Forward and inverse modeling of Black spruce tree ring series: applications to paleohydrology and paleoclimatology

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The long-term objective of this NSERC DG is to produce hydro-climatic, tree-ring-based reconstructions in regions, such as the temperate and the boreal zones, where knowledge on past hydro-fluctuations is critically lacking. These reconstructions will provide a more accurate baseline to which recent changes can be compared and will cast a new light on the origin of long-term hydro-climatic variability in these understudied regions. To achieve this, we will propose a novel proxy-model fusion approach, specifically designed to overcome the difficulties encountered by dendrochronologists in regions where trees grow far from environmentally stressful conditions. In the past few decades, dendrochronology has greatly contributed to the understanding of long-term hydro-climatic changes. However, it led to equivocal results in regions where both temperature and precipitation control growth rates. The basic approach to climate reconstruction dictates a trade off between spatial resolution and climate-sensitivity, and favours the inclusion of either temperature-sensitive (restricted to latitudinal / altitudinal tree lines) or precipitation-sensitive (restricted to semi-arid regions) forest stands as proxies for past climates. However, refining our understanding of past instabilities will force tree ring scientists to extract robust hydro-climatic signals from regions that have contributed less to our understanding of global changes. Indeed, such regions are typically characterized by complex, multivariate and time-varying (divergent) proxy-climate relationships that represent major obstacles to the production of tree-ring-based reconstructions. The novel reconstruction approach will be implemented in North-Eastern North-America (NENA)’s boreal forest which represents an archetypal example of a region where ambiguous tree-ring-to-climate relationships prevail. The approach will be based on two complementary aspects. First, climatic signal extraction will rely on multiple proxies (tree ring widths, carbon and oxygen stable isotopes) in order to strengthen the analysis of both temperature and precipitation signals. Second, relationships between proxies and climate will be assessed using process-based modeling anchored to a vast network of multi-proxy measurements that integrate all temporal scales relevant to infer past climates from tree rings: infra-annual, inter-annual, decadal, millennial. This research program will generate crucial information on long-term hydro-climate variability and persistence in NENA, which will help refine our capacity to predict (and adapt to) future changes. The program will rely on a strong and active network of collaborations at the national and international levels and will represent an excellent opportunity for trainees to be involved in a research initiative that represents the cutting edge of paleoclimatology.