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Drought reduces the carbon (C) assimilation of trees and decouples aboveground from belowground carbon fluxes, but little is known about the response of drought-stressed trees to rewetting. This study aims to assess dynamics and patterns of C allocation in beech saplings under dry and rewetted soil conditions. In October 2010, 5-year-old beech saplings from a forest site were transplanted into 20 l pots. In 2011, the saplings were subjected to different levels of soil drought ranging from non-limiting water supply (control) to severe water limitation with soil water potentials of less than -1.5 MPa. As a physiologically relevant measure of drought, the cumulated soil water potential (i.e., drought stress dose (DSD)) was calculated for the growing season. In late August, the saplings were transferred into a climate chamber and pulse-labeled with (13)C-depleted CO2 (δ(13)C of -47‰). Isotopic signatures in leaf and soil respiration were repeatedly measured. Five days after soil rewetting, a second label was applied using 99 atom% (13)CO2. After another 12 days, the fate of assimilated C in each sapling was assessed by calculating the (13)C mass balance. Photosynthesis decreased by 60% in saplings under severe drought. The mean residence time (MRT) of recent assimilates in leaf respiration was more than three times longer than under non-limited conditions and was positively correlated to DSD. Also, the appearance of the label in soil respiration was delayed. Within 5 days after rewetting, photosynthesis, MRT of recent assimilates in leaf respiration and appearance of the label in soil respiration recovered fully. Despite the fast recovery, less label was recovered in the biomass of the previously drought-stressed plants, which also allocated less C to the root compartment (45 vs 64% in the control). We conclude that beech saplings quickly recover from extreme soil drought, although transitional after-effects prevail in C allocation, possibly due to repair-driven respiratory processes.

Soil drought influences the C turnover as well as the fine‐root system of tree saplings. Particularly during the period of establishment, the susceptibility to drought stress of saplings is increased because of incompletely developed root systems and reduced access to soil water. Here, we subjected beech saplings (Fagus sylvatica L.) to different levels of drought stress. Beech saplings were planted in rhizotrons, which were installed in the soil of a Norway spruce forest before bud burst. Soil moisture was manipulated in the following year during May to September. We measured photosynthetic net CO2 uptake, volume production of fine roots, and rhizosphere respiration during the growing season. Biometric parameters of the fine‐root system, biomass, and nonstructural carbohydrates were analyzed upon harvest in October. Photosynthesis and rhizosphere respiration decreased with increasing drought‐stress dose (cumulated soil water potential), and cumulative rhizosphere respiration was significantly negatively correlated with drought‐stress dose. Fine‐root length and volume production were highest at moderate soil drought, but decreased at severe soil drought. The proportion of fine‐roots diameter < 0.2 mm and the root‐to‐shoot ratio increased whereas the live‐to‐dead ratio of fine roots decreased with increasing drought‐stress dose. We conclude that the belowground C allocation as well as the relative water‐uptake efficiency of beech saplings is increased under drought.

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&lt;p&gt;Accurate parametrization and validation of SVAT- or evapotranspiration-models requires robust estimates of transpiration and conductivity on the level of individual leaves. Such estimates are commonly made from measurements with mobile gas exchange systems, which allow precise measurements of leaf transpiration. However, this method has some decisive practical (expensive and labor intensive, both constraining feasible number of replicates) as well as methodological limitations (destruction of the leaf boundary layer). In order to validate the FO3REST model &amp;#8211; which estimates the phytotoxic ozone uptake of forest stands &amp;#8211; a sensor was required that continuously measures leaf transpiration and conductivity with a high number of replicates. Within the valORTree project, which was carried out from 2019-2021 in the climate chambers of the TUMmesa ecotron facility (J&amp;#225;kli et al. 2021), a novel, low-cost leaf sensor (&quot;TransP&quot;) was developed that enables continuous &lt;em&gt;in-situ &lt;/em&gt;determination of transpiration and conductivity for the important forest tree species beech (&lt;em&gt;Fagus sylvatica&lt;/em&gt; L.) and Norway spruce (&lt;em&gt;Picea abies &lt;/em&gt;(L.) H. Karst.) over the entire growing season. The sensor records different temperatures in the leaf/needle environment and was calibrated against the gravimetrically determined transpiration rate (r&lt;sup&gt;2&lt;/sup&gt; = 0.74 for beech; r&lt;sup&gt;2&lt;/sup&gt; = 0.84 for spruce). Measurement inaccuracies can be compensated for by using many of the inexpensive sensors in parallel. The sensor output was validated against measurements using Li-6400 and Li-6800 gas exchange systems (Licor, USA). Differences in the outputs of the two methods could be explained by the fact that the Licor systems measures transpiration based on stomatal conductance, whereas TransP includes the &lt;em&gt;in-situ&lt;/em&gt; boundary layer resistance. So far, the sensor has been applied under low-wind conditions in indoor applications and is currently further developed for application in the field.&lt;/p&gt;&lt;p&gt;However, we clearly show that measuring transpiration of beech leaves and spruce needles with the TransP sensor provides robust data. Since TransP operation is minimally invasive and the leaf boundary layer is preserved during measurements, it is assumed that the sensor provides a realistic representation of the &lt;em&gt;in-situ&lt;/em&gt; transpiration of individual leaves/needles. In addition, the high temporal resolution of the measurements provides the ability to accurately integrate transpiration over the entire period of the measurement.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;Reference&lt;/p&gt;&lt;p&gt;J&amp;#225;kli, B., Meier, R., Gelhardt, U., Bliss, M., Gr&amp;#252;nhage, L., &amp; Baumgarten, M. (2021). Regionalized dynamic climate series for ecological climate impact research in modern controlled environment facilities.&amp;#160;&lt;em&gt;Ecology and evolution&lt;/em&gt;,&amp;#160;&lt;em&gt;11&lt;/em&gt;(23), 17364-17380.&lt;/p&gt;