Annual growth ofCassiope tetragonaas a proxy for Arctic climate: developing correlative and experimental transfer functions to reconstruct past summer temperature on a millennial time scale

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Abstract
Annual growth of the polar evergreen shrubCassiope tetragonaon Svalbard was evaluated as a proxy for Arctic summer temperatures. Transfer functions were derived from temperature‐growth correlations of shoots and from a temperature‐growth response, obtained from experimental warming using open top chambers (OTC) in high Arctic tundra vegetation at Isdammen approximately 1.5 km southeast of Longyearbyen, Svalbard (78°N, 15 E) and in Longyeardalen, 3 km west of Isdammen from 2004 to 2006. Air temperatures, monitored throughout the summer months, were 1.3 °C higher inside the OTCs than in the control plots. Annual stem growth was measured by tagging stems and leaves, and in the lab with shoots harvested from OTCs and control plots. Annual growth parameters assessed were leaf production, sum of length and weight of individual leaves, and stem length increment derived from leaf scar distances and the distances between wintermarksepta in the stem. Wintermarksepta are formed at the end of the summer growth period when the pith is narrowing and consist of dense and dark tissue (Fig. 1b). The variation of annual growth in a 34‐year site chronology (based onCassiopeshoots from the surroundings of the OTCs and control plots) correlated strongly with the mean summer temperature on Svalbard. The number of leaf pairs, leaf length and stem length also increased in the OTC warmed plots in the second and third year of warming. Transfer functions were derived from the temperature‐annual growth correlations from a single shoot from Longyeardalen, from the cross‐dated Isdammen site chronology and from the growth response to experimental warming. Based on leaf scar distances and distances between wintermarksepta of well‐preserved subfossil shoots in arctic tundra soil, annual stem length increase was assessed for the layers of a soil core collected at the Isdammen site. Based on the derived transfer functions summer temperature of the period relating to the 15 cm deep tundra soil core layer, radiocarbon dated at 4230±40 bp, may have been 3.0 °C lower than the present‐day 6.2 °C value. These results indicate that the transfer functions can be used to reconstruct past temperatures, beyond the time range of instrumental temperature and ice core records of Svalbard. (a) Morphology of shoot ofCassiope tetragonacollected May 26, 2007, Longyeardalen. Removal of the front row of leaves shows four leaf pairs of the 2006 summer, the leaf primordium for the 2007 growing season and the stem length increase summer 2006. (b) Wintermarksepta, darker colored than the pith tissue, indicating the winterperiod in a longitudinal section of an air‐dried shoot collected August 26, 2006, Longyeardalen. image

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