These Alaska spruce trees and deciduous shrubs show evidence of heavy browsing by snowshoe hares. [Image 3 of 6 related images. See Image 4.]
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With Alaska's warming climate, forests are moving upward to higher elevations and northward to higher latitudes. Scientists at the National Science Foundation (NSF) Bonanza Creek Long-Term Ecological Research (LTER) site in Alaska, one of 28 such NSF LTER sites, are working to understand interactions between changing tree lines and plant-eating animals such as the snowshoe hare.
"This study is a reminder that there will be winners and losers as climate changes, and that species' interactions with their environments will play a critical role in how the landscape changes," says Colette St. Mary, an NSF LTER program director.
Scientists Knut Kielland and Justin Olnes of the University of Alaska Fairbanks reconstructed the pattern of spruce tree growth at the Bonanza Creek LTER site from 1970 to 2009. They found that fewer young spruce trees had taken root during periods when snowshoe hares were abundant.
Kielland says the habitats and ranges of snowshoe hares and white spruces overlap. Hares and spruces are common residents of forested floodplains, but hares, being mobile, are often faster than spruce seedlings in reaching the best habitat. And because spruce twigs are an important winter food for snowshoe hares, when the hares can get at them, these herbivores may nibble every branch in sight.
Snowshoe hares have now made their way north to Alaska's shrubby environment beyond the state's forest tree line. As spruce trees follow behind, they must pass through a "snowshoe hare filter." Olnes says the filter was especially evident after the last hare population peak in 2009. With greater numbers of hares feeding on spruce trees, fewer spruce seedlings were able to grow to adulthood.
"Spruces became well-established on more open floodplain sites due to the lower hare population there, but those sites placed the trees at greater risk of drying out during the hot summer months," said Kielland. "The hares are effectively pushing spruce trees into unfavorable habitats."
The research is providing scientists with a unique opportunity to study the interplay among rising temperatures, shifting tree lines and changing habitats.
[Research supported by NSF grant DEB 10-26415.]
To learn more about this research, see the NSF Discovery story Race across the tundra: White spruce vs. snowshoe hare. (Date image taken: 2016-2017; date originally posted to NSF Multimedia Gallery: April 17, 2018)