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Spanish Government, Grant/Award Number: AGL2015‐68274 ‐C3 ‐3‐R; Sixth Framework Programme, Grant/AwardNumber: EVK2‐2001 ‐00237; Seventh Framework Programme, Grant/AwardNumber: COST ‐STSM ‐ECOST ‐STSM ‐FP1304‐140915‐066395 and ERANET‐Mundus program (Grant agreement 20112573)

We investigated the interaction between fungal communities of soil and dead wood substrates. For this, we applied molecular species identification and stable isotope tracking to both soil and decaying wood in an unmanaged boreal Norway spruce-dominated stand. Altogether, we recorded 1990 operational taxonomic units, out of which more than 600 were shared by both substrates and 589 were found to exclusively inhabit wood. On average the soil was more species-rich than the decaying wood, but the species richness in dead wood increased monotonically along the decay gradient, reaching the same species richness and community composition as soil in the late stages. Decaying logs at all decay stages locally influenced the fungal communities from soil, some fungal species occurring in soil only under decaying wood. Stable isotope analyses suggest that mycorrhizal species colonising dead wood in the late decay stages actively transfer nitrogen and carbon between soil and host plants. Most importantly, Piloderma sphaerosporum and Tylospora sp. mycorrhizal species were highly abundant in decayed wood. Soil- and wood-inhabiting fungal communities interact at all decay phases of wood that has important implications in fungal community dynamics and thus nutrient transportation.

Summary Microbial respiration in dead wood contributes substantially to the long‐lived forest carbon (C) pool and has a significant role in the forest nitrogen (N) cycle. Wood N content has been found to increase during the decay process; however, temporal dynamics and the sources of this external N remain unclear. To examine N dynamics at various stages of decomposition, we combined high variety of analytical methods on Norway spruce logs, including wood δ15N, N%, 14C‐dating, fungal composition and N2 fixation rate. For N2 fixation rate, we also determined its dependency on ambient temperature and decay class, when estimating annual N2 fixation rates for our study site. N2 fixation was observed to have a major role in increasing wood N content during decay. For the most decayed wood, it accounted for 60% of the total N accumulation. Compared to other reports, where the annual temperature was similar to our site, the calculated annual fixation rate of 85 g N ha−1 year−1 is a low estimate. However, previous studies have not taken appropriately into account the dependency of N2 fixation rate on ambient temperature and decay class. Our δ15N model describing the sources of external N, statistical analysis and the fungal DNA composition of decayed wood suggest that other sources of external N accumulating in wood were soil‐foraging wood‐decay fungi and mycorrhizal fungi. Our study improves knowledge of the temporal dynamics of N accumulation in wood with advancing wood decay, the potential sources of external N and their relative significance. All of these factors are important for nitrogen as well as carbon models dealing with ecosystem responses to climate change. Lay Summary

Stable oxygen isotope ratios (δ¹⁸O) in trees from high latitude ecosystems are valuable sources of information for recent and past environmental changes, but the interpretation is hampered by the complex hydrology of forests growing under permafrost conditions, where only a shallow layer of soil thaws in summer. We investigated larch trees (Larix gmelinii) at two sites with contrasting soil conditions in Siberia and determined δ¹⁸O of water from different soil depths, roots, twigs, and needles as well as δ¹⁸O of soluble carbohydrates regularly over two growing seasons. A comparison of results from the 2 yrs revealed an unexpected ‘inverse’ climate‐isotope relationship, as dry and warm summer conditions resulted in lower soil and root δ¹⁸O values. This was due to a stronger uptake of isotopically depleted water pools originating from melted permafrost or previous winter snow. We developed a conceptual framework that considers the dependence of soil water profiles on climatic conditions for explaining δ¹⁸O in needle water, needle soluble carbohydrates and stem cellulose. The negative feedback of drought conditions on the source isotope value could explain decreasing tree‐ring δ¹⁸O trends in a warming climate and is likely relevant in many ecosystems, where a soil isotope gradient with depth is observed.

