ABSTRACT: Three millennia of climate, hydrology, and cliff ecology of the Great Lakes Basin of North America: long tree-ring records from cliff-dwelling trees.

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Abstract

We present a 2,787 year ring-width chronology from cliffside eastern white cedar (Thuja occidentalis L.) from Ontario's Bruce Peninsula. This long time-series is a 1,390 year extension of a previously-published chronology from the dolomitic limestone cliffs of the Niagara Escarpment. The record has been extended through the inclusion of non-living stem material that was salvaged from ledges and cliff-base talus. Earlier climate response modelling indicates a statistically-significant response to climate, primarily as an inverse response to temperature and a (weakly) significant response to precipitation in the season prior to growth. We now interpret this chronology as a proxy for summer rainfall and drought, based on additional climate-response modelling and an interpretation of a growing body of ecological data. A preliminary, inferred reconstruction of drought, in the form of the Palmer Drought Severity Index (PDSI), over the past 2,700 years is presented and we analyse the decadal to century scale variability of available moisture over that timescale. An exploration of two important statistical measures of chronology signal strength (RBAR and Expressed Population Signal, EPS) indicates periods of decreased reliability that need to be addressed. We therefore discuss strategies for improving the temporal integrity of our chronology, and for improving the reconstruction of PDSI. The potential to further extend the chronology through the inclusion of additional subfossil material found on cliffs, on cliff talus, in paleo shoreline caves, and preserved in situ below present-day lake levels is clearly demonstrated. Radiocarbon dating of trees, rooted in the substrate of the upper 40 meters of Lake Huron, yields ages as old as 10,000 years before present, suggesting the rare possibility for developing a chronology of Holocene length. We are currently developing a network of cedar chronologies across the entire breadth of the Niagara Escarpment, spanning at least 500 to 1,000 years. We are exploring the possibility of using these chronologies for creating statistically-robust reconstructions of past climate and hydrology from the Great Lakes region over multiple centuries.

Bibliography of Canadian Geomorphology