Holly Hit Hard by Pollution
Decorating the house with sprigs of holly is as much a pleasurable Christmas ritual for some folk as snuggling up by the fireside with an atmospheric Charles Dickens novel.
'The impact on the hollies is easily visible to the layman's eye'
Holly features prominently in popular carols and Christmas cards and its spiky leaves, and radiantly long-lasting crimson berries, are among the most enduring motifs of the festive season.
At others times of year, holly - a rare example of a bush that can thrive in dense, dark undergrowth - takes more of a back seat. Its prickly, cattle-deterring leaves are a positive menace if you are weaving your way through thick woodland.
For all that, the idea of a countryside without holly is hard to imagine. But recent research suggests that the long term health of hollies in Europe could be in danger.
Dr Jon Ranford of Staffordshire University, Stoke-on-Trent, and his colleague Dr Kevin Reiling, have been looking at the effects of increasing levels of ozone pollution on hollies, and the results are not good.
Their findings suggest that not only does the species shed more leaves than usual when ozone levels are high, but that it also suffers stunted growth in the following season, seriously affecting its long-term ability to survive.
"We have been conducting studies on the impact of ozone on plant growth at the university," says Dr Ranford.
"We decided to look at holly because, as a native evergreen that retains its leaves over a long period - four to five years in a mature tree - it can be more easily monitored for pollution effects than a tree shedding its leaves every autumn."
The first half of the experiment involved taking 200 seedlings grown outdoors and placing them in plant growth chambers where temperature, light and water conditions were controlled. Some of the hollies were then dosed with 70 parts per billion ozone levels - typical of recent British summers - while others were given clean air, so that growth comparisons could be made.
After 28 days, the seedlings were planted outside in natural conditions. They were then monitored for the next two years. The researchers found that while a young holly will generally retain its leaves for two years, the ozone-polluted trees were shedding their leaves at around 18 months.
"This had a further impact the next year," says Dr Ranford. "Because they didn't have as many leaves, their ability to produce sugars through photosynthesis was impaired and this constricted their root and shoot growth."
He says the impact on the hollies is easily visible to the layman's eye. "If you placed all the plants in the experiment in a row, you could easily tell which had been subjected to the ozone. They were not as big or as bushy and generally looked less healthy."
A second experiment tested how the plants coped with the effects of frost - the ozone hollies were effectively "hammered", according to Dr Ranford.
"Plants and insects naturally have to cope with frost but it was clear that ozone pollution interfered with the process of defending themselves against ice formation."
The hollies were also "leaking" water through holes made in their cell membranes by the ozone.
Dr Ranford wants to take the experiments a stage further to see if rising pollution levels have an effect on holly regeneration in the countryside.
"There are many horticultural varieties of holly and, while we believe these will also be affected, nursery-grown trees and shrubs are cared for. But in their natural habitats there is much greater competition for resources, and the ability of young hollies to establish themselves needs to be looked at."
So far, there have been no publicised reports of older hollies in the wild showing signs of the effects, although Sarah Bierley of the Shropshire Wildlife Trust says our increasingly long, hot summers are not in their favour. "Hollies like a lot of rain and are not good in drought conditions," she says.
The Trust is currently running a campaign to raise £250,000 by the end of March to buy a 90-acre site on the bleakly exposed ridge at Stiperstones, Shropshire, home to 200 cracked and windblown hollies which, at 400 years old, are the most ancient examples in Europe.
"There's nowhere else like it in the country," says Bierley, "because all the other holly forests where pollarding was carried out to provide a cattle crop have been grubbed up or have fallen into neglect."
These ancient hollies have survived because they are part of the Stiperstones Site of Special Scientific Interest. As such, they have benefited from the life-lengthening practice of pollarding, carried out by Natural England.
But, according to Bierley, there has been little regeneration of new holly seedlings, a process that rising temperatures and ozone levels are unlikely to assist. The Trust, if successful in its bid, plans to reintroduce special holly-gathering days for people to come and pick holly branches for Christmas.
Dr Ranford says veteran hollies such as those at the Stiperstones could be as vulnerable to rising ozone levels as young ones, given that ageing trees are less able to handle changes in growth conditions.
His is not an entirely gloomy Christmas message, however. "I'm not saying this will be the end of hollies by any means. Plants will always adapt. But ozone levels are projected to continue rising until 2100, which means that the background concentrations of ozone will be the same all year round as we now experience in summer. It really needs a global effort to combat the issue."