United Kingdom (UK) summers dominated by anti-cyclonic circulation patterns are characterised by clear skies, warm temperatures, low precipitation totals, low air humidity and more enriched oxygen isotope ratios (δ 18 O) in precipitation. Such conditions usually result in relatively more positive (enriched) oxygen isotope ratios in tree leaf sugars and ultimately in the tree-ring cellulose formed in that year, the converse being true in cooler, wet summers dominated by westerly air flow and cyclonic conditions. There should therefore be a strong link between tree-ring δ 18 O and the amount of summer precipitation. Stable oxygen isotope ratios from the latewood cellulose of 40 oak trees sampled at eight locations across Great Britain produce a mean δ 18 O chronology that correlates strongly and significantly with summer indices of total shear vorticity, surface air pressure, and the amount of summer precipitation across the England and Wales region of the United Kingdom. The isotope-based rainfall signal is stronger and much more stable over time than reconstructions based upon oak ring widths. Using recently developed methods that are precise, efficient and highly cost-effective it is possible to measure both carbon (δ 13 C) and oxygen (δ 18 O) isotope ratios simultaneously from the same tree-ring cellulose. In our study region, these two measurements from multiple trees can be used to reconstruct summer temperature (δ 13 C) and summer precipitation (δ 18 O) with sufficient independence to allow the evolution of these climate parameters to be reconstructed with high levels of confidence. The existence of long, well-replicated oak tree-ring chronologies across the British Isles mean that it should now be possible to reconstruct both summer temperature and precipitation over many centuries and potentially millennia.

How will carbon source–sink relations of 35-yr-old larch trees (Larix decidua) at the alpine treeline respond to changes in atmospheric CO2 and climate? We evaluated the effects of previously elevated CO2 concentrations (9 yr, 580 ppm, ended the previous season) and ongoing soil warming (4 yr, + 4°C). Larch branches were pulse labeled (50 at% 13CO2) in July 2010 to trace fresh assimilates through tissues (buds, needles, bark and wood) and non-structural carbon compounds (NCC; starch, lipids, individual sugars) using compound-specific isotope analysis. Nine years of elevated CO2 did not lead to increased NCC concentrations, nor did soil warming increase NCC transfer velocities. By contrast, we found slower transfer velocities and higher NCC concentrations than reported in the literature for lowland larch. As a result of low dilution with older carbon, sucrose and glucose showed the highest maximum 13C labels, whereas labels were lower for starch, lipids and pinitol. Label residence times in needles were shorter for sucrose and starch (c. 2 d) than for glucose (c. 6 d). Although our treatments showed no persistent effect on larch carbon relations, low temperature at high altitudes clearly induced a limitation of sink activities (growth, respiration, root exudation), expressed in slower carbon transfer and higher NCC concentrations.

The oxygen isotope ratio of precipitation and tree rings is a complex function of climate variables and atmospheric dynamics, which often makes the interpretation of δ18O for palaeoclimate research challenging. Here we analyzed monthly precipitation δ18O series for 1973–2004 and annually resolved tree ring δ18O chronologies for 1945–2004 for three sites in Switzerland: one north of the Alps, one at high‐elevation within the Alps, and one south of the Alps. The goal of the study was to improve the understanding of the tree ring archive by a systematic analysis of nonlocal parameters related to atmospheric circulation, in particular, geopotential height field anomalies and the frequency of synoptic weather situations, in addition to the usual local climate parameters like temperature, sunshine duration, and relative humidity. We observed that on average high‐pressure situations during summer were associated with relatively high δ18O and low‐pressure situations were associated with relatively low δ18O, for both the isotope ratio in precipitation and tree rings. However, correlations to the frequency of weather types were not higher than simple correlations to local temperature. Accordingly, we constructed a combined index from temperature and air pressure that proved to be a good predictor of δ18O in precipitation and used this as the source water term in a tree ring isotope fractionation model. This enabled us to use the model beyond the period where isotope values for precipitation are available, opening new perspectives in the interpretation of long tree ring δ18O chronologies. Key Points Systematic investigation of circulation patterns and tree ring isotope ratios Application of a new tree ring oxygen isotope fractionation model Importance of the frequency of weather types for oxygen isotope variability

