Of all the trees that are in the wood, the holly may well bear the crown. Sadly, it鈥檚 a crown that is now tarnished by ozone pollution.
All 400 species of the Christmas favourite could be affected, says Jonathan Ranford of Staffordshire University, Stoke on Trent, UK. Holly trees in northern temperate zones across the US, Canada and Europe are especially vulnerable, he adds.
Ranford and his colleague Kevin Reiling dosed European holly saplings (Ilex aquifolium) with 70 parts per billion of ozone 鈥� typical of peak summer levels in Europe 鈥� for seven hours a day, over a month. The plants grew fewer new leaves and shed more existing ones than usual. The pollutant 鈥� a component of smog 鈥� also weakens holly trees鈥� ability to withstand cold in winter.
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Ranford and Reiling performed two growth experiments. In the first, they planted ozone-dosed saplings outside alongside untreated saplings and monitored them for over two years. In their first year outside, the ozone-treated trees produced 40 per cent fewer leaves than the other saplings, and in the second, 30 per cent fewer, showing that the impact of the ozone exposure persisted beyond any immediate damage.
Although holly is evergreen, it does shed some leaves in winter, and the team found that the treated plants lost half their leaves up to four months earlier than the untreated trees. 鈥淎s well as losing more leaves, they didn鈥檛 grow as many, so they suffered a double whammy,鈥� says Ranford. The lack of leaves stunted the ability of the treated trees to create sugar through photosynthesis for root and shoot growth.
In the second experiment, treated and untreated saplings were grown in refrigerated chambers, in temperatures as low as -15 掳C. The ozone-treated trees were 鈥渓eaky鈥�, says Ranford, because water drains away through holes punched in cell membranes by ozone. And the stomata, the leaves鈥� pores, can鈥檛 close properly to retain water because of damage to their guard cells. As a result, 35 per cent more of the treated trees died at -5 掳C with 20 per cent more perishing at -10 掳C. The results of both studies appear in Environmental Pollution (vol 145, p 171 and 355).
