Flood-Rings Production Modulated by River Regulation in Eastern Boreal Canada

In northeastern boreal Canada, the long-term perspective on spring flooding is hampered by the absence of long gage records. Changes in the tree-ring anatomy of periodically flooded trees have allowed the reconstruction of historical floods in unregulated hydrological systems. In regulated rivers, the study of flood rings could recover past flood history, assuming that the effects of hydrological regulation on their production can be understood. This study analyzes the effect of regulation on the flood-ring occurrence (visual intensity and relative frequency) and on ring widths in Fraxinus nigra trees growing at five sites distributed along the Driftwood River floodplain. Driftwood River was regulated by a dam in 1917 that was replaced at the same location in 1953. Ring width revealed little, to no evidence, of the impact of river regulation, in contrast to the flood rings. Prior to 1917, high relative frequencies of well-defined flood rings were recorded during known flood years, as indicated by significant correlations with reconstructed spring discharge of the nearby Harricana River. After the construction and the replacement of the dam, relative frequencies of flood rings and their intensities gradually decreased. Flood-ring relative frequencies after 1917, and particularly after 1953, were mostly composed of weakly defined (less distinct) flood rings with some corresponding to known flood years and others likely reflecting dam management. The strength of the correlations with the instrumental Harricana River discharge also gradually decrease starting after 1917. Compared with upper floodplain trees, shoreline trees at each site recorded flood rings less frequently following the construction of the first but especially of the second dam, indicating that water level regulation limited flooding in the floodplains. Compared with the downstream site to the dam, the upstream ones recorded significantly more flood rings in the postdam period, reemphasizing the importance of considering the position of the site along with the river continuum and site conditions in relation to flood exposure. The results demonstrated that sampling trees in multiple riparian stands and along with various hydrological contexts at a far distance of the dams could help disentangle the flooding signal from the dam management signal.

Multi-Century Spring Flood Reconstruction in Eastern Boreal Canada from Novel Application of Wood-Cell Anatomy

<p>The streamflow regimes of eastern boreal Canada are snow-melt and ice-melt driven with the highest flows occurring in spring. Over the last few decades, a positive streamflow trend has been observed, with increasing severity and frequency of spring flooding. Further changes in flood dynamics are projected as a consequence of global climate change. The validity of projections is restricted by the lack of long and spatially well-replicated observations. High-resolution proxy records are needed to better understand the natural range of variability in spring runoff and associated atmospheric controls.</p><p>Recent research has shown that riparian black ash trees (Fraxinus nigra Marsh.) exposed to periodic submersion produce “flood rings” whose earlywood cross-sectional vessel area is linearly associated with the severity of flooding. Twelve continuous chronologies of ring width and earlywood vessel anatomy were developed for Lake Duparquet to extend the record of Harricana River mean spring flow. A visually determined index of flood rings was also developed to determine i) the spatial coherency of the spring flood signal and ii) the coherency of the flood signal among natural, regulated and unflooded rivers.</p><p>The reconstruction spans the period 1770-2016 and captures more than 65% of the variance of Harricana river spring flow. Trend analysis indicates an increase in both magnitude and frequency of the major floods starting at the end of the Little Ice Age (LIA, 1850-1890), with highest peaks after 1950. Time-frequency analysis shows non-stationarity: a stable 30-year periodicity during the LIA is replaced by a decadal pattern starting around 1850, and evolves into a more high-frequency pattern after 1930. The signal is strongly coherent between watersheds for natural rivers and weaker for regulated basins. Field correlations with gridded climate data indicate the broad spatially coherent pattern of spring high flows across much of central/eastern north Canada is positively associated with April-May precipitation and snow cover, and negatively associated with March-April maximum temperature.</p><p>These large-scale associations support atmospheric forcing of inter-annual hydroclimatic variability. While the Artic and North Atlantic Oscillations have previously been found to influence winter and spring climate conditions in eastern Quebec, our results contrast with a significant negative association with El-Niño Southern Oscillation from January to May, and the Pacific Decadal Oscillation from December to February. In Lake Duparquet, warm and wet air from Pacific-South Ocean (El-Niño) are associated with early spring and small floods, while cold and dry air masses (La-Niña) correlate to late thaw and high floods in spring. The association with sea surface temperature and 200mb geopotential field heights reveal a clear atmospheric connection between eastern north boreal Canada and the tropical Pacific Ocean.</p><p>The novel application of wood-cell anatomy to hydroclimatology underscores an increase in flood frequency and severity since the end of the 18<sup>th</sup> century in northeastern Canada. More broadly, the application highlights how analysis of tree rings from riparian trees can be used to extend the flood history of boreal rivers.</p>

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

<p>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 <em>Quercus</em> and <em>Fraxinus</em> species. More recently, continuous measurement of earlywood cross-sectional vessel area in riparian black ash trees <em>(Fraxinus nigra</em> 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.</p><p>In this study, both ring-porous black ash and diffuse-porous [trembling aspen (<em>Populus tremuloides</em> Michx.) and balsam poplar (<em>Populus balsamifera</em> 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.</p><p>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.  </p>

TREE-RING FEATURES: INDICATORS OF EXTREME EVENT IMPACTS

Wood anatomical features may be visible on the microscopic as well as on the macroscopic scale. While the former can often be quantified by detailed wood anatomical analyses, the latter are often treated as qualitative features or as binary variables (present/absent). Macroscopic tree-ring features can be quantified in terms of frequency, intensity, or classified according to their position within a tree ring, like intra-annual density variations (IADFs) in conifers or frost rings in earlywood or latewood. Although some of these tree-ring features, like e.g. missing rings or IADFs are often seen as anomalies, hampering dendrochronologists to perform proper crossdating of tree-ring series, many of these properties are formed under extreme environmental stress or heavy impact, and could mark these extreme events by the manifestation in the wood anatomical structures throughout the lifespan of trees. The described tree-ring features form discrete time-series of extreme events. For example, flood rings may be marked by lunar-shaped earlywood vessels or enlarged latewood vessels in ring-porous oaks. White earlywood rings and light rings indicate reduced cell wall thickness and lignification occurring in very cold years. Frost rings result from cambial cell death during abrupt cooling events in the growing season. Missing rings and IADFs are mainly caused by drought events. Characteristic variations in earlywood vessel size, shape, or number in ring-porous oak species are markers for flood events, defoliation, heat stress, or drought. Traumatic resin ducts may be triggered by a range of biotic or environmental stressors, including wounding, fires or mechanical factors. Reaction wood is indicative of mechanical stress, often related to geomorphic events. In many cases anatomical responses are unspecific and may be caused by different stressors or extreme events. Additionally, the sensitivity of trees to form such features may vary between species, or between life stages within one species. We critically evaluate the indicative value of different wood anatomical tree-ring features for environmental reconstructions.