Summer drought reduces total and litter-derived soil CO2 effluxes in temperate grassland - clues from a 13C litter addition experiment

Current climate change models predict significant changes in rainfall patterns across Europe. To explore the effect of drought on soil CO2 efflux (FSoil) and on the contribution of litter to FSoil we used rain shelters to simulate a summer drought (May to July 2007) in an intensively managed grassland in Switzerland by reducing annual precipitation by around 30% similar to the hot and dry year 2003 in Central Europe. We added 13C-depleted as well as unlabelled grass/clover litter to quantify the litter-derived CO2 efflux (FLitter). Soil CO2 efflux and the 13C/12C isotope ratio (δ13C) of the respired CO2 after litter addition were measured during the growing season 2007. Drought significantly decreased FSoil in our litter addition experiment by 59% and FLitter by 81% during the drought period itself (May to July), indicating that drought had a stronger effect on the CO2 release from litter than on the belowground-derived CO2 efflux (FBG, i.e. soil organic matter (SOM) and root respiration). Despite large bursts in respired CO2 induced by the rewetting after prolonged drought, drought also reduced FSoil and FLitter during the entire 13C measurement period (April to October) by 26% and 37%, respectively. Overall, our findings show that drought decreased FSoil and altered its seasonality and its sources. Thus, the C balance of temperate grassland soils respond sensitively to changes in precipitation, a factor that needs to be considered in regional models predicting the impact of climate change on ecosystems C balance

Climatic sensitivity of δ18O in the wood and cellulose of tree rings: Results from a mixed stand of Acer pseudoplatanus L. and Fagus sylvatica L.

The oxygen isotope composition in tree rings is commonly used to assess the impact of climatic factors on tree growth. However, the relationships between environmental variables and δ18O in whole wood and the wood components (e.g., cellulose, lignin) are still not completely clear - particularly for deciduous species. It is currently common procedure to consider only one wood constituent, a process that saves time. To test whether the δ18O of cellulose relates better to environmental variables than that of whole wood, we measured the oxygen isotopic composition of tree ring whole wood and cellulose, for the period 1916-1950, for two different broad-leaved species (Fagus sylvatica L. and Acer pseudoplatanus L.) on Monti Picentini (Southern Italy). Data show that δ18O in cellulose is enriched (4.7 ± 1.1‰) compared to the corresponding whole wood. The relationship between the δ18O of the cellulose and of whole wood is generally poor, especially for A. pseudoplatanus suggesting that the sources of variability for δ18O differ strongly between the whole wood and the cellulose constituents. Furthermore, we find that the cellulose fraction correlates strongly with climatic variables, while the whole wood fraction generally does not. Monthly temperature during the growing season is strongly correlated with δ18O of cellulose for F. sylvatica (r = 0.82) and A. pseudoplatanus (r = 0.63) but not with δ18O of whole wood. We found a relevant difference in the isotopic signals of the two species that could be related with difference in seasonal timing of growth. In contrast to F. sylvatica, early spring conditions have an influence on A. pseudoplatanus at our study site. This is evidenced by the high correlation between δ18O of cellulose and April and May precipitation, and the positive relationship between tree ring width and March and April mean annual temperature. The different climatic sensitivities of the two investigated species suggest that physiological properties, i.e. root system and stomatal conductance responses, or a variation in the proportions of wood constituents play an important role for the oxygen isotopic signal. We conclude that, even when partially obscured by site parameters (soil depth and structure, nutrient availability, etc.), δ18O in the cellulose compartment strongly reflects climate information. Our study suggests that cellulose extraction is a necessary step in studies seeking to investigate the climatic signatures in the δ18O of broad-leaved species. © 2008 Elsevier B.V. All rights reserved

Impact of different nitrogen emission sources on tree physiology as assessed by a triple stable isotope approach

The importance that nitrogen (N) deposition has in driving the carbon (C) sequestration of forests has recently been investigated using both experimental and modeling approaches. Whether increased N deposition has positive or negative effects on such ecosystems depends on the status of the N and the duration of the deposition. By combining delta(13)C, delta(18)O, delta(15)N and dendrochronological approaches, we analyzed the impact of two different sources of NO(x) emissions on two tree species, namely: a broad-leaved species (Quercus cerris) that was located close to an oil refinery in Southern Italy, and a coniferous species (Picea abies) located close to a freeway in Switzerland. Variations in the c(i)/c(a) ratio and the distinction between stomatal and photosynthetic responses to NO(x) emissions in trees were assessed using a conceptual model, which combines delta(13)C and delta(18)O, delta(15)N in leaves, needles and tree rings was found to be a bioindicator of N input from anthropogenic emissions, especially at the oil refinery site. We observed that N fertilization had a stimulatory effect on tree growth near the oil refinery, while the opposite effect was found for trees at the freeway site. Changes in the c(i)/c(a) ratio were mostly related to variations in delta(13)C at the freeway site and. thus, were driven by photosynthesis. At the oil refinery site they were mainly related to stomatal conductance, as assessed using delta(18)O. This study demonstrates that a single method approach does not always provide a complete picture of which physiological traits are more affected by N emissions. The triple isotope approach combined with dendrochronological analyses proved to be a very promising tool for monitoring the ecophysiological responses of trees to long-term N deposition. (C) 2008 Elsevier Ltd. All rights reserved

Recent atmospheric drying in Siberia is not unprecedented over the last 1,500 years

