Carbon allocation to root exudates is maintained in mature temperate tree species under drought.

Carbon (C) exuded via roots is proposed to increase under drought and facilitate important ecosystem functions. However, it is unknown how exudate quantities relate to the total C budget of a drought-stressed tree, that is, how much of net-C assimilation is allocated to exudation at the tree level. We calculated the proportion of daily C assimilation allocated to root exudation during early summer by collecting root exudates from mature Fagus sylvatica and Picea abies exposed to experimental drought, and combining above- and belowground C fluxes with leaf, stem and fine-root surface area. Exudation from individual roots increased exponentially with decreasing soil moisture, with the highest increase at the wilting point. Despite c. 50% reduced C assimilation under drought, exudation from fine-root systems was maintained and trees exuded 1.0% (F. sylvatica) to 2.5% (P. abies) of net C into the rhizosphere, increasing the proportion of C allocation to exudates two- to three-fold. Water-limited P. abies released two-thirds of its exudate C into the surface soil, whereas in droughted F. sylvatica it was only one-third. Across the entire root system, droughted trees maintained exudation similar to controls, suggesting drought-imposed belowground C investment, which could be beneficial for ecosystem resilience.

High resilience of carbon transport in long-term drought-stressed mature Norway spruce trees within 2 weeks after drought release.

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 13 C labeling experiment in parallel with watering. We determined the arrival time of current photoassimilates in major C sinks by tracing the 13 C 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 13 C 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.

Dynamics of initial carbon allocation after drought release in mature Norway spruce-Increased belowground allocation of current photoassimilates covers only half of the carbon used for fine-root growth.

After drought events, tree recovery depends on sufficient carbon (C) allocation to the sink organs. The present study aimed to elucidate dynamics of tree-level C sink activity and allocation of recent photoassimilates (Cnew ) and stored C in c. 70-year-old Norway spruce (Picea abies) trees during a 4-week period after drought release. We conducted a continuous, whole-tree 13 C labeling in parallel with controlled watering after 5 years of experimental summer drought. The fate of Cnew to growth and CO2 efflux was tracked along branches, stems, coarse- and fine roots, ectomycorrhizae and root exudates to soil CO2 efflux after drought release. Compared with control trees, drought recovering trees showed an overall 6% lower C sink activity and 19% less allocation of Cnew to aboveground sinks, indicating a low priority for aboveground sinks during recovery. In contrast, fine-root growth in recovering trees was seven times greater than that of controls. However, only half of the C used for new fine-root growth was comprised of Cnew while the other half was supplied by stored C. For drought recovery of mature spruce trees, in addition to Cnew , stored C appears to be critical for the regeneration of the fine-root system and the associated water uptake capacity.

Acclimation of mature spruce and beech to five years of repeated summer drought - The role of stomatal conductance and leaf area adjustment for water use.

Forests globally are experiencing severe droughts, leading to significant reductions in growth, crown dieback and even tree mortality. The ability of forest ecosystems to acclimate to prolonged and repeated droughts is critical for their survival with ongoing climate change. In a five-year throughfall exclusion experiment, we investigated the long-term physiological and morphological acclimation of mature Norway spruce (Picea abies [L.] KARST.) and European beech (Fagus sylvatica L.) to repeated summer drought at the leaf, shoot and whole tree level. Throughout the drought period, spruce reduced their total water use by 70 % to only 4-9 L per day and tree, while beech was less affected with about 30 % reduction of water use. During the first two summers, spruce achieved this by closing their stomata by up to 80 %. Additionally, from the second drought summer onwards, spruce produced shorter shoots and needles, resulting in a stepwise reduction of total leaf area of over 50 % by the end of the experiment. Surprisingly, no premature leaf loss was observed. This reduction in leaf area allowed a gradual increase in stomatal conductance. After the five-year drought experiment, water consumption per leaf area was the same as in the controls, while the total water consumption of spruce was still reduced. In contrast, beech showed no significant reduction in whole-tree leaf area, but nevertheless reduced water use by up to 50 % by stomatal closure. If the restriction of transpiration by stomatal closure is sufficient to ensure survival of Norway spruce during the first drought summers, then the slow but steady reduction in leaf area will ensure successful acclimation of water use, leading to reduced physiological drought stress and long-term survival. Neighboring beech appeared to benefit from the water-saving strategy of spruce by using the excess water.

Physiological recovery of tree water relations upon drought release-response of mature beech and spruce after five years of recurrent summer drought.

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 (gs) and the reversal of osmoregulation will be significantly delayed in the more isohydric spruce. Furthermore, we hypothesized that the xylem sap flow density (udaily) 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, udaily and increased osmoregulation in leaves, but only isohydric spruce displayed increased leaf abscisic acid concentrations. In line with our hypothesis, ΨPD and gs recovered within 1 day in beech. Conversely, isohydric spruce showed delayed increases in ΨPD and gs. The delay in recovery of spruce was partially related to the replenishment of the stem water reservoir, as indicated by the missing response of udaily at the crown base compared with DBH level upon watering. However, udaily 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., udaily, ΨPD and gs, 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.