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The 18 O signature of atmospheric water vapour (δ18 OV ) is known to be transferred via leaf water to assimilates. It remains, however, unclear how the 18 O-signal transfer differs among plant species and growth forms. We performed a 9-hr greenhouse fog experiment (relative humidity ≥ 98%) with 18 O-depleted water vapour (-106.7‰) on 140 plant species of eight different growth forms during daytime. We quantified the 18 O-signal transfer by calculating the mean residence time of O in leaf water (MRTLW ) and sugars (MRTSugars ) and related it to leaf traits and physiological drivers. MRTLW increased with leaf succulence and thickness, varying between 1.4 and 10.8 hr. MRTSugars was shorter in C3 and C4 plants than in crassulacean acid metabolism (CAM) plants and highly variable among species and growth forms; MRTSugars was shortest for grasses and aquatic plants, intermediate for broadleaf trees, shrubs, and herbs, and longest for conifers, epiphytes, and succulents. Sucrose was more sensitive to δ18 OV variations than other assimilates. Our comprehensive study shows that plant species and growth forms vary strongly in their sensitivity to δ18 OV variations, which is important for the interpretation of δ18 O values in plant organic material and compounds and thus for the reconstruction of climatic conditions and plant functional responses.

Stable isotope ratios in tree rings have become an important proxy for palaeoclimatology, particularly in temperate regions. Yet temperate forests are often characterized by heterogeneous stand structures, and the effects of stand dynamics on carbon (δ13 C) and oxygen isotope ratios (δ18 O) in tree rings are not well explored. In this study, we investigated long-term trends and offsets in δ18 O and δ13 C of Picea abies and Fagus sylvatica in relation to tree age, size, and distance to the upper canopy at seven temperate sites across Europe. We observed strong positive trends in δ13 C that are best explained by the reconstructed dynamics of individual trees below the upper canopy, highlighting the influence of light attenuation on δ13 C in shade-tolerant species. We also detected positive trends in δ18 O with increasing tree size. However, the observed slopes are less steep and consistent between trees of different ages and thus can be more easily addressed. We recommend restricting the use of δ13 C to years when trees are in a dominant canopy position to infer long-term climate signals in δ13 C when relying on material from shade-tolerant species, such as beech and spruce. For such species, δ18 O should be in principle the superior proxy for climate reconstructions.

Carbon reserves are important for maintaining tree function during and after stress. Increasing tree mortality driven by drought globally has renewed the interest in how plants regulate allocation of recently fixed C to reserve formation. Three-year-old seedlings of two species (Tilia platyphyllos and Pinus sylvestris) were exposed to two intensities of experimental drought during ~10 weeks, and 13 C pulse labelling was subsequently applied with rewetting. Tracking the 13 C label across different organs and C compounds (soluble sugars, starch, myo-inositol, lipids and cellulose), together with the monitoring of gas exchange and C mass balances over time, allowed for the identification of variations in C allocation priorities and tree C balances that are associated with drought effects and subsequent drought release. The results demonstrate that soluble sugars accumulated in P. sylvestris under drought conditions independently of growth trends; thus, non-structural carbohydrates (NSC) formation cannot be simply considered a passive overflow process in this species. Once drought ceased, C allocation to storage was still prioritized at the expense of growth, which suggested the presence of 'drought memory effects', possibly to ensure future growth and survival. On the contrary, NSC and growth dynamics in T. platyphyllos were consistent with a passive (overflow) view of NSC formation.

Almost no δ(18) O data are available for leaf carbohydrates, leaving a gap in the understanding of the δ(18) O relationship between leaf water and cellulose. We measured δ(18) O values of bulk leaf water (δ(18) OLW ) and individual leaf carbohydrates (e.g. fructose, glucose and sucrose) in grass and tree species and δ(18) O of leaf cellulose in grasses. The grasses were grown under two relative humidity (rH) conditions. Sucrose was generally (18) O-enriched compared with hexoses across all species with an apparent biosynthetic fractionation factor (ebio ) of more than 27‰ relative to δ(18) OLW , which might be explained by isotopic leaf water and sucrose synthesis gradients. δ(18) OLW and δ(18) O values of carbohydrates and cellulose in grasses were strongly related, indicating that the leaf water signal in carbohydrates was transferred to cellulose (ebio = 25.1‰). Interestingly, damping factor pex px , which reflects oxygen isotope exchange with less enriched water during cellulose synthesis, responded to rH conditions if modelled from δ(18) OLW but not if modelled directly from δ(18) O of individual carbohydrates. We conclude that δ(18) OLW is not always a good substitute for δ(18) O of synthesis water due to isotopic leaf water gradients. Thus, compound-specific δ(18) O analyses of individual carbohydrates are helpful to better constrain (post-)photosynthetic isotope fractionation processes in plants.

