An experimental facility for free air humidity manipulation (FAHM) can alter water flux through deciduous tree canopy

A facility for free air humidity manipulation (FAHM) was established to investigate the effect of increased air humidity on trees’ performance and their canopy functioning with respect to rising air humidity predicted for Northern Europe. The FAHM system enables air relative humidity (RH) to be increased up to 18 units (%) over the ambient level during mist fumigation, depending on the wind speed inside the experimental stand. Water was dispersed inside 14 × 14 m experimental plots in the form of mist with an average particle size of 50 μm from June to August in 2008, and from May to September in 2009. The average increase in RH was 7 units (%) over the whole period of humidification in 2008 ( P F ) in silver birch ( Betula pendula Roth.) trees was 24.8% ( P P P > 0.05) in silver birch on the days without misting. In hybrid aspen ( Populus tremula L. × P. tremuloides Michx.) the average difference in F between C and H plots was 61.1% ( P P F as a result of long-term acclimation to fumigation. The leaves of silver birch in a humidified plot demonstrated up to 2.4 °C lower ( P T L ) compared to the control plot in 2009. The decline in T L decreased the humidity gradient between leaf and air by about 1/3, whereas 2/3 of the effect was caused directly by changes in air humidity in the leaf boundary layer. Our preliminary data suggest that the FAHM experimental facility enables water fluxes through a deciduous tree canopy to be reduced and this effect is attributable both to the increased air humidity and decreased leaf temperature. Changes in these two basic factors may create considerable differences in the physiology, anatomy and nutrition of a whole tree, also affecting forest functioning in the light of global climate change.