Hygroscopic aerosols — particles in the air that attract water — could be causing forest decline around the world, according to experiments performed in Germany. Researchers believe that aerosol accumulation on trees enables thin bridges of liquid to form between the leaf interior and the leaf surface, causing the plants to dry out much more rapidly.
“In the atmosphere, aerosols act as cloud condensation nuclei,” says Juergen Burkhardt of the University of Bonn, Germany. “Deposited aerosols on leaf surfaces act almost the same way but attract water from inside the plant.”
Plants have developed sophisticated mechanisms for taking up carbon dioxide from the air for photosynthesis without losing too much water but, as the scientists note, it’s a delicate balance. And one that appears to be upset by rising levels of airborne particles.
“Global aerosol concentrations have roughly doubled compared with natural conditions, and the concentration increase over the continents is even higher,” says Burkhardt. “Our results show that aerosols deposited on leaves interfere with this delicate balance, pointing to a direct mechanism by which air pollution can reduce the drought tolerance of plants.”
Burkhardt and colleagues grew three species of tree — Scots pine, silver fir and common oak — for two years in two greenhouses, one ventilated with ambient air and the other fed with air filtered to remove 99% of aerosols. Seedlings grown under filtered conditions had superior drought tolerance to those raised in ambient air, the team found.
By Alan Duffy
Volatile gases emitted by trees scatter light to increase its availability to leaves.
That “pine-smell” you enjoy during a walk in the woods doesn’t just lighten your mood. It lights the entire forest itself.
Research published in the journal Nature Geoscience Letters, and led by Alexandru Rap of the University of Leeds in the UK, has found that the “smell” of a forest, caused by vast quantities of biogenic volatile organic compounds (BVOCs), increases the scattering of direct sunlight and allows it to reach the wider plant-canopy.
Illuminating more of the canopy leads to increased overall growth, more than offsetting the substantial cost in creating the volatiles to the plants, that captures an additional 1.23 billion tonnes of carbon each year.
The impact of these BVOCs was estimated in a simulation framework that included a global aerosol model to track their release across different habitats. It also employed a radiation model that changed the resultant sunlight, and a land surface scheme to model the resulting growth. These models have previously been used to estimate the impact of mass burn-offs in the Amazon region.
The modelling allowed researchers to explore the relatively uncertain range of global BVOCs production, as well as their changing impact on plant response – with some regions actually suffering a small decrease in growth.
The greatest impacts were in South America and central Africa, where direct sunlight scattered into an additional 10 watts of diffuse lighting across each square metre. This drove growth which sequestered an additional 0.2 grams of carbon per day per square metre. Taken globally, the growth more than compensated for the decline in certain regions.