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With the aim of evaluating the influence of glacial meltwater signature on tree-ring stable isotopes, we analysed δ18 O and δ13 C in the tree rings of Larix decidua Mill. specimens growing in the area of an ice-contact lake ( Lago Verde, at Miage Glacier, European Alps). Additionally, we analysed δ18 O in the glacial meltwater of the lake and of the glacier stream and compared it with the δ18 O of precipitation predicted by a spatial model. We found that tree-ring cellulose of trees fed by glacial meltwaters ( LVW site) is significantly more depleted in δ18 O than at a control site LVM (−0.91‰) fed only by precipitation, thus reflecting the measured higher depletion of glacial meltwaters with respect to local precipitation. δ13 C values did not show significant differences in mean values between the two sites but an anomalous correlation with summer temperature was found at the LVW site, probably due to the different responses of trees stomatal conductance. Over the 30-year period of analysis, four years at LVW (1992, 1995, 2003 and 2009) were markedly depleted in δ18 O. These years are those when the highest summer temperatures were recorded in the area (the ones during which glacier ablation usually increases and more depleted meltwaters fill the lake), with the exception of 1995 during which high water levels occurred following the year with the second highest summer temperature (1994). Overall, our analysis demonstrates that tree-ring δ18 O, driven by the glacial meltwater signature in the lake, can be used for detecting past major glacier runoff events. The proposed approach could also be used for quantifying past glacier runoff and for defining past distribution areas of glacial meltwaters in glacier forefields, thus contributing to past environmental reconstructions and to hazard assessment. [ABSTRACT FROM AUTHOR]

Abstract: Three sites at about 1400 m a.s.l., were chosen for this study along the Mt. Blanc Motorway in Italy. Chronologies of stable isotope ratios (δ13C, δ18O, δ15N), total N concentration and ring width of Larix decidua Mill. were analyzed to observe changes in growth and climatic signals in tree rings after significant changes in air pollution emissions occurred locally over time. The tunnel opened in 1968 and was closed for three years from March 1999 to March 2002. The obtained series from the three sites (Close = C, High = H and Far = F from the highway) for the analyzed periods, 1950–1970 (only sites H and F) and 1985–2008 (all sites), did not show any particular long-term change except site H that showed significant changes in δ18O (enrichment), δ15N (depletion) and total N (increase). δ13C values at site C were enriched in the first year of the tunnel closure (1999), showing an opposite trend in δ13C at site C, in comparison to the two control sites H and F, which cannot be explained by climatic factors. Since no great differences in δ18O were recorded in 1999, this enrichment in δ13C could be related to an enhancement in photosynthetic rate during periods of low air pollution loads. Opposite to δ15N, total N concentration shows a generally good correlation between sites, and site F was measured as the most N enriched. In a correlation analysis performed on the two study periods between the climatic parameters and ring width, we did not find any clear relationships, whereas for the series of stable isotopes and total N, we found the strongest and most significant relationships only between δ13C and summer (June to August) temperature (positive correlations) and precipitation (negative correlations) at sites H and F. These same relationships at site C were, instead, mostly insignificant, indicating an alteration of the climatic signal recorded in the δ13C chronology, caused by direct exposure to the high level of air pollution at this site. Given that site C is more affected by pollution with respect to the other two sites (whereas the same climatic conditions influence tree growth at all sites), the lack of a climatic signal in the δ13C chronology at this site can be ascribed to air pollution. Few other long-term changes were recorded by tree rings (e.g. at site H), indicating that trees probably record better the pollution events or the worsening of the environmental conditions rather than a lack of pollution for a relatively short time period in a polluted environment. [Copyright &y& Elsevier]

Higher atmospheric CO2 concentrations (c(a)) can under certain conditions increase tree growth by enhancing photosynthesis, resulting in an increase of intrinsic water-use efficiency (i WUE) in trees. However, the magnitude of these effects and their interactions with changing climatic conditions are still poorly understood under xeric and mesic conditions. We combined radial growth analysis with intra- and interannual δ(13)C and δ(18)O measurements to investigate growth and physiological responses of Larix decidua, Picea abies, Pinus sylvestris, Pinus nigra and Pseudotsuga menziesii in relation to rising c(a) and changing climate at a xeric site in the dry inner Alps and at a mesic site in the Swiss lowlands. (i)WUE increased significantly over the last 50 yr by 8-29% and varied depending on species, site water availability, and seasons. Regardless of species and increased (i)WUE, radial growth has significantly declined under xeric conditions, whereas growth has not increased as expected under mesic conditions. Overall, drought-induced stomatal closure has reduced transpiration at the cost of reduced carbon uptake and growth. Our results indicate that, even under mesic conditions, the temperature-induced drought stress has overridden the potential CO2 'fertilization' on tree growth, hence challenging today's predictions of improved forest productivity of temperate forests., (© 2014 The Authors. New Phytologist © 2014 New Phytologist Trust.)

We tested three hypotheses related to the functioning of mountain plants, namely their reproductive effort, leaf surface structure and effectiveness of CO(2) assimilation, using archive material from contrasting elevations. Analysis of elevational trends is at risk of suffering from two major biases: a phylogenetic bias (i.e. an elevational change in the abundance of taxonomic groups), and covariation of different environmental drivers (e.g. water, temperature, atmospheric pressure), which do not permit a mechanistic interpretation. We solved both problems in a subcontinental survey of elevational trends in key plant traits in the European Alps and the high Arctic (northern Sweden, Svalbard), using herbarium samples of 147 species belonging to the genera Carex, Saxifraga and Potentilla. We used both species and phylogenetically independent contrasts as data points. The analysis revealed enhanced reproductive efforts at higher elevation in insect-pollinated taxa (not in wind-pollinated taxa), no increase in leaf pubescence at high elevation (as is often assumed), and a strong correlation between (13)C discrimination and elevation. Alpine taxa operate at a smaller mesophyll resistance to CO(2) uptake relative to diffusive resistance (stomata). By comparison with congeneric low altitude polar taxa (low temperature, but high atmospheric pressure), the response could be attributed to the elevational decline in atmospheric pressure rather than temperature (a mean increase in delta(13)C by 1.4 per thousand km(-1)). The signal is consistent within and across genera and within species, suggesting rapid adjustment of leaf physiology to reduced partial pressure of CO(2). These results offer answers to long-debated issues of plant responses to high elevation life conditions.