TREES AS INDICATORS OF CLIMATE CHANGE, GEOMORPHOLOGICAL PROCESSES AND ANTHROPOGENIC IMPACT IN NORTHERN ITALY

Climate is changing due to both natural and anthropogenic causes. Among the several natural \u201csentinels\u201d of climate change, trees potentially represent excellent indicators for reconstructing the changing climatic conditions in the recent and remote past and for monitoring the impacts of the current global warming and the related environmental conditions. Trees respond to changes in climate (air temperature, precipitation rate, water availability, etc.) and environment (soil nutrients, pathogen infections, mechanical wounding, etc.) both rapidly, modifying their tree-ring growth rate and physiological processes, and slowly, modifying their distribution. Chemical elements and pollutants deriving from human activity can mask the climatic signal but, at the same time, trees become precious collectors of data that can be used for multidisciplinary research. In this Ph.D. project, I aimed at testing the potential use of trees as indicators of climate, environmental and human impacts in different morphoclimatic conditions, and to investigate if natural and anthropogenic conditions can mask the climatic signal recorded in trees. For this purpose, I selected different key sites in northern Italy. I applied remote sensing techniques, dendrochronological and dendroisotopical approach and the investigation of Volatile Organic Compounds both in tree leaves and in tree rings at the Miage Glacier (AO), the widest debris-covered glacier in Italy. This site has been chosen because of its uniqueness: the Miage Glacier is the only glacier, in the southern side of the Alps, characterized by the presence of abundant supraglacial vegetation that confers an important ecological value to the site. The Miage Glacier has been recognised as a geomorphosite for its scientific, scenic, cultural, economic and educational values. The results show that high-resolution satellite images allow the rapid detection of supraglacial trees whenever their density is high and that tree establishment is driven by supraglacial slope, debris thickness, glacier thickness and surface velocity. Supraglacial trees are characterized by tree-ring width, stable carbon and oxygen isotopes and needle volatile organic compounds that are significantly different compared to trees located outside the glacier. The dendrochronological approach also resulted to be successful for defining areas affected by glacial melting water and, as a consequence, for assessing geomorphological hazards in glacial environments. Tree-ring width and terpenes in annual tree rings were found to be valuable indicators of fungal infection in mountain environments. I also performed magnetic and mineralogical analyses of tree bark samples both at the head of a sample Alpine valley, in Santa Caterina Valfurva (SO), near one of the widest Alpine glacier, the Forni Glacier, and in an urban polluted context, the city of Milan, in order to identify the accumulation rate of magnetic particles and compare different morphoclimatic environments. The results show that outer tree bark can be useful for monitoring the distribution of pollutants with magnetic properties and suggest the role of trees as PM sinks in urban areas. Overall, the results presented in this thesis represent a contribution for a better understanding of the potential use of trees both in high mountain environments and in urban areas for monitoring the impacts of the current climatic and environmental changes. Some of the proposed approaches represent scientific novelties because were never applied in extreme environments or were never considered in the context of the current climate change

Novel indicators of environmental change from trees in the debris-covered glacier foreland: the case study of the miage glacier (Mont Blanc Massif, Italian Alps)

Supraglacial trees are a useful source of data for reconstructing past glacier surface movements and debris-coverage instability. Proglacial trees also represent a useful tool for the identification and dating of changes in the glacial stream discharge and wide-spreading of melting water. Dendroglaciology is currently applied not only for reconstructing glacier fluctuations but also for investigating glacier surface dynamics at decadal scale. Trees and dated tree-ring characteristics such as scars, growth rate and reaction wood may provide information about glacier movements, discharge and hydrology. The Miage Glacier in the Mont Blanc Massif (Italy), represents a unique situation in the southern side of the Alps, due to the presence of abundant supraglacial vegetation. The density and distribution of trees is strictly linked to glacier surface velocity, thickness of debris-coverage, ablation rate, grain size distribution, slope and ice thickness, as documented by the results obtained during field surveys, data analysis and remote sensing techniques. The most recent studies show that supraglacial trees can also be considered environmental and climatic stress indicators. Leaf VOC (Volatile Organic Compounds) emissions and tree-ring carbon and oxygen stable isotopes show significant differences in trees located on the supraglacial debris with respect to trees on the lateral moraine, and these results suggest the possibility to apply these techniques in the identification of areas affected by glacio-geomorphological and climatic stress. Tree-ring characteristics may also be analyzed in order to reconstruct the past hydrology of debris-covered glaciers with annual resolution. In particular, trees fed by glacial meltwater of the Lago Verde (Miage Glacier) show that tree-ring cellulose is more depleted in \u3b418O compared to trees fed by other water sources and, moreover, tree-ring width is narrower in trees affected by lake-level fluctuations

