Summary• Recent 13 CO 2 canopy pulse chase labeling studies revealed that photosynthesis influences the carbon isotopic composition of soil respired CO 2 (d 13 C SR ) even on a diel timescale. However, the driving mechanisms underlying these short-term responses remain unclear, in particular under drought conditions.• The gas exchange of CO 2 isotopes of canopy and soil was monitored in drought ⁄ nondrought-stressed beech (Fagus sylvatica) saplings after 13 CO 2 canopy pulse labeling. A combined canopy ⁄ soil chamber system with gas-tight separated soil and canopy compartments was coupled to a laser spectrometer measuring mixing ratios and isotopic composition of CO 2 in air at high temporal resolution. The measured d 13 C SR signal was then explained and substantiated by a mechanistic carbon allocation model.• Leaf metabolism had a strong imprint on diel cycles in control plants, as a result of an alternating substrate supply switching between sugar and transient starch. By contrast, diel cycles in drought-stressed plants were determined by the relative contributions of autotrophic and heterotrophic respiration throughout the day. Drought reduced the speed of the link between photosynthesis and soil respiration by a factor of c. 2.5, depending on the photosynthetic rate.• Drought slows the coupling between photosynthesis and soil respiration and alters the underlying mechanism causing diel variations of d 13 C SR .

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ABSTRACT

Biophysical and biochemical traits of foliage and twigs at the top of tree canopies provide essential information on trees' ecophysiology. Conventional methods used for canopy sampling are typically time consuming and costly, while the very canopy tops are still out of reach unless a canopy crane is used. Thus, we developed a novel twig sampling method using a device attached to a drone that allows to grasp and cut small twigs and bring them to the ground for immediate analysis. This "Flying Tree Top Sampler" (FTTS) complements existing methods used in tree canopy research. This paper describes the method to make it openly available to interested users, provides the necessary technical details, and reports on a proof-of-concept application by probing a single tree during senescence. In this demonstration example we show that the ability to investigate the top part of the tree leads to the clear finding that chlorophyll content of the top leaves is significantly lower than that of leaves in the lower canopy. Without the samples collected by the FTTS, the decrease of chlorophyll content with height would not have been of statistical significance using a p = 0.05 significance threshold. Besides its advantages, the existing limitations of the FTTS are discussed, and suggestions for future developments are provided.

On-line measurements of photosynthetic carbon isotope discrimination ((13)Δ) under field conditions are sparse. Hence, experimental verification of the natural variability of instantaneous (13)Δ is scarce, although (13)Δ is, explicitly and implicitly, used from leaf to global scales for inferring photosynthetic characteristics. This work presents the first on-line field measurements of (13)Δ of Fagus sylvatica branches, at hourly resolution, using three open branch bags and a laser spectrometer for CO₂ isotopologue measurements (QCLAS-ISO). Data from two August/September field campaigns, in 2009 and 2010, in a temperate forest in Switzerland are shown. Diurnal variability of (13)Δ was substantial, with mean diurnal amplitudes of ~9‰ and maximum diurnal amplitudes of ~20‰. The highest (13)Δ were generally observed during early morning and late afternoon, and the lowest (13)Δ during midday. An assessment of propagated standard deviations of (13)Δ demonstrated that the observed diurnal variation of (13)Δ was not a measurement artefact. Day-to-day variations of (13)Δ were summarized with flux-weighted daily means of (13)Δ, which ranged from 15‰ to 23‰ in 2009 and from 18‰ to 29‰ in 2010, thus displaying a considerable range of 8-11‰. Generally, (13)Δ showed the expected negative relationship with intrinsic water use efficiency. Diurnal and day-to-day variability of (13)Δ was, however, always better predicted by that of net CO₂ assimilation, especially in 2010 when soil moisture was high and vapour pressure deficit was low. Stomatal control of leaf gas exchange, and consequently (13)Δ, could only be identified under drier conditions in 2009.