Global warming is accompanied by changes in atmospheric water vapour content and precipitation rate, although there will be pronounced regional differences in their magnitude and direction . Over a period from 1900 to 2005 precipitation has significantly increased in northern Europe and continuation of this trend with larger increase in the frequency than in the magnitude of precipitation is predicted from climatic models. Climate change scenarios predict by the end of the century increases in air temperature by 3.5–5ºC and precipitation by 5–30% in boreal and northern temperate regions of Europe [2, 3]. Increase in atmospheric relative humidity (RH), the inevitable result of more frequent rainfall events, will reduce water loss through transpiration [4, 5], and affect both the delivery of nutrients to root absorbing surface and nutrient uptake by trees due to diminished water fluxes through the vegetation [6, 7].
On the other hand, climate extremes including heat waves and droughts across Europe are projected to become more frequent and enduring over the 21st century [1, 8]. Because trees have adapted to local average climatic conditions, extreme events have consequences on forest health and productivity across site conditions [9, 10]. Plants growing in humid air have less effective stomatal control over transpirational water loss [4, 11, 12] and demonstrate higher vulnerability to xylem cavitation, i.e. have narrow hydraulic safety margin [13, 14]. In addition, Okamoto et al.  demonstrated that high air humidity induces abscisic acid (ABA) 8′-hydroxylase in stomata and vasculature, followed by the reduction of ABA levels - a plant hormone, which promotes stomatal closure under water deficit .
Water deficit decreases stomatal conductance before leaf water potential (ΨL) falls below critical values, to avoid adverse consequences on leaf tissues (dehydration of protoplasm) and water transport system (hydraulic dysfunction through runaway xylem cavitation). However, the mechanisms by which stomata respond to and control ΨL are still unclear [14, 17]. The classical view suggests that a primary signal of water shortage is ABA, produced by roots situated in dry soil and transported to shoots . As a result, a considerable time lag is expected in the response of stomata to changing soil water status. Soil drying concentrates ABA in both the xylem sap and leaves [19–21]. This is followed by water efflux from guard cells and stomatal closure . Stricter stomatal control leads to increasing short-term (intrinsic water-use efficiency ) and long-term water-use efficiency (carbon isotope discrimination ).
In Arabidopsis, shoot vascular tissues appear to be a major site of ABA biosynthesis and suggest tissue-autonomous ABA synthesis in addition to its long-distance root-to-shoot movement [16, 25]. Bauer et al.  report that guard cells possess the entire ABA biosynthesis pathway and that cell-autonomous synthesis is sufficient for stomatal closure. Thus, effects of fast changes in leaf water status do not involve chemical signals from roots, but rather are predominantly hydraulic [22, 27, 28]. Guard cells respond to changes in ΨL either directly or via a signal generated close by . Stomatal closure, in turn, will increase stomatal limitation to photosynthesis. At severe water deficit, efficiency of photosystem II will decrease as well [12, 30, 31] further impelling decline of CO2 assimilation.
The structure and function of the water transport system govern the productivity and survival of land plants because the vascular architecture places a physical limit on plant functioning [29, 32]. Therefore, the water pathway from the soil-root interface to the sites of evaporation in leaves is critical to maintain leaf water status and hold stomata open, keeping a positive carbon budget. Water deficit will induce cavitation of xylem elements in roots, stems and leaf veins [10, 33, 34], thereby reducing water supply to foliage and amplifying water deficit effects on stomatal conductance and photosynthetic performance. Tissue dehydration also impacts aquaporin (AQP) expression controlling hydraulic conductance of the leaf symplastic compartment . Furthermore, as the concentration of ABA increases in the xylem, AQP activity in the bundle sheath cells is down-regulated, thereby reducing water flow into the leaf as demonstrated by Shatil-Cohen et al. .
We analysed the impact of water deficit on plant water status, gas exchange and hydraulic conductance on saplings of silver birch (Betula pendula Roth) under artificially manipulated air humidity in field conditions. Silver birch is distributed widely over almost all of Europe, and in northern Europe it is among the most important commercial tree species. Because trees growing in moist atmosphere experience less water loss and have higher stomatal openness, we hypothesize that physiological characteristics in trees acclimated to higher humidity exhibit higher susceptibility to rapidly-induced water deficit. The primary aim of this study was to test this hypothesis experimentally. Secondly we tested whether the putative trade-off between plant hydraulic capacity and water-use efficiency (WUE) is observable on a short time scale. We aimed this study to broaden the understanding of the ability of trees to acclimate with the increasing atmospheric humidity predicted for northern Europe.