Abstract. Measurements of the stable carbon isotope ratio (δ 13 C) on annual tree rings offer new opportunities to evaluate mechanisms of variations in photosynthesis and stomatal conductance under changing CO 2 and climate conditions, especially in conjunction with process-based biogeochemical model simulations. The isotopic discrimination is indicative of the ratio between the CO 2 partial pressure in the intercellular cavities and the atmosphere (c i /c a ) and of the ratio of assimilation to stomatal conductance, termed intrinsic wateruse efficiency (iWUE). We performed isotope-enabled simulations over the industrial period with the land biosphere module (CLM4.5) of the Community Earth System Model and the Land Surface Processes and Exchanges (LPX-Bern) dynamic global vegetation model. Results for C3 tree species show good agreement with a global compilation of δ 13 C measurements on leaves, though modeled 13 C discrimination by C3 trees is smaller in arid regions than measured. A compilation of 76 tree-ring records, mainly from Europe, boreal Asia, and western North America, suggests on average small 20th century changes in isotopic discrimination and in c i /c a and an increase in iWUE of about 27 % since 1900. LPXBern results match these century-scale reconstructions, supporting the idea that the physiology of stomata has evolved to optimize trade-offs between carbon gain by assimilation and water loss by transpiration. In contrast, CLM4.5 simulates an increase in discrimination and in turn a change in iWUE that is almost twice as large as that revealed by the tree-ring data. Factorial simulations show that these changes are mainly in response to rising atmospheric CO 2 . The results suggest that the downregulation of c i /c a and of photosynthesis by nitrogen limitation is possibly too strong in the standard setup of CLM4.5 or that there may be problems associated with the implementation of conductance, assimilation, and related adjustment processes on long-term environmental changes.

Wood material from the sapwood of the studied tree species is as useful as cellulose for studying environmental effects on tree-ring δ(18)O and δ(13)C values at a short-term scale as considered in most ecophysiological studies. The more variable response of oak may require further investigations.

The 18O signature of atmospheric water vapour (δ18OV) is known to be transferred via leaf water to assimilates. It remains, however, unclear how the 18O‐signal transfer differs among plant species and growth forms. We performed a 9‐hr greenhouse fog experiment (relative humidity ≥ 98%) with 18O‐depleted water vapour (−106.7‰) on 140 plant species of eight different growth forms during daytime. We quantified the 18O‐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 δ18OV variations than other assimilates. Our comprehensive study shows that plant species and growth forms vary strongly in their sensitivity to δ18OV variations, which is important for the interpretation of δ18O values in plant organic material and compounds and thus for the reconstruction of climatic conditions and plant functional responses.

The present study aims at providing standard values for the exploration type (ET)-specific quantification of extramatrical mycelium (EMM) of ectomycorrhizal fungi applicable to ecological field studies. These values were established from mycelial systems of ectomycorrhizae (ECM) synthesized in rhizotrons with near-to-natural peat substrate. Based on image analysis, the "Specific Potential Mycelial Space Occupation" (sPMSO), i.e. the ET-specific complete area that is covered by the EMM systems (mm 2 cm −1 ECM −1 ), and the "Specific Actual Mycelial Space Occupation" (sAMSO), i.e. the projection area of mycelial systems (mm 2 cm −1 ECM −1 ), were analyzed as an extension of a previously described approach. The "Specific Extramatrical Mycelial Length" (sEML) [m cm −1 ECM −1 ] and the "Specific Extramatrical Mycelial Biomass" (sEMB) (μg cm −1 ECM −1 ) were calculated for each of the ET via the proportion of hyphal projected area, hyphal length and biomass, the latter two being derived from previous measurements on Piloderma croceum, a "Medium-Distance" (MD)-ET. Both sPMSO and sAMSO were highest for the "Long-Distance" (LD)-ET, whereas those of the "Short-Distance" (SD)-ET and MD-ET were similar, although showing high variation. In contrast, mycelial density per occupied area of the MD-ET was twice as high as that of the LD-ET. Proportional to the sAMSO, the EMM length and biomass differed considerably between the three ET with values of the MD-ET being 1.9 times higher than those of SD-ET, and those of the LD-ET being 15 times higher than those of the SD-ET. These standards in relation to ECM length may ease quantification of mycelial space occupation and biomass in a relatively simple way. Thereby, the ET-specific contribution of EMM can be distinguishedalso of non-cultivable species-and up-scaling to large-scale estimation of cost/benefit relations is possible.

