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Several climate change scenarios predict increasing precipitation for northern latitudes and several other regions in the globe, leading to increase in atmospheric water vapour content [expressed as increased relative humidity (RH) or as decreased water vapour pressure deficit (VPD)] and environmental wetness. Plants are known to be sensitive to high humidity (low VPD) as indicated by changes in stomatal function, transpiration, mineral nutrient uptake, growth, development, sugar metabolism and leaf epicuticular wax composition in several species and studies. To understand the impact of increasing humidity (lower VPD) on forest ecosystems, a long-term and large-scale field experiment (FAHM, Free Air Humidity Manipulation) is conducted in Estonia with silver birch and hybrid aspen, representing widespread deciduous species in northern Europe. The experiment revealed that high humidity is an important climatic factor, causing reduction in growth rate of the aboveground parts, leaf biomass and area, bud size, sap flux and earlier bud break and delayed leaf fall. In the belowground parts, root biomass (fine-root biomass in particular) was increased although the root compartment was exposed to anaerobic conditions, increased soil pH and soil water potential, with lowered soil respiration and altered microbial and fungal communities. In the leaves, high humidity shifted the metabolism towards nonstructural carbohydrates, antioxidants and phenolic compounds, while nutrient (N and P) content, photosynthesis, dark respiration, hydraulic conductance and the density of glandular trichomes were reduced. The change in the chemical composition of the surface wax layer resulted in lower hydrophobicity, exposing the leaves for fungal pathogen attacks. In the stem, N and P content in wood and number of living parenchyma cells were increased, while the stem wood density was reduced and wood chemistry altered. Reduction in the growth rates under high humidity was more pronounced in hybrid aspen than in silver birch. Birch showed more efficient adjustment of leaf and root morphology, hydraulic architecture, primary carbon and nitrogen metabolism and leaf surface properties mitigating the negative impacts of high humidity. At the ecosystem level, high humidity altered soil processes (moisture content, microbiota, nitrification), with potential to affect competition and species composition. A complementary laboratory study with birch demonstrated that additional N supply can counteract the effects of low VPD on cellular metabolism and leaf surface wax quality.