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As climate change progresses, the frequency and duration of drought stress events are increasing. While the mechanisms of drought acclimation of trees has received considerable attention in recent years, the recovery processes remain critically understudied. We used a unique throughfall exclusion experiment in a mature temperate mixed forest consisting of the more isohydric Norway spruce and more anisohydric European beech, to study the recovery and resilience after drought release. We hypothesized that pre-dawn water potential (ΨPD) of both species will increase within 1 day after watering, while the recovery of stomatal conductance (g s) and the reversal of osmoregulation will be significantly delayed in the more isohydric spruce. Furthermore, we hypothesized that the xylem sap flow density (u daily) will not fully recover within the growing season due to the strong drought impact. After 5 years of summer drought, trees showed significantly reduced ΨPD, u daily and increased osmoregulation in leaves, but only isohydric spruce displayed increased leaf abscisic acid concentrations. In line with our hypothesis, ΨPD and g s recovered within 1 day in beech. Conversely, isohydric spruce showed delayed increases in ΨPD and g s. The delay in recovery of spruce was partially related to the replenishment of the stem water reservoir, as indicated by the missing response of u daily at the crown base compared with DBH level upon watering. However, u daily fully recovered only in the next growing season for beech and was still reduced in spruce. Nevertheless, in both species, osmotic acclimations of leaves were reversed within several weeks. While both species displayed full resilience to drought stress in water-related physiology, the recovery time was in several cases, e.g. u daily, ΨPD and g s, shorter for beech than for spruce. With future increases in the frequency of drought events under ongoing climate change, tree species that recover more quickly will be favored. [ABSTRACT FROM AUTHOR]

Under ongoing global climate change, drought periods are predicted to increase in frequency and intensity in the future. Under these circumstances, it is crucial for tree's survival to recover their restricted functionalities quickly after drought release. To elucidate the recovery of carbon (C) transport rates in c. 70‐year‐old Norway spruce (Picea abies [L.] KARST.) after 5 years of recurrent summer droughts, we conducted a continuous whole‐tree 13C labeling experiment in parallel with watering. We determined the arrival time of current photoassimilates in major C sinks by tracing the 13C label in stem and soil CO2 efflux, and tips of living fine roots. In the first week after watering, aboveground C transport rates (CTR) from crown to trunk base were still 50% lower in previously drought‐stressed trees (0.16 ± 0.01 m h−1) compared to controls (0.30 ± 0.06 m h−1). Conversely, CTR below ground, that is, from the trunk base to soil CO2 efflux were already similar between treatments (c. 0.03 m h−1). Two weeks after watering, aboveground C transport of previously drought‐stressed trees recovered to the level of the controls. Furthermore, regrowth of water‐absorbing fine roots upon watering was supported by faster incorporation of 13C label in previously drought‐stressed (within 12 ± 10 h upon arrival at trunk base) compared to control trees (73 ± 10 h). Thus, the whole‐tree C transport system from the crown to soil CO2 efflux fully recovered within 2 weeks after drought release, and hence showed high resilience to recurrent summer droughts in mature Norway spruce forests. This high resilience of the C transport system is an important prerequisite for the recovery of other tree functionalities and productivity. [ABSTRACT FROM AUTHOR]

Hydraulic redistribution (HR) can buffer drought events of tree individuals, however, its relevance for neighbouring trees remains unclear. Here, we quantified HR to neighbouring trees in single‐ and mixed‐species combinations. We hypothesized that uptake of HR water positively correlates with root length, number of root tips and root xylem hydraulic conductivity and that neighbours in single‐species combinations receive more HR water than in phylogenetic distant mixed‐species combinations. In a split‐root experiment, a sapling with its roots split between two pots redistributed deuterium labelled water from a moist to a dry pot with an additional tree each. We quantified HR water received by the sapling in the dry pot for six temperate tree species. After 7 days, one quarter of the water in roots (2.1 ± 0.4 ml), stems (0.8 ± 0.2 ml) and transpiration (1.0 ± 0.3 ml) of the drought stressed sapling originated from HR. The amount of HR water transpired by the receiving plant stayed constant throughout the experiment. While the uptake of HR water increased with root length, species identity did not affect HR as saplings of Picea abies ((L.) Karst) and Fagus sylvatica (L.) in single‐ and mixed‐species combinations received the same amount of HR water. We quantified hydraulically redistributed water towards neighbouring saplings of six temperate tree species. One quarter of the water found in roots, shoots and transpiration originated from HR, with no differences due to species mixture, emphasizing the major importance of HR for temperate trees. [ABSTRACT FROM AUTHOR]

Phloem sustains maintenance and growth processes through transport of sugars from source to sink organs. Under low water availability, tree functioning is impaired, i.e. growth/photosynthesis decline and phloem transport may be hindered. In a 3-year throughfall exclusion (TE) experiment on mature European beech (Fagus sylvatica L.) we conducted 13CO2 branch labeling to investigate translocation of recently fixed photoassimilates under experimental drought over 2 years (2015 and 2016). We hypothesized (H1) that mean residence time of photoassimilates in leaves (MRT) increases, whereas (H2) phloem transport velocity (V phloem) decreases under drought. Transport of carbohydrates in the phloem was assessed via δ13C of CO2 efflux measured at two branch positions following 13CO2 labeling. Pre-dawn water potential (ΨPD) and time-integrated soil water deficit (iSWD) were used to quantify drought stress. The MRT increased by 46% from 32.1 ± 5.4 h in control (CO) to 46.9 ± 12.3 h in TE trees, supporting H1, and positively correlated (P < 0.001) with iSWD. Confirming H2, V phloem in 2016 decreased by 47% from 20.7 ± 5.8 cm h–1 in CO to 11.0 ± 2.9 cm h–1 in TE trees and positively correlated with ΨPD (P = 0.001). We suggest that the positive correlation between MRT and iSWD is a result of the accumulation of osmolytes maintaining cell turgor in the leaves under longer drought periods. Furthermore, we propose that the positive correlation between V phloem and ΨPD is due to a lower water uptake of phloem conduits from surrounding tissues under increasing drought leading to a higher phloem sap viscosity and lower V phloem. The two mechanisms increasing MRT and reducing V phloem respond differently to low water availability and impair trees' carbon translocation under drought. [ABSTRACT FROM AUTHOR]