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Bonn, Germany (SPX) Jun 24, 2013
Bonn University scientists demonstrate that hygroscopic air pollutants decrease tree drought tolerance. Air pollution is related to forest decline and also appears to attack the protecting wax on tree leaves and needles.
Bonn University scientists have now discovered a responsible mechanism: particulate matter salt compounds that become deliquescent because of humidity and form a wick-like structure that removes water from leaves and promotes dehydration. These results are published in "Environmental Pollution".
Nature conservationists call it "lingering illness", and the latest report on the North-Rhine Westphalian forest conditions confirms ongoing damage. Bonn University scientists have now shown that salt deposits on leaves may decrease the drought tolerance of trees, thereby contributing to forest decline.
"Our study reveals that so-called wax degradation on pine needles may develop from deposited particulate matter", says Dr. Jurgen Burkhardt from the Institute of Crop Science and Resource Conservation. Wax helps to protect leaves and needles from water loss.
It has long been known that air pollutants accelerate wax ageing and that "wax degradation" is closely related to forest damage. "Wax degradation was addressed by many studies in the 1980s and 90s, but sound explanations for both the degradation mechanism and the high correlation with forest damage have yet been missing", Dr. Burkhardt reports.
Previous approaches assumed chemical reactions for wax degradation, whereas the present study reveals physical reasons. "The deposition of hygroscopic salts is capable of decreasing the drought tolerance of trees", co-author Shyam Pariyar says.
Accelerated dehydration of needles treated with salt solutions
The deliquescent salts form very thin liquid connections between the surface and interior of the needle, and water is removed from the needles by these wick-like structures. Because the plants are unable to counteract this removal of water, the plants dehydrate more rapidly.
Therefore, polluted air containing large amounts of particulate matter may directly reduce the drought tolerance of trees. Simultaneously, the deliquescent salts make wax appear "degraded". "This newly described mechanism was not considered in earlier explanations of Central European forest decline", states Dr. Burkhardt.
Conceivable aggravation of forest decline by climate change
Recently, regional forest damage has been reported in the western USA and other parts of the world. A relationship with increasing climate change-type drought has been proposed, but the newly discovered mechanism involving particulate matter might contribute to the regional forest damage. "Particularly because air concentrations of hygroscopic particles have largely increased within the last decades", says Dr. Burkhardt.
Publication: Particulate pollutants are capable to `degrade' epicuticular waxes and to decrease the drought tolerance of Scots pine (Pinus sylvestris L.), "Environmental Pollution", DOI: 10.1016/j.envpol.2013.04.041
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