Flood rings, earlywood vessels and hydrological signal in Fraxinus pennsylvanica trees growing along the central Assiniboine river floodplain, southcentral Canada

Abstract

In southcentral Canada, paleoflood reconstructions have mainly focussed on identifying flood rings in upper terrace bur oak (Quercus macrocarpa) trees from the Red and Assiniboine rivers. In contrast to upper terrace trees, floodplain tree species such as green ash (Fraxinus pennsylvanica) were said a few decades ago to provide ‘noisy’ flood proxies due to a greater flood exposition. This unverified assumption about floodplain trees was tested using green ash trees growing along the floodplain of the Assiniboine river. In each sample and for each year, flood rings were visually identified, vessels with a cross-sectional area >1000 µm2 as well as earlywood, latewood and total ring width were measured. Results indicated that flood rings were replicable and that they correspond to tree rings characterized by a high number of earlywood vessels having a reduced mean area (less porous earlywood) and, in which, radial multiples were abundant. Years in which flood rings were most abundant corresponded to documented high-magnitude floods. Flood-ring, earlywood vessel (except density and total area) and ring-width chronologies were all significantly correlated to spring mean discharge. Floodplain green ash trees do capture hydrological events including extreme floods and should thus not be ignored in paleoflood studies.