Led by the University of Bonn, Germany, with contributions from the Centre for Ecology & Hydrology in West Lothian as well as from Croatia and the US, the glasshouse study compared the growth of seedlings of Scots pine, silver fir and common oak in air free of particulate matter and in unfiltered air.
The seedlings in filtered air transpired less water than those in Bonn's "moderately polluted" air, they found.
Atmospheric particulate matter tends to be hygroscopic, absorbing water from the air. The researchers suggest that this manifests itself as a fine film of salt solution on leaf surfaces, visible via electron microscope, which can spread into leaf stomata to form a wick that carries water to the outer surface of the leaf.
Dr Juergen Burkhardt of the Institute of Bonn University's Crop Science and Resource Conservation (INRES) said: "The stomata partially lose control of transpiration and plants become more at risk from drought."
The research was prompted by the phenomenon being increasingly observed globally of forest damage after dry years.
Burkhardt added: "There have been different explanations for these regional events. These experiments point to a direct, but so far missing, link between air pollution and drought vulnerability."
The findings are published in Environmental Research Letters.