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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^2 = 0.34 and
R^2 = 0.47, respectively. The instrumental record (1912-2008) extended with
these reliable mean July temperature estimates (1876-1911) reveals a
significant warming trend on Svalbard since 1876 of 0.07 ^oC decade^-^1 on
average. This study shows that the climate-growth relationships in C.
tetragona, its longevity, its annual resolution, the availability of
(sub)fossil fragments in tundra soil cores and its circumartic distribution
make it a very valuable tool for climate reconstructions beyond the
instrumental record and in areas lacking meteorological data, throughout
the Arctic.
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