on subalpine and alpine plants. For example, the lack of flowers in midsummer could affect populations of bumblebees and ants, which feed on nectar, and populations of seed predators like tephritid flies.

Since 1975, the glacier lily has advanced its flowering date by 3.2 days per decade, slightly more than other species of mountain wildflowers that have been studied. Its duration of flowering has increased, too, and unlike other wildflowers, its flowering abundance appears to be independent of climate, likely being controlled instead by factors such as nutrient availability or the previous year’s fruit set. The glacier lily’s unique response to climate change highlights the amazing diversity of montane wildflowers, and its resiliency is a symbol of hope for the future of these beautiful mountain plants.
About Kara Rogers

Kara is a freelance science writer and senior editor of biomedical sciences at Encyclopaedia Britannica, Inc. She is the author of Out of Nature: Why Drugs From Plants Matter to the Future of Humanity (University of Arizona Press, 2012).

The views expressed are those of the author and are not necessarily those of Science Friday.

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