Little is known about the dynamics of concentrations and carbon isotope ratios of individual carbohydrates in leaves in response to climatic and physiological factors. Improved knowledge of the isotopic ratio in sugars will enhance our understanding of the tree ring isotope ratio and will help to decipher environmental conditions in retrospect more reliably. Carbohydrate samples from larch (Larix gmelinii) needles of two sites in the continuous permafrost zone of Siberia with differing growth conditions were analysed with the Compound-Specific Isotope Analysis (CSIA). We compared concentrations and carbon isotope values (δ13C) of sucrose, fructose, glucose and pinitol combined with phenological data. The results for the variability of the needle carbohydrates show high dynamics with distinct seasonal characteristics between and within the studied years with a clear link to the climatic conditions, particularly vapour pressure deficit. Compound-specific differences in δ13C values as a response to climate were detected. The δ13C of pinitol, which contributes up to 50% of total soluble carbohydrates, was almost invariant during the whole growing season. Our study provides the first in-depth characterization of compound-specific needle carbohydrate isotope variability, identifies involved mechanisms and shows the potential of such results for linking tree physiological responses to different climatic conditions.

Erica arborea (L) is a widespread Mediterranean species, able to cope with water stress and colonize semiarid environ- ments. The eco-physiological plasticity of this species was evaluated by studying plants growing at two sites with differ- ent soil moistures on the island of Elba (Italy), through dendrochronological, wood-anatomical analyses and stable isotopes measurements. Intra-annual density fluctuations (IADFs) were abundant in tree rings, and were identified as the key parameter to understand site-specific plant responses to water stress. Our findings showed that the formation of IADFs is mainly related to the high temperature, precipita- tion patterns and probably to soil water availability, which differs at the selected study sites. The recorded increase in the 13 C-derived intrinsic water use efficiency at the IADFs level was linked to reduced water loss rather than to increas- ing C assimilation.The variation in vessel size and the differ- ent absolute values of δ 18 O among trees growing at the two study sites underlined possible differences in stomatal control of water loss and possible differences in sources of water uptake. This approach not only helped monitor sea- sonal environmental differences through tree-ring width, but also added valuable information on E. arborea responses to drought and their ecological implications for Mediterranean vegetation dynamics.

We investigated whether stand structure modulates the long-term physiological performance and growth of Pinus halepensis Mill. in a semiarid Mediterranean ecosystem. Tree radial growth and carbon and oxygen stable isotope composition of latewood (δ(13)C(LW) and δ(18)O(LW), respectively) from 1967 to 2007 were measured in P. halepensis trees from two sharply contrasting stand types: open woodlands with widely scattered trees versus dense afforested stands. In both stand types, tree radial growth, δ(13)C(LW) and δ(18)O(LW) were strongly correlated with annual rainfall, thus indicating that tree performance in this semiarid environment is largely determined by inter-annual changes in water availability. However, trees in dense afforested stands showed consistently higher δ(18)O(LW) and similar δ(13)C(LW) values compared with those in neighbouring open woodlands, indicating lower stomatal conductance and photosynthesis rates in the former, but little difference in water use efficiency between stand types. Trees in dense afforested stands were more water stressed and showed lower radial growth, overall suggesting greater vulnerability to drought and climate aridification compared with trees in open woodlands. In this semiarid ecosystem, the negative impacts of intense inter-tree competition for water on P. halepensis performance clearly outweigh potential benefits derived from enhanced infiltration and reduced run-off losses in dense afforested stands.

Current models of leaf water enrichment predict that the differences between isotopic enrichment of water at the site of evaporation (Delta(e)) and mean lamina leaf water enrichment (Delta(L)) depend on transpiration rates (E), modulated by the scaled effective length (L) of water isotope movement in the leaf. However, variations in leaf parameters in response to changing environmental conditions might cause changes in the water path and thus L. We measured the diel course of Delta(L) for (18)O and (2)H in beech seedlings under well-watered and water-limited conditions. We applied evaporative enrichment models of increasing complexity to predict Delta(e) and Delta(L), and estimated L from model fits. Water-limited plants showed moderate drought stress, with lower stomatal conductance, E and stem water potential than the control. Despite having double E, the divergence between Delta(e) and Delta(L) was lower in well-watered than in water-limited plants, and thus, L should have changed to counteract differences in E. Indeed, L was about threefold higher in water-limited plants, regardless of the models used. We conclude that L changes with plant water status far beyond the variations explained by water content and other measured variables, thus limiting the use of current evaporative models under changing environmental conditions.

Drought-induced forest decline, like the Scots pine mortality in inner-Alpine valleys, will gain in importance as the frequency and severity of drought events are expected to increase. To understand how chronic drought affects tree growth and tree-ring delta(13)C values, we studied mature Scots pine in an irrigation experiment in an inner-Alpine valley. Tree growth and isotope analyses were carried out at the annual and seasonal scale. At the seasonal scale, maximum delta(13)C values were measured after the hottest and driest period of the year, and were associated with decreasing growth rates. Inter-annual delta(13)C values in early- and latewood showed a strong correlation with annual climatic conditions and an immediate decrease as a response to irrigation. This indicates a tight coupling between wood formation and the freshly produced assimilates for trees exposed to chronic drought. This rapid appearance of the isotopic signal is a strong indication for an immediate and direct transfer of newly synthesized assimilates for biomass production. The fast appearance and the distinct isotopic signal suggest a low availability of old stored carbohydrates. If this was a sign for C-storage depletion, an increasing mortality could be expected when stressors increase the need for carbohydrate for defence, repair or regeneration.