Abstract. Records of stable oxygen isotope ratios in tree rings are valuable tools to reconstruct past climatic conditions and investigate the response of trees to those conditions. So far the use of stable oxygen isotope signatures of tree rings has not been systematically evaluated in dynamic global vegetation models (DGVMs). DGVMs integrate many hydrological and physiological processes and their application could improve proxy-model comparisons and the interpretation of oxygen isotope records. Here we present an approach to simulate leaf water and stem cellulose δ18O of trees using the LPX-Bern DGVM (LPX-Bern). Our results lie within a few per mil of measured tree ring δ18O of 31 different forest stands mainly located in Europe. Temporal means over the last 5 decades as well as interannual variations for a subset of sites in Switzerland are captured. A sensitivity analysis reveals that relative humidity, temperature, and the water isotope boundary conditions have the largest influence on simulated stem cellulose δ18O, followed by all climatic factors combined, whereas increasing atmospheric CO2 and nitrogen deposition exert no impact. We conclude that simulations with LPX-Bern are useful for investigating large-scale oxygen isotope patterns of tree ring cellulose to elucidate the importance of different environmental factors on isotope variations and therefore help to reduce uncertainties in the interpretation of δ18O of tree rings.

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 (δ C) and oxygen isotope ratios (δ O) in tree rings are not well explored. In this study, we investigated long-term trends and offsets in δ O and δ 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 δ C that are best explained by the reconstructed dynamics of individual trees below the upper canopy, highlighting the influence of light attenuation on δ C in shade-tolerant species. We also detected positive trends in δ 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 δ C to years when trees are in a dominant canopy position to infer long-term climate signals in δ C when relying on material from shade-tolerant species, such as beech and spruce. For such species, δ O should be in principle the superior proxy for climate reconstructions.

Elevated tropospheric CO(2) concentrations may increase plant carbon fixation. In ectomycorrhizal trees, a considerable portion of the synthesized carbohydrates can be used to support the mutualistic fungal root partner which in turn can benefit the tree by increased nutrient supply. In this study, Norway spruce seedlings were inoculated with either Piloderma croceum (medium distance "fringe" exploration type) or Tomentellopsis submollis (medium distance "smooth" exploration type). We studied the impact of either species regarding fungal biomass production, seedling biomass, nutrient status and nutrient use efficiency in rhizotrons under ambient and twice-ambient CO(2) concentrations. A subset was amended with ammonium nitrate to prevent nitrogen imbalances expected under growth promotion by elevated CO(2). The two fungal species exhibited considerably different influences on growth, biomass allocation as well as nutrient uptake of spruce seedlings. P. croceum increased nutrient supply and promoted plant growth more strongly than T. submollis despite considerably higher carbon costs. In contrast, seedlings with T. submollis showed higher nutrient use efficiency, i.e. produced plant biomass per received unit of nutrient, particularly for P, K and Mg, thereby promoting shoot growth and reducing the root/shoot ratio. Under the given low soil nutrient availability, P. croceum proved to be a more favourable fungal partner for seedling development than T. submollis. Additionally, plant internal allocation of nutrients was differently influenced by the two ECM fungal species, particularly evident for P in shoots and for Ca in roots. Despite slightly increased ECM length and biomass production, neither of the two species had increased its capacity of nutrient uptake in proportion to the rise of CO(2). This lead to imbalances in nutritional status with reduced nutrient concentrations, particularly in seedlings with P. croceum. The beneficial effect of P. croceum thus diminished, although the nutrient status of its host plants was still above that of plants with T. submollis. We conclude that the imbalances of nutrient status in response to elevated CO(2) at early stages of plant development are likely to prove particularly severe at nutrient-poor soils as the increased growth of ECM cannot cover the enhanced nutrient demand. Hyphal length and biomass per unit of ectomycorrhizal length as determined for the first time for P. croceum amounted to 6.9 m cm(-1) and 6.0 μg cm(-1), respectively, across all treatments.

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