Forni glacier fluctuations: influence on the biological system in the glacier foreland

Glacier retreat is among the most evident impacts of the current climate change. The phases of glacier shrinkage can be reconstructed through geomorphological investigations conducted in the glacier foreland, a suitable site where to evaluate the relationship existing between glacier retreat, geomorphological processes and colonization of newly formed terrains performed by biological forms. The most recent studies conducted in one of the most representative recently deglaciated areas of the Italian Alps are here presented. The expansion of the proglacial area of the Forni Valley (Stelvio National Park, Italy) was analyzed for the period comprised between the Little Ice Age (LIA) and nowadays, through the use of historical images, orthophotos and field data. In particular, the area and rate of expansion were estimated for the four time intervals defined by the dated moraines, and the results show that the expansion rate was nearly double in the last 88 years compared to the previous years, thus highlighting an acceleration in the expansion rate starting from the beginning of the 20th century. As the glacier foreland expands, different organisms colonize progressively older terrain, including trees, if the area is located below the treeline. Tree ecesis time and germination year were estimated by means of dendrochronological approach and whorls branch counting, performed on living conifers growing in the most recent deglaciated area, close to the current position of the glacier front. The results of this study show an acceleration of the average ecesis in the last few years, with values ranging between 5 and 11 years, and with an average value of 7 years. On the other hand, glacier advances destroy forests. The study of buried logs and peat (performed through radiocarbon dating and dendrochronological techniques) contribute to a better understanding of past glacier fluctuations and related climate change. In particular, a buried log found in the Forni Valley revealed information about the Subboreal climatic transition and the related glacier fluctuations. Overall, these findings evidence some of the complex interactions between abiotic and biotic systems in glacial environments, and the precious contribution of arboreal vegetation in dating glacier changes and monitoring velocity of processes in the glacier foreland over time

Glacier features influencing the presence and abundance of supraglacial trees: the case study of the miage debris-covered glacier (Mont Blanc Massif, Italian Alps)

The number of debris-covered glaciers featuring supraglacial tree vegetation is increasing worldwide, as a response of high mountain environments to the current climate warming. At the debriscovered surface of these glaciers, trees can be found thus giving peculiar landscape and ecosystems. Their distribution is not homogeneous, thus suggesting that some glacier parameters influence germination and growth of trees. This study was performed on the widest Italian debris-covered glacier, the Miage Glacier in the Mont Blanc massif, where herbaceous and tree vegetation is present at the surface of the glacier tongue. We analyzed the ablation area in the range from 1730 m to 2400 m a.s.l., where a quite continuous debris coverage is present and colonized by trees (mainly Larix decidua Mill. and Picea abies Karst), also reaching an age of 60 years close to the terminus. By remote sensing investigations and through field surveys we obtained a record of glacier parameters (debris thickness, debris-surface temperature, slope, aspect, elevation, ablation rate, surface velocity, debris-NDMI, variation in ice thickness over several years) to be analyzed with respect to the presence and abundance of trees in 15 plots (plot size: 15 m x 15 m). Our results show that supraglacial trees are present at the Miage Glacier: 1) whenever exceeding a debris thickness threshold ( 6519 cm), 2) with a quite gentle slope ( 64 22\ub0), 3) with a low glacier surface velocity ( 64 7.0 m/year), 4) where the ice thinning due to surface ablation is moderate (ranging between -1.8 m/year and -0.7 m/year) and 5) where the vertical changes due to glacier dynamics are positive (i.e. prevalent increase due to both slow debris accumulation and then preservation of ice flow inputs that we found ranging from +7 m and +28 m over a period 28 years long). The analysis of the same parameters, conducted on other debris-covered glaciers featuring supraglacial trees, may provide new data in order to evaluate if such conditions are local ones or if they are actual and general factors driving germination and growth of trees

