Air pollution is a strong driver of forest decline and tree death throughout much of the world — but the exact mechanisms haven’t been completely clear. It was already known that common air pollutants appear to ‘attack’ the protective wax on tree leaves and needles — but how exactly?
New research from Bonn University appears to have an answer to that question — particulate matter salt compounds “that become deliquescent because of humidity and form a wick-like structure that removes water from leaves and promotes dehydration.” This — not surprisingly — makes the trees much less resilient, decreasing their drought tolerance.
“Our study reveals that so-called wax degradation on pine needles may develop from deposited particulate matter,” states Dr Jürgen Burkhardt from the Institute of Crop Science and Resource Conservation. “Wax helps to protect leaves and needles from water loss.”
“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 continues. “Previous approaches assumed chemical reactions for wax degradation, whereas the present study reveals physical reasons.”
Bonn University continues:
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,” says Dr Burkhardt.
“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.
The new research was just published in the journal Environmental Pollution.