ECOLOGICAL CLIMATOLOGY ALONG AN ELEVATIONAL TRANSECT IN THE OUTER BELT OF THE ITALIAN ALPS: MODERN POLLEN, VEGETATION AND CLIMATE

Montane vegetation is traditionally known to be particularly sensitive to climate changes. The strong elevational climatic gradient that characterizes mountain areas results in a steep ecological slope, with several ecotones occurring in a small area. Modern pollen deposition is significantly predicted by both vegetation cover and pollen production; in turn, each predictor is significantly predicted by elevation and climate. Analyses of modern pollen deposition are essential to calibrate fossil pollen sequences accounting for site topography and depositional process, and thus for pollen-based palaeoenvironment and palaeoclimate reconstructions. This study analyzes the relationships among modern pollen assemblages, vegetation and climate along an elevational gradient in the outer belt of the European Alps. Results of Canonical Correspondence Analysis (CCA) demonstrated a general good agreement with previous studies, which identified elevation as the main gradient in the variation of modern pollen and vegetation assemblages in elevational transects. Modern pollen assemblages have been studied in pollen traps and moss samples from different vegetation communities along an elevational transect (stretching from 1240 to 2390 m asl), as well as the vegetation using the Braun-Blanquet system up to the 10 m radius scale, field vegetation surveys and aerial photographs for plant cover of the main species for larger surfaces. Moss samples are assumed to record an average of several years of pollen deposition and can be profitably used as analogues for fossil pollen assemblages; while pollen traps can be expressed as Pollen Accumulation Rates (PAR) and used as a modern reference to estimate past plant population densities. Alnus viridis, the main woody species forming dwarf forests in the oceanic-type timberline ecotone, shows a specific elevational PAR arrangement under modern climate conditions. Strong pollen producers (e.g. Pinus sylvestris/mugo, Picea, Castanea, Corylus and Ostrya) display enhanced uphill wind-transport to subalpine and alpine zones leading to wider pollen belts with less defined boundaries than vegetation. To overcome these limitations, potential indicator pollen taxa of alpine/subalpine belts (Vaccinium, Rhododendron, Loiseleuria) documented in this study and PAR of timberline species (e.g. Alnus viridis) could be useful. Thus, if it is possible to identify the major vegetation types and ecotones by means of their modern pollen deposition (e.g. timberline ecotone), other limits, poorly marked by changes in pollen dispersal (e.g. treeline) are not resolved by pollen proxies alone.