Assessing glacier features supporting supraglacial trees : a case study of the Miage debris-covered Glacier (Italian Alps)

The number of debris-covered glaciers featuring supraglacial trees is increasing worldwide as a response of high mountain environments to climate warming. Generally, their distribution on the glacier surface is not homogeneous, thus suggesting that some glacier parameters influence germination and growth of trees. In this study, we focused our attention on the widest Italian debris-covered glacier, the Miage Glacier (Mont Blanc massif). We analyzed the ablation area in the range from 1730 to 2400 m a.s.l. where continuous debris coverage is present and trees are found. Using data obtained by remote sensing investigations and field surveys, we defined a record of glacier parameters to be analyzed with respect to the presence and abundance of trees. We found that supraglacial trees are present at the Miage Glacier (1) whenever exceeding a debris thickness threshold (\u2a7e19 cm), (2) with a gentle slope (\u2a7d10\ub0), (3) with a low glacier surface velocity (\u2a7d7.0 m/yr), and (4) where the vertical changes due to glacier dynamics are positive (i.e. prevalent increase ranging between +7 and +28 m over 28 years due to both slow debris accumulation and preservation of ice flow inputs). The statistical analysis supports our findings. The analysis of the same parameters might be conducted on other debris-covered glaciers featuring supraglacial trees, in order to evaluate whether such conditions are local ones or whether they are general factors driving germination and growth of trees. By identifying the features supporting the presence and growth of trees in these environments, and their thresholds, a contribution is given for a better understanding of the importance of debris-covered glaciers and, in general, of debris-covered ice, as a refuge for trees during glacial and warm intervals of the Holocene

Investigating distribution patterns of airborne magnetic grains trapped in tree barks in Milan, Italy : insights for pollution mitigation strategies

High levels of air particulate matter (PM) have been positively correlated with respiratory diseases. In this study, we performed a biomonitoring investigation using samples of bark obtained from trees in a selected study area in the city of Milan (northern Italy). Here, we analyse the magnetic and mineralogical properties of the outer and inner barks of 147 trees, finding that magnetite is the prevalent magnetic mineral. The relative concentration of magnetite is estimated in the samples using saturation isothermal remanent magnetization (SIRM) and hysteresis parameters. We also make a first-order estimate of absolute magnetite concentration from the SIRM. The spatial distribution of the measured magnetic parameters is evaluated as a function of the distance to the main sources of magnetic PM in the study area, for example, roads and tram stops. These results are compared with data from a substantially pollution-free control site in the Central Italian Alps. Magnetic susceptibility, SIRM and magnetite concentration are found to be the highest in the outer tree barks for samples that are closest to roads and especially tram stops. In contrast, the inner bark samples are weakly magnetic and are not correlated to the distance from magnetite PM sources. The results illustrate that trees play an important role acting as a sink for airborne PM in urban areas

Tree-ring volatile terpenes show potential to indicate fungal infection in asymptomatic mature Norway spruce trees in the Alps

Volatile terpenes (VTs) content in tree-ring resin, in response to natural infection by Heterobasidion spp. in asymptomatic mature Norway spruce (Picea abies) trees was investigated. Twenty-three randomly selected mature trees were sampled in a stand in the Western Italian Alps by extracting cores using an increment borer. Based on fungal isolations from cores and molecular typing using taxon-specific competitive-priming (TSCP)-polymerase chain reaction, 12 out of the 23 trees were identified as infected by Heterobasidion parviporum. Tree-ring growth patterns and VT content in tree rings were determined. Analysis of VT content was performed by means of gas chromatography mass spectrometry on a subset of trees. Results show slightly but not significantly lower tree-ring width in infected compared with non-infected trees in the past two decades. Total concentrations of sesquiterpenes (SQTs) and relative proportions of -pinene, -pinene and longifolene were significantly greater in infected trees; while relative proportions of camphene, 3-carene, -cymene, sesquiterpene 15.90 and -farnesene were significantly lower. This is the first study showing that VTs in tree-ring resin may indicate infection of trees by a fungal forest pathogen, even when trees are mostly asymptomatic