Forests and plants in general can remove pollutants but may be at a hard cost. 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.

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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.



Salt is a very corrosive material in a number of situations. For example salt spray at the sea shore will lead to rapid degradation of any building there. So why would plants be exempt?



"Our study reveals that so-called wax degradation on pine needles may develop from deposited particulate matter", says Dr. JÃ¼rgen 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 aging 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.



The scientists sprayed salt solutions on Scots pine needles and recorded their weight loss after abscission. The needles treated with salt solutions dried out significantly faster than the untreated control needles. Using an electron microscope, the scientists observed the salts becoming deliquescent and moving into the stomata of the needles.



Stomata are tiny pores used by plants to take up carbon dioxide for photosynthesis and release water vapor and oxygen. 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.



For further information see Forest Wax.



Salt image via Wikipedia.