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Spring drought is becoming a frequently occurring stress factor in temperate forests. However, the understanding of tree resistance and resilience to the spring drought remains insufficient. In this study, European beech (Fagus sylvatica L.) seedlings at the early stage of leaf development were moderately and severely drought stressed for 1 month and then subjected to a 2-week recovery period after rewatering. The study aimed to disentangle the complex relationships between leaf gas exchange, vascular anatomy, tree morphology and patterns of biomass allocation. Stomatal conductance decreased by 80 and 85% upon moderate and severe drought stress, respectively, which brought about a decline in net photosynthesis. However, drought did not affect the indices of slow chlorophyll fluorescence, indicating no permanent damage to the light part of the photosynthetic apparatus. Stem hydraulic conductivity decreased by more than 92% at both drought levels. Consequently, the cambial activity of stressed seedlings declined, which led to lower stem biomass, reduced tree ring width and a lower number of vessels in the current tree ring, these latter also with smaller dimensions. In contrast, the petiole structure was not affected, but at the cost of reduced leaf biomass. Root biomass was reduced only by severe drought. After rewatering, the recovery of gas exchange and regrowth of the current tree ring were observed, all delayed by several days and by lower magnitudes in severely stressed seedlings. The reduced stem hydraulic conductivity inhibited the recovery of gas exchange, but xylem function started to recover by regrowth and refilling of embolized vessels. Despite the damage to conductive xylem, no mortality occurred. These results suggest the low resistance but high resilience of European beech to spring drought. Nevertheless, beech resilience could be weakened if the period between drought events is short, as the recovery of severely stressed seedlings took longer than 14 days.

Abscisic acid (ABA) is one of the most common stress signals that appear in plant organs in response to soil drying. Equilibrium between ABA biosynthesis and catabolism regulates ABA accumulation in plants under water stress. The aim of our work was to explore the dynamics of changes in ABA metabolites as well as other stress-induced phytohormones such as jasmonic acid, indole-3-acetic acid, and their respective metabolites in hop [Humulus lupulus (L.)] plants during drying and to identify among them potential signals involved in drought signalling. We showed that the concentrations of all ABA metabolites (except the concentration of ABA glucosyl ester in leaves) increased in the same manner in leaves and xylem sap approximately at the same level of soil water content when the relative water content of leaves decreased. The predominant metabolites in leaves and xylem sap were phaseic acid and dihydroxyphaseic acid. ABA glucosyl ester was not a source of the increased concentration of ABA in leaves and xylem sap because of its considerably lower concentration compared to ABA. The concentration of jasmonates decreased in leaves of hop plants. Changes in auxin concentration suggest that this hormone is involved in the response of hop plants to soil drying.

Spring drought episodes are becoming more frequent and intensive in European temperate forests. To study tree resilience to spring drought, Norway spruce seedlings were exposed to three levels of drought stress (well-watered (W), moderately stressed (M), and severely stressed (S)) for 42 days and then fully irrigated for 14 days. Drought strongly reduced gas exchange parameters for both M and S seedlings. After 42 days, stomatal conductance was lower by 83% and 97% in M and S, respectively, than in W seedlings. Respiration prevailed over photosynthesis in S seedlings at the end of the drought period. Drought mostly reduced longitudinal growth, especially in shoots and needles. Xylem growth reduction was caused mainly by lower number of newly produced tracheids, not by changes in their size. Norway spruce seedlings showed good resilience to spring drought, as the observed physiological parameters started to recover after rewatering and seedlings started to sprout and form new tracheids. In M seedlings, all physiological traits recovered to the level of W seedlings during the 14-day irrigation period but the recovery took longer in S seedlings. Shoots and needles did not regrow in length but leaf mass per area increased during the recovery phase. To conclude, Norway spruce seedlings showed good resilience to spring single-drought-event, but time necessary to full recovery from stress could make seedlings more vulnerable to recurrent drought events.

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