Climate change-induced rise of air temperatures and the increase of extreme climatic events, such as droughts, will largely affect plant growth and hydraulics, leading to mortality events all over the globe. In this study, we investigated the growth and hydraulic responses of seedlings of contrasting functional types. Pinus sylvestris, Quercus spp. and Castanea sativa seedlings were grown in a common garden experiment under four treatments: control, air warming, drought and their combination

more »

... ing two consecutive growing periods. Height and diameter increments, stomatal conductance and stem water potentials were measured during both growing seasons. Additionally, hydraulic parameters such as xylem-specific native and maximum hydraulic conductivities, and native percentage of loss of conductivity were measured at the end of the entire experiment. Our results clearly pointed to different adaptive strategies of the studied species. Scots pine displayed a relatively isohydric behavior with a strict stomata control prohibiting native embolism whereas sweet chestnut and oak as relatively anisohydric species displayed an increased loss of native conductivity as a results of low water potentials. Seasonal timing of shoot and diameter growth also differed among functional types influencing drought impacts. Additionally, the possibility of embolism reversal seemed to be limited under the study conditions. understanding of climate change-induced impacts, such as species-specific physiological thresholds of drought severity and duration [6] . Forests acclimation comprises molecular, physiological, and structural adjustments [7] . In plants, the water movement is initiated by transpiration through the stomata pulling water from the roots to the leaves through the xylem. This pathway connects soil and atmosphere through the plant and is well known as the soil-plant-atmosphere-continuum (SPAC) [8] . Plants have developed different strategies to cope with drought by adjusting different traits along the SPAC such as a reduction of leaf area [9] or an increase of the sapwood area-to-leaf area ratio [10] . Therefore, the study of plant hydraulic traits has lately become the main approach to understand plant vulnerability to fast changing conditions [11] . Plants have different physiological strategies to cope with drought. During dry spells, stomatal closure is the main plant mechanism to limit water loss, and thus, to maintain water potentials within the safety margins. Therefore, species have been classified in two groups based on the degree of stomatal closure in response to drought [12, 13] , although recent studies claim that there is a continuum between isohydric and anisohydric behaviors [14, 15] . On the one side, relatively isohydric species show moderate constant maximum values of water potentials which means that stomatal closure occurs faster, avoiding water loss through transpiration. These species have been described as more prone to suffer carbon starvation during prolonged dry spells [6], since the early stomatal closure also stops assimilation, and thus, plants are obliged to rely on carbon reserves [16] . On the other side, relatively anisohydric species perform a more relaxed stomatal regulation allowing water potentials to reach more negative values during drought conditions. As a consequence, carbon assimilation is not interrupted, but during intense drought events cavitation may occur inside the xylem conduits. Cavitation breaks the continuity of water columns and hence the water supply to transpiring leaves. Xylem embolism reduces hydraulic conductance and may ultimately result in the hydraulic failure of the plant hydraulic system [17] . These two different strategies include several trade-offs between the level of xylem tension and carbon uptake through photosynthesis. Thus, the strategy followed by a given tree species and the intensity and duration of the dry spells are crucial to determine tree performance and survival. Several studies have claimed the possibility of refilling gas-filled conduits driven by over-atmospheric root/stem pressures originating during night or specific periods of the year [18, 19] or even when the xylem is still under tension driven by phloem unloading [20] . Although the refilling of the gas-filled conduits has been better explained in angiosperms, it also seems to occur in conifers [21] , but the mechanisms behind might differ from those proposed for angiosperms due to xylem structural differences. However, the re is some controversy about conduit refilling since it could be affected by measurement artifacts [22] . Nevertheless, the study of hydraulic traits such as hydraulic conductivity, stomatal conductance and water potentials of different functional tree types (isohydric vs. anisohydric, conifer vs. angiosperm), and the species-specific ability of conduits refilling under controlled conditions is a prerequisite to assess species resilience under future climatic conditions. Plants may have benefited from the rise in temperature, for instance, due to an extension of the growing season [23, 24] . Although tree responses are usually species-specific, the increase of temperature alone can also enhance plant growth [25] through higher carbon assimilation if water availability is not restricted [26] , but can promote drought stress by raising the water vapor deficit [27] . Drought impacts on tree physiology describe a circle that starts by affecting water transport and assimilation and continues with allocation processes such as growth [28] . Several studies have associated the drought-induced growth reduction to physiological tree decline and with overall tree mortality [29, 30] . In fact, tissue formation is inhibited by drought long before carbon supply falls short because of drought-induced limitations of gas exchange [31] . Moreover, plant-plant interactions seem to have an interspecific effect on the drought tolerance of tree species to cope with future climate scenarios [32] [33] [34] , since competition decreases individual radial growth as a result of reduced nutrient and water availability [35] . Previous studies have suggested that tree-to-tree competition, as a long-term predisposing stressor, may be an additional risk factor for drought-induced mortality in