Are periodic tangential band of vessels a new anatomical marker of floods in diffuse-porous tree rings?

Flood rings (FR) in ring-porous species have been widely used to identify flood events in boreal and temperate regions. Flood rings also have been experimentally reproduced in both Quercus and Fraxinus species. More recently, continuous measurement of earlywood cross-sectional vessel area in riparian black ash trees (Fraxinus nigra Marsh.) have shown that not only were FR associated with flood events but that the year-to-year variation in chronologies derived from earlywood cross-sectional vessel area also reflected that in mean spring flow data. These findings led to the reconstruction of the Harricana river spring flow for the period 1770-2016 with more than 65% of the variance in the gauge streamflow data captured (See Nolin et al. presentation at EGU2020). Compared to ring-porous species, anatomical variations in diffuse-porous species in relation to flood events has been little studied. In this study, both ring-porous black ash and diffuse-porous [trembling aspen (Populus tremuloides Michx.) and balsam poplar (Populus balsamifera L.)] trees were sampled in three floodplain sites located on the shore of Lake Duparquet, northern Quebec. Within each floodplain site, trees were selected so to represent a gradient of exposure to spring flooding. Given that the response of black ash to flooding is well documented (FR), paired sampling was used so each Populus tree was paired with a nearby black ash tree. When feasible, cross-sections from dead trees were also collected. For each tree, the elevation of the tree base to lake water level and the height of extracted cores were noted. The main objective of the study was to assess if diffuse-porous trembling aspen and balsam poplar growing on floodplains responded like ring-porous black ash to annual spring flooding. All wood samples were prepared following standard dendrochronological procedures with visual crossdating validated using program COFECHA. In addition to ring-width measurements, a visual determination of the intensity of FR was done for each black ash tree. In diffuse-porous species, a newly observed tree-ring anomaly referred to as tree ring with “periodic tangential band of vessels” (PTBV) were systematically compiled using a two-part numerical code; the first digit corresponding to the start position of the banding sequence within a tree ring and the second digit referring to the number of consecutive bands within a sequence. Two observers independently compiled their observations. The main hypotheses were that years recording PTBV will correspond to FR years and that they will also be associated with those hydroclimatic variables leading to major spring floods. Preliminary analyses indicated that FR and PTBV years display synchronicity. Both anomalies are also associated with hydroclimatic conditions leading to major spring flooding. The absence of a perfect match between ring-porous and diffuse-porous species however as well as the observed variability in the banding patterns still need to be analyzed in relation to flood exposure and core height. The discovery of a new potential flood marker in diffuse-porous tree species opens the door for the novel application of wood-cell anatomy in dendrohydrology and especially when ring-porous species are absent.

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