Dendrochronology in the High Arctic: July air temperatures reconstructed from annual shoot length growth of the circumarctic dwarf shrub Cassiope tetragona.

The instrumental Arctic climate record is both temporally and spatially limited. Therefore, there is a need for reliable climate proxies to increase knowledge of past and future Arctic climate change. Annual shoot length increase of the circumarctic dwarf shrub species Cassiope tetragona represents such a new climate proxy. We measured annual shoot length increase of 32 plant samples of the circumarctic dwarf shrub species C. tetragona using the presence of wintermarksepta within the stems, resulting in a 169 year growth chronology (1840-2008) for a High Arctic site on Svalbard. This is the longest growth chronology for dwarf shrub species produced up to now. Relationships between climate and Cassiope growth were investigated through correlative, response function and forward stepwise multiple regression analysis. July average air temperature was found to be the most important factor determining growth, by itself capable of explaining 41% of the variance in shoot growth between 1912 and 2008. The second best predictors were previous year September precipitation sums and average air temperatures, along with several previous growth parameters. A multiple regression model explaining growth with current July and previous year September temperature, combined with previous growth of lag 1, 2 and 5 years as predictors explains 70% of the observed variance in growth. July temperatures and previous year September precipitation sums alone explain 59% of the variance in standardized growth. Mean July air temperature was reconstructed for the period between 1876 and 2007 by a growth-temperature transfer model, using current and following year's growth. The estimated temperatures correlated well with measured temperatures over the calibration (1912-1959) and verification (1960-2007) period: R