Статья из журнала.Newly developed millennial δ13C larch tree-ring chronology from Siberia allows reconstruction of summer (July) vapor pressure deficit (VPD) changes in a temperature-limited environment. VPD increased recently, but does not yet exceed the maximum values reconstructed during the Medieval Warm Anomaly. The most humid conditions in the Siberian North were recorded in the Early Medieval Period and during the Little Ice Age. Increasing VPD under elevated air temperature affects the hydrology of these sensitive ecosystems by greater evapotranspiration rates. Further VPD increases will significantly affect Siberian forests most likely leading to drought and forest mortality even under additional access of thawed permafrost water. Adaptation strategies are needed for Siberian forest ecosystems to protect them in a warming world. © 2020, The Author(s)

Compound-specific carbon isotopes and concentrations of carbohydrates and organic acids as indicators of tree decline in mountain pine

We investigated seasonal variations in d13C values and concentrations of carbohydrates and organic acids in needles of declining mountain pine (Pinus mugo) trees from the Swiss National Park (SNP), using compound-specific isotopes analysis (CSIA). Our goal was to study the impact of climatic drivers on the individual compounds and understand the reasons of partial tree declines in relation to healthy mountain pine trees under seasonal weather patterns. We found that temperature is the main climate driver determining the seasonal carbon dynamics at the needle level. Lower seasonal d13C variability and lower concentration levels of sucrose in needles suggest less photosynthetic activity and sink carbon demand in declining compared to healthy mountain pine trees. Higher concentration levels of hexose (glucose and fructose) can play a reserve function for surviving mechanisms of mountain pine trees. Seasonal patterns of organic acid (malate and citrate) suggest increasing investment in maintenance and repair mechanism. The seasonal course of carbohydrates and organic acids can therefore be considered an indicator for a modified carbon metabolism within the leaves and possibly within the other tree tissues, partially explaining the decline of mountain pine trees

Tree-ring δ18O from an alpine catchment reveals changes in glacier stream water inputs between 1980 and 2010

The tree-ring stable carbon and oxygen isotope chronologies from two forest sites located in the Forni Glacier forefield (Italy)—one along the glacier stream (GL) and the other toward the valley slope (SL)—were analyzed with the aim of disentangling the precipitation and glacier meltwater inputs in source water δ18O, as reflected by the tree-ring cellulose δ18O. The cellulose δ18O from the GL trees has a negative correlation with winter and summer temperatures, whereas the cellulose δ18O from the SL trees has a positive correlation with precipitation δ18O. The isotopic signature of the source water at the GL site is also influenced by waters of glacial origin, as confirmed by the18O-depleted glacier meltwater inputs (GMWI_δ18O) estimated by means of an isotope model. The GMWI_δ18O values are consistent with the mean difference measured between the δ18O in the glacier stream and in the precipitation and the winter and summer temperature explains up to 37 percent of the GMWI_δ18O variance. Our results show an increasing influence of glacier meltwater throughout the past decade for the GL site. Our analysis opens new opportunities to reconstruct changes in water regimes of the glacier streams by means of the tree-ring cellulose δ18O

Picea-FACE_synthesis_gas_exchange

Needle gas exchange rates measured on 1-year-old needles in ambient (A) and elevated (E) trees at both ambient and elevated CO2 levels (400 and 550 ppm) during five summer field campaigns (18 June, 2 July, and 19 September 2013; 23 and 26 September 2014)

The relationship between needle sugar carbon isotope ratios and tree rings of larch in Siberia

Significant gaps still exist in our knowledge about post-photosynthetic leaf level and downstream metabolic processes and isotopic fractionations. This includes their impact on the isotopic climate signal stored in the carbon isotope composition (delta C-13) of leaf assimilates and tree rings. For the first time, we compared the seasonal delta C-13 variability of leaf sucrose with intra-annual, high-resolution delta C-13 signature of tree rings from larch (Larix gmelinii Rupr.). The trees were growing at two sites in the continuous permafrost zone of Siberia with different growth conditions. Our results indicate very similar low-frequency intra-seasonal trends of the sucrose and tree ring delta C-13 records with little or no indication for the use of 'old' photosynthates formed during the previous year(s). The comparison of leaf sucrose delta C-13 values with that in other leaf sugars and in tree rings elucidates the cause for the reported C-13-enrichment of sink organs compared with leaves. We observed that while the average delta C-13 of all needle sugars was 1.2% more negative than delta C-13 value of wood, the delta C-13 value of the transport sugar sucrose was on an average 1.0% more positive than that of wood. Our study shows a high potential of the combined use of compound-specific isotope analysis of sugars (leaf and phloem) with intra-annual tree ring delta C-13 measurements for deepening our understanding about the mechanisms controlling the isotope variability in tree rings under different environmental conditions.201

Compound-specific carbon isotope patterns in needles of conifer tree species from the Swiss National Park under recent climate change

Текст статьи не публикуется в открытом доступе в соответствии с политикой журнала.As a result of climate change, elevated CO2 along with rising temperature and water deficits can lead to changes in tree physiology and leaf biochemistry. These changes can increase heat- and drought-induced tree mortality. We aim to reveal impacts of climatic drivers on the individual carbon compounds at the leaf level among European larch (Larix decidua) and mountain pine (Pinus mugo) trees, which are widely distributed at high elevations. We investigated seasonal carbon isotope composition (δ13C) and concentration patterns of carbohydrates and organic acids in needles of these two different species from a case study in the Swiss National Park (SNP). We found that average and minimum air temperatures were the main climatic drivers of seasonal variation of δ13C in sucrose and glucose as well as in concentrations of carbohydrates and citric acid/citrate in needles of both tree species. The impact of seasonal climatic drivers on larch and mountain pine trees at the needle level is in line with our earlier study in this region for long-term changes at the tree-ring level. We conclude that the species-specific changes in δ13C and concentrations of carbohydrates and organic acids are sensitive indicators of changes in the metabolic pathways occurring as a result of climatic changes