Evaporation from forests in cloud enhances the effects of acid deposition

Declining growth rates and injury to foliage have been observed in forests at high elevations in areas of Europe and North America 1,2 ; in such areas the trees are regularly enveloped in cloud and mist. Concentrations of dissolved chemicals in cloud drops are often much larger than in rain, so the capture of cloud droplets by leaves may give rise to significant rates of ‘occult deposition’ 3 of acidic pollutants and heavy metals on forests 3,4 . I point out here that evaporation of intercepted cloudwater from forest canopies, proceeding simultaneously with occult deposition, can lead to chemical concentrations on leaf surfaces that are substantially larger than those measured in cloud drops themselves. These large chemical concentrations may be a cause of the observed injury in forests where wind-driven cloud is common.