The project will explore structure-function relationships of the water transport pathway of commercially important woody species (Populus tremula × P. tremuloides, Betula pendula) in light of global climate change focusing on effects of increasing air humidity predicted for northern Europe. Plant structural responses to artificially elevated air humidity will be examined on saplings with respect to plant functioning in field conditions. The measurements will comprise relevant anatomical characteristics of leaves, branches and main stems as well as multiple physiological traits. The aim is to estimate the contribution of plant structural changes to acclimation of water relations, hydraulic capacity and gas exchange to altered environmental conditions. We hypothesize that reduced sap flow in trees growing under elevated relative humidity (RH) will induce changes in plant anatomical characteristics having impact on functional traits. Preliminary results of the FAHM experiment confirm the humidification effect on both functional and structural characteristics. We assume that the shifts in wood density result from changes in xylem anatomical structure having an inevitable impact on hydraulic properties: branch and stem hydraulic conductance, xylem conductivity and its vulnerability to cavitation. Humid conditions should lead to higher xylem functional vulnerability and stomatal sensitivity to vapour pressure deficit, thus making the plants more susceptible to environmental fluctuations. To test the hypotheses, we seek answers to the following questions: (1) What changes in leaf anatomical structure will be induced under elevated RH and how will they influence leaf gas exchange parameters? (2) To what extent leaf structural modifications affect leaf hydraulic conductance and how the latter interacts with stomatal conductance? (3) What changes in xylem structure will be induced by reduced water flux in trees and whether they have consequences to xylem hydraulic efficiency?