Wild Holly, Mistletoe, Spread With Warmer Winters

James Owen
for National Geographic News
December 7, 2005
Christmas greenery is bringing festive color to new parts of Europe as
holly and mistletoe growing in the wild begin to take root outside of
their traditional ranges.

Two recent studies suggest that a trend toward milder winters in northern Europe is likely tied to the spread of these plants, which are common decorations during the winter holidays.

The research shows that holly is heading north across Scandinavia, while mistletoe is scaling snowy alpine slopes.

Common holly (Ilex aquifolium), a spiky-leaved, red-berried evergreen, has advanced across southern Scandinavia and northern Germany in recent decades, according to researchers.

The shrub has increased its range by 75 miles (120 kilometers) in Germany and 80 miles (130 kilometers) in Denmark since the 1940s, says Gian-Reto Walther, a vegetation ecologist at the University of Hanover in Germany.

"In Norway the northward range expansion is about 30 kilometers [19 miles]," Walther added. "There are also new occurrences along the Swedish coast."

A study published earlier this year in the journal Proceedings of the Royal Society B compared the plant's current distribution in Scandinavia and Germany with records from 1944.

On the Move

The team found that holly, which can stand winter freezes down to minus 4°F (minus 20°C), has been spreading as temperatures rise due to climate change.

"Climate stations included in our study have experienced an average temperature rise of approximately one degree Celsius [1.8°F] in the coldest month," Walther said.

This finding is in line with figures from the Intergovernmental Panel on Climate Change based in Geneva, Switzerland. The panel has documented an average temperature increase of 1 to 1.2°F (0.6 to 0.8°C) per decade in northern Europe since the 1970s.

The distribution of some plants—especially evergreen, broad-leaved woody species—is strictly limited by climatic factors, Walther says.

"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."

Free E-Mail News Updates
Sign up for our Inside National Geographic newsletter. Every two weeks we'll send you our top stories and pictures (see sample).


© 1996-2008 National Geographic Society. All rights reserved.