The vapor pressure deficit reflects the difference between how much moisture the atmosphere could and actually does hold, a factor that fundamentally affects evapotranspiration, ecosystem functioning, and vegetation carbon uptake. Its spatial variability and long-term trends under natural versus human-influenced climate are poorly known despite being essential for predicting future effects on natural ecosystems and human societies such as crop yield, wildfires, and health. Here we combine regionally distinct reconstructions of pre-industrial summer vapor pressure deficit variability from Europe’s largest oxygen-isotope network of tree-ring cellulose with observational records and Earth system model simulations with and without human forcing included. We demonstrate that an intensification of atmospheric drying during the recent decades across different European target regions is unprecedented in a pre-industrial context and that it is attributed to human influence with more than 98% probability. The magnitude of this trend is largest in Western and Central Europe, the Alps and Pyrenees region, and the smallest in southern Fennoscandia. In view of the extreme drought and compound events of the recent years, further atmospheric drying poses an enhanced risk to vegetation, specifically in the densely populated areas of the European temperate lowlands. The atmosphere has dried across most regions of Europe in recent decades, a trend that can be attributed primarily to human impacts, according to tree ring records spanning 400 years and Earth system model simulations.

Dissimilation of carbon sources during plant respiration in support of metabolic processes results in the continuous release of CO₂. The carbon isotopic composition of leaf dark-respired CO₂ (i.e. δ13CR ) shows daily enrichments up to 14.8‰ under different environmental conditions. However, the reasons for this 13C enrichment in leaf dark-respired CO₂ are not fully understood, since daily changes in δ 13C of putative leaf respiratory carbon sources (δ13CRS ) are not yet clear. Thus, we exposed potato plants (Solanum tuberosum) to different temperature and soil moisture treatments. We determined δ13CR with an in-tube incubation technique and δ13CRS with compound-specific isotope analysis during a daily cycle. The highest δ13CRS values were found in the organic acid malate under different environmental conditions, showing less negative values compared to δ13CR (up to 5.2‰) and compared to δ13CRS of soluble carbohydrates, citrate and starch (up to 8.8‰). Moreover, linear relationships between δ13CR and δ13CRS among different putative carbon sources were strongest for malate during daytime (r²=0.69, P≤0.001) and nighttime (r²=0.36, P≤0.001) under all environmental conditions. A multiple linear regression analysis revealed δ13CRS of malate as the most important carbon source influencing δ13CR . Thus, our results strongly indicate malate as a key carbon source of 13C enriched dark-respired CO₂ in potato plants, probably driven by an anapleurotic flux replenishing intermediates of the Krebs cycle.

Purpose Wood-inhabiting fungi play an important role in wood decomposition, and their community characteristics are influenced by both inherent wood nutrients and external nutrient inputs. However, how inherent nutrients from tree taxonomic groups interact with external nutrient inputs to influence fungal community characteristics remains unclear. Methods Here, we conducted a three-year field experiment in subtropical forest with eight tree species under four nutrient additions (no addition, nitrogen (N) addition, phosphorus (P) addition, and combined N and P addition), and investigated the effects of N and P additions on fungal communities in both overall wood and wood from different tree taxonomic groups (angiosperms and gymnosperms). Results Results showed that P addition had a stronger impact than N addition on fungal community structure, network complexity, and interaction strength between fungal species in overall wood. Considering tree taxonomic groups, fungal community structure in angiosperms was strongly affected by N addition, while in gymnosperms, it was predominantly affected by P addition. N addition significantly increased Ascomycota abundance, fungal diversity and network complexity in angiosperms, while P addition significantly decreased Ascomycota abundance, pathogenic fungi abundance, fungal diversity and network complexity in gymnosperms. These divergent responses of fungal communities were primarily driven by nutrient-induced changes in community’s demand for carbon (C) and nutrients, with angiosperms increasing fungal demand for wood C and gymnosperms decreasing fungal demand for wood P. Conclusion Our results highlight the key role of tree taxonomic groups in regulating the responses of wood-inhabiting fungal communities to nutrient additions through shifts in microbial nutrient demands.