PALEOSUOLI ED ALTRI ARCHIVI PALEOAMBIENTALI PER LA RICOSTRUZIONE DELLE FLUTTUAZIONI OLOCENICHE DELLA TREELINE ALPINA E APPENNINICA
Creators: name: COMPOSTELLA, CHIARA
Publisher: name: Universit� degli Studi di Milano
Year: 2012

description: The aim of this research is the reconstruction of Holocene environmental changes at the treeline in three study areas along a gradient from continental to oceanic climate; the study was carried out in two areas in the Central Italian Alps (Valtellina and Val Camonica) and in one in the Northern Apennines (Mt. Cusna area).
To outline the complex interactions between soil, vegetation, climate and human activity, a multidisciplinary approach was necessary, combining pedological and paleopedological investigation, soil micromorphology, pedoanthracology, macroremains analysis and dendrochronology.
Firstly, we characterized current soils to outline present relations between soil, climate and vegetation. These results were necessary to correctly interpret data from the subsequent paleosol analyses. Paleopedological study, through the identification of forest paleosols above the current treeline, allowed to identify the altitudinal shift of the treeline during the Holocene. Micromorphological analyses highlighted processes driving soil development; we could analyze the parent material effect on soil development, correlate different sampling site and identify different climatic phases that have affected study areas.
Macroremains and charcoals analyses allowed to reconstruct vegetation compositions during the different climatic phases. Macroremains analysis was performed on peat samples and allowed a detailed environmental reconstruction. Charcoals were extracted from soil samples and were identified to reconstruct forest composition at the treeline; we performed also the identification of insect exoskeleton fragments found in soils, which provided detailed paleoecological information.
Dendrochronological analysis was performed along altitudinal transects, in order to estimate the effect of the current warming on vegetation and to assess how treelines respond to climate change, in order to better interpret data about the past fluctuations.
In the Alpine sites we focused on Podzols, typical soils linked to conifer forest cover; podzols were sampled at different elevations, along an altitudinal transect from closed forest to alpine grassland; we compared their macroscopical, physical, chemical and micromorphological characteristics, to asses if the profiles above the present treeline are in equilibrium with the present environmental conditions or if they are relict soils formed under more favorable climate conditions with higher treeline. Our analysis confirmed the second hypothesis; Podzol above present treeline are relict soil, very well preserved, with the same translocation rate of the lower ones and without traces of regressive pedogenesis. The identification of relict Podzols allowed to estimate a minimum rise of 250 m of the treeline probably during the Holocene climatic optimum.
In this study area we performed also a macroremains analysis in two peatlands, to correlate vegetation history with forest paleosols; a tree log found at the base of one peat section was AMS dated to 9090y cal BP, suggesting that the rise of the treeline to higher elevations occurred in the early Holocene.
On the Apennines we performed two main studies, the first with multidisciplinary approach (field observation, pedoanthracological analysis, soil micromorphology and dendrochronology) to reconstruct Late Holocene soil evolution and environmental changes at the treeline, and the second with micromorphological approach, to outline processes of soil evolution and to outline how micromorphology can help in reconstructing environmental change in areas where paleosols are available as proxy archives.
In the first study, we described nine soil and paleosols profiles across the present treeline; physical and chemical analyses, pedoanthracological and micromorphological observations were carried out and tree ring analysis was performed to outline recent disturbance events. All studied soils consisted of two superimposed units. The lowermost, with a well developed B horizon, is the result of intense brunification process, the superficial unit, up to 1 m thick, consists of colluvium deposits with poorly developed pedogenesis. Below the present treeline, Abies and Fagus dominated the anthracological assemblages in the buried B horizon and Abies charcoals were AMS dated to 790-670 y cal BP. Above the treeline, Abies and Laburnum dominated the anthracological assemblages in the buried B horizon and Laburnum charcoals were dated to 3920-3700 cal BP; the anthracological assemblages in the buried overlying Ab horizon was dominated by Abies, Laburnum and Vaccinium with abundant insect remains dated to late Middle Age. Our analyses indicate the occurrence of forests well above the present treeline in the Early-Mid Holocene;. the lowering of the treeline probably started during Late Holocene but open forest or treed heathland occurred at high altitude until recent times. Colluvial episodes and the burial of paleosols probably took place through successive events during Modern Age; tree ring analysis indicated that such phase occurred at least during the 18th and early 19th century.
In the second study on the Apennines, we focused on the potentiality of micromorphology as a tool for paleoenvironmental reconstruction in areas were paleosols are available as proxy archives. We studied nine soil profiles across the present treeline; from field observations it was possible to divide all the profiles into two main units: the upper one, the recent soil of colluvial origin, and the deeper one, the underlying buried, more developed paleosol. The micromorphological approach provided detailed information concerning the nature of the two sequences: we could reconstruct four principal phases of paleosol development, identify multiple colluvial layers and determine the origin of the colluvial material. So we demonstrated that the micromorphology approach represents a powerful tool in multidisciplinary paleoenvironmental study, due to the high level of resolution in outlining the successive phases of soil evolution.
Finally, we analyzed the response of the treeline to the current global warming; so, we performed a detailed, individual-based survey of different treeline types in our study areas, to assess treeline structure and dynamics in areas with different treeline type, species composition and bioclimatic conditions.
All the tree individuals were recorded and described along altitudinal transects from the closed forest to the species limit; the age of each individual was assessed through a core or estimated through the individuation of annual growth units, using a method purposely developed. The structure and dynamics of the treeline ecotone was described through the main altitudinal limits and the abruptness pattern of tree height, density and age. Our study showed that treeline was markedly different in the three areas, with diffuse treeline on the Alps, but with advancing dynamics only in one site (Valtellina), and abrupt and completely static treeline on the Apennines, even if the climatic parameters at the species limit were much more favourable than the alpine ones. A main role is probably played by species composition which is in turn determined by bioclimatic features and historical events.
In conclusion, we demonstrate the importance of a multidisciplinary approach to paleoclimate studies, to overcome the not uniform distribution of paleoenvironmental archives; an approach combining abiological and biological disciplines has proved to be useful to understand the response of the treeline to the past and present climate changes. Soils have proved to be excellent palaeoenvironmental archives; micromorphological analysis, in particular, is able to show the temporal succession of the different phases of soil evolution and the corresponding environmental.
The present research also underlines the importance of monitoring the current situation in order to better interpret the palaeoenvironmental evidence; the responses of vegetation to climate change at high altitude are not linear, although they are obviously climate-dependent. It has been observed how the treeline position is influenced by the bioclimatic profile of the different study areas and by the species composition of arboreal vegetation at its altitudinal limit. This information must be taken into account, both for the interpretation of palaeoenvironmental data and in the case of predictive studies, aiming to model the response of ecosystems to current climate change.