"With climate change, these limits are expected to shift accordingly," he added. "Hence, knowledge of the distribution of a particular species in the past and the observed shift in [its] range at present may provide a visible ecological fingerprint of climate change."
The researchers say holly's rapid response to changes in average temperature makes it an ideal "bioindicator" of global warming.
The same goes for European mistletoe (Viscum album), which is gaining altitude in response to climate change, according to researchers at the Swiss Federal Institute for Forest, Snow and Landscape Research.
The scientists compared the current upper limit for mistletoe in the Swiss Alps with records from a survey in 1910.
The study, published this year in the International Journal of Biometeorology, revealed the plant has climbed some 656 feet (200 meters) higher in the last hundred years, reaching 4,100 feet (1,250 meters).
The team notes that in Switzerland temperatures rose more than twice the global average in the 20th century, with most of that increase coming in the last 20 years. They say the upward spread of mistletoe is more closely linked to warmer winters than to hotter summers.
Given future predicted temperature increases, the team expects the plant to scale as high as 5,250 feet (1,600 meters) by 2030.
Deck the Halls
Holly and mistletoe have long been used to celebrate winter festivals in countries, such as Great Britain, where pre-Christian Druids considered both plants to be sacred.
Under Christian rule, holly's blood-red berries and spiked leaves came to symbolize the crown of thorns worn by Christ. Mistletoe kept its association as a fertility symbol, with couples kissing under a hanging sprig.
Such traditions aren't as deep-rooted in countries where the plants are less prevalent, according to Martin Sykes, a plant ecologist at Lund University in Sweden.
Sykes, who co-authored the holly distribution study, says holly is not a common Christmas decoration in Sweden.
"Other, more traditional things, such as juniper or Vaccinium"the genus of plants that includes cranberry and bilberry"are used to make rings for the door, for example."
But while holly is spreading in Sweden, other studies suggest the region's Vaccinium species are increasingly at risk from warming.
Observations in northeast Sweden indicate that bilberry is becoming more susceptible to spring frosts, because mild winters are bringing the plant into bud too early.
And while mistletoe might be spreading in alpine Europe, further climate warming could cause the parasitic plant to disappear in Britain, says Chris Jeffree, a botanist at the Institute of Molecular Plant Sciences at Edinburgh University in Scotland.
If British winters continue to get warmer, Jeffree said, "the temperature range would be unsuitable for mistletoe, and it would retreat into continental Europe."
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