Long-term trends in European tree growth over the past 1000 years - an interspecies comparison

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Annually resolved and absolutely dated terrestrial climate proxies such as tree rings constitute the basis of high-resolution paleoclimatology covering the past millennia. For Europe, temperature variability inferred from tree rings from high altitudes and latitudes was and still is intensively studied. Conversely, research of precipitation sensitive tree-ring records from low elevation sites (<1000 m a.s.l.) has only recently gained attraction, although hydroclimate variability (i.e. precipitation and drought) is a crucial element in the ongoing assessment of past and future climate change. Potential forcing factors, the amplitude and frequency of long-term (i.e. multi-decadal to centennial) growth trends, which are essential to reconstruct the full range of hydroclimate variability, are merely studied and not yet fully understood.In the proposed project, a large-scale multi-species comparison of long-term growth trends across Europe, spanning the last millennium, will be conducted for the first time. During the past decades tree-ring measurements from low-elevation and precipitation sensitive tree species in Europe such as oak, beech, alder, ash, elm, fir, pine and spruce have been collected from historical and archaeological timbers, and linked to the present with material from living trees to produce multiple species chronologies that span the last millennium. This unique data set of over 60.000 tree-ring measurements, including the most ecologically and economically important European tree species is available through a dense and active network of collaboration partners.The aim of the proposed project is to better understand the spatiotemporal variability of low frequency trends in tree growth and identify those significant drivers (i.e. climate and/or volcanic, CO2 or solar forcing) affecting tree growth over Europe. Methods applied will involve recently developed standardization methods, trend and spectral analyses, as well as different filtering techniques; all optimized to detect and describe low-frequency variations in the climate signal of tree-ring chronologies. The extracted long-term growth trends will be compared across different regions within Europe at the intra- and inter-species levels. To identify drivers of decadal to centennial trends, time series derived from other proxy archives (e.g. sediments) will be tested for their coherency with tree growth, as well as the influence of various climate parameters including stream flow and groundwater changes, and forcing anomalies.New findings gained in the proposed project of climate forced, low-frequency trends in tree growth and their underlying causes will improve climate reconstructions, model simulations and the quantification of long-term changes in the carbon cycle at continental to global scale.