SOILS EVOLUTION AND TREELINE FLUCTUATIONS ON THE SOUTHERN SIDE OF THE ALPS UNDER HOLOCENE CLIMATE CHANGE

The climate in the Alps has been characterized, from the late 19th century until now, by an increase of the mean annual temperatures by about 2°C. It is known that the climate plays an important role in the ecosystems, particularly in the Alps, where meteorological factors, combined with severe morphological conditions, often show extreme behaviours. The climate is a principal factor governing the natural environment of mountains on short-time scales, and it characterizes the location and the intensity of biological, physical and chemical processes. Since mountains environments are susceptible to the impacts of a rapidly changing climate, they provide interesting locations for the early detection and the study of the signals of climate change and its impacts on the environmental systems. Indeed, in mountain landscapes, biological and abiological responses to variations in climatic conditions are particularly noticeable. The main evidences concern the glacier shrinkage, characterized by a general reduction of glaciers, by a widening of proglacial areas followed by colonization of the vegetation, by the upward shift of the vegetation belts and, in particular, of the treeline. Although altitudinal treelines are widely recognized as climatic boundaries, geomorphological processes and human activities can locally limit the treeline position, conditioning its altitude and dynamics. In fact, in mountain regions, trees establishment and growth at the highest altitudes may be greatly affected by geomorphological processes and/or local human impacts. The aim of this PhD Project is the characterization of the maximum altitude of the treeline for the central part of the Italian Alps, the spatial and statistical investigations of the role of geomorphological processes and related landforms on the treeline position and, finally, the reconstruction of late Holocene soil evolution and environmental changes at the treeline in some sample areas. In order to cover the central and the western part of the Italian Alps and also the northern part of Apennines, five different sample areas were selected: two in the Upper Valtellina (Mt. Confinale and proglacial area of Forni glacier), two in Aosta Valley (Becca di Viou and Saint Nicolas) and one in the Northern Apennines (Mt. Cusna). In order to perform a more complete reconstruction of the environmental changes at the treeline a multidisciplinary approach was used. First, a detailed mapping of the treeline and the determination of the main factors (i. e., geomorphological constraints, climate and human impact) that limit the maximum altitude of the treeline by means of GIS software was performed to reconstruct the treeline position and to choose the sample areas. Survey and sampling of soils was carried out in the sample areas, with the purpose of outlining the general characteristics of the soils and of searching for paleosols and other potential proxy archives. The analysis of the identified soil profiles took advantage of multiple laboratory and microscope techniques to provide both maximum accuracy of data and a wider overview of the aspects involved, with also new methods of analysis like the Rock-Eval pyrolysis and the stable carbon and nitrogen isotopes composition. In addition, a detailed reconstruction of the treeline altitudinal dynamics using a dendrochronological approach and an analysis of climate-growth response using a dendroclimatic approach were performed. Whereas, in the sample areas with high geomorphological influence, the soil erosion rates was investigated using a dendrogeomorphological approach, sampling trees and related exposed roots. From this multidisciplinary approach, the following results are emerged. The treeline elevation in the Upper Valtellina region is primarily limited (82% of trees) by geomorphological constraints (treeline mean elevation 2355 m a.s.l.), whereas 15% of the analyzed trees belong to climatic treeline (treeline mean elevation 2530 m a.s.l.) and human impact influences only 3% of the analyzed trees (treeline mean elevation 2335 m a.s.l.). The results also indicate that the current position of treelines is not only lowered by geomorphological constraints in general but, more specifically, can also be differently affected by different groups or types of geomorphological processes. In this study area, gravitational processes affect 87% of the treeline trees, running and/or channelized water affects 8% of trees, and ice affects only 5% of trees. Gravitational processes represent the most destructive processes of the treeline ecotone and tend to reduce the ecotone width, leading to coinciding treeline and timberline. Furthermore, the landforms related to gravitational processes can create considerable obstacles to the treeline upward shift, especially when they merge (e.g., merging talus cones). As far as the site scale is concerned, in Mt. Confinale study area, the reconstruction of the altitudinal dynamics shows that the treeline elevation over time increased from 2505 m a.s.l. (period 1990–1999) to 2531 m a.s.l. (period 2000–2009) and, finally, to 2545 m a.s.l. (in 2013) with a rates of ongoing treeline upward shift for the period 2000–2009, of up to 2.6 m/y. On the Mt. Confinale study site the presence of geomorphological factors may combine with climatic ones to promote an upward shift in the treeline and also may influence the soil evolution, especially if active geomorphological processes are less intense. The soils, although showing a good degree of development, do not reach the characteristics of a typical forest soil. The presence of not particulary developed soil in the treeline ecotone suggests that the colonization of the Pinus cembra, favored by ecologic, microtopographic and microclimatic factors, is followed by the pedogenesis. The latter, in turn, may favor a more active colonization that could lead to a more marked shift to higher altitude of the vegetation belt. In the proglacial area of the Forni glacier, the soil development fit quite well with the chronosequence approach, although not all pedological parameters seem to follow the age-related trend. In fact, even if these soils have a young age, in only 150 years of pedogenesis some transformations could be noticed. For example in the superficial horizons the organic carbon and total nitrogen content increased as well as the values of crystalline iron oxides, while the content of the amorphous iron oxides decreased according to the deglaciation time. For what concerns the Becca di Viou study sites, the literature attests an upward shift of the treeline in the last century; the obtained pedological and the dendroclimatic data seem to stress a changing in treeline ecotone position, following by a possible shift of the timberline and also by a shift of the soil type. The pedological results also highlight that forest, in the past, could reach higher altitude than today. In Saint Nicolas study area the soil analyses results underline the occurrence of colluvial events that influence the vegetation colonization and soil development. Moreover, the results show the great influence of water driven processes on the environmental evolution and also on soil erosion rates, the latter determined by means of trees roots exposure (local average erosion rates: 1.16 cm/year). Instead, at Mt. Cusna study site, the soil analyses underline the presence of different pedogenetic phases: a first, strongly evident, phase leading to the formation of a brown soil under forest vegetation cover, which was interrupted by a colluvial event; a second phase characterized by the presence of a stable forest cover, which favored the formation of a brown soil, also interrupted by a colluvial events; the present day pedogenetic phase, associated to a vegetation change, in particular at higher altitude the presence of shrub as Vaccinium myrtillus, promoting the cryptopodzolization processes. In conclusion, the multidisciplinary study of all examined sites enlightens the following common traits: the geomorphological processes and related landforms are the most important factor influencing the treeline position; in the areas with a climatic constraint (Mt. Confinale, Progracial area of the Forni glacier and Becca di Viou) an upward shift of vegetation belt and of the soil type has been observed; in the areas with geomorphological constraints (Saint Nicolas and Mt. Cusna) the vegetation and soil responses to climate change are masked by the effects of geomorphological processes, but a different response could be noted between Alpine and Apennine study case due to the slope dynamics and the climatic features.