Dendrochronology in the High Arctic: development of a tree-ring like climate proxy

By Stef Weijers, Rob Broekman and Jelte Rozema
Systems Ecology, Department of Ecological Science, Faculty of Earth and Life Sciences, VU University Amsterdam, The Netherlands

The low distribution of instrumental and proxy climate data in the Arctic hampers clear distinction between natural climate variability and anthropogenic-induced change. This emphasizes the need for proxy climate data to fill in spatial and temporal gaps in the arctic climate record. Previous studies have shown that annual growth parameters of the long-lived circumarctic evergreen dwarf shrub species Cassiope tetragona might serve as an arctic climate proxy. Leaves of C. tetragona grow in opposite pairs, which alternate in 90o angles, forming four rows of leaves along its stems (hence tetragona). Annual leaf growth parameters (leaf length, number of leaves, average leaf length) can be measured as smaller leaves are formed by this species at the start and end of each growing season, resulting in wave-like leaf patterns along it stems with each wave representing one year of growth. Annual stem length growth can similarly be derived by measuring and summing the distances between leaf scars per year. An alternative measure for annual stem length growth is the distance between wintermarksepta (WMS). WMS are dark bands in the pith of C. tetragona stems, which demark the end of each growing season.
We created stem length growth chronologies for three sites along a high to subarctic climatic gradient: near C. tetragona’s cold tolerance limit (Ny-Ålesund, Svalbard), at its assumed climatic optimum (Endalen, Svalbard), and near its European southern limit (Abisko, Sweden), which together represent the entire temperature gradient of this species. As monthly means are a coarse unit for prediction of growth in marginal regions with short growing seasons, we tested growing degree-days (GDD) as a potential alternative to monthly mean temperatures. GDD is a measure for growing season intensity and defined as the cumulative temperature above a given threshold, usually set at 5 degree C.
GDD5 turned out to be a strong predictor of stem length growth on the high arctic archipelago Svalbard, but less so at C. tetragona’s southern range margin. A growth-GDD5 transfer function was created through calibration between the two Svalbard growth chronologies and the local instrumental record (available for the periods 1912-1930 and 1957-2008). These stem length chronologies from Ny-Ålesund and Endalen are 154 and 169 years long, respectively, and are the longest growth chronologies developed for this species thus far. We reconstructed GDD5 at Svalbard beyond its instrumental record (1885-2008). Reconstructed GDD5 shared more than 60% variance with the instrumental data. Our reconstruction of Svalbard Airport GDD5 shows that summers on Svalbard have become more intense from the early 1960’s onward, while before that time the intensity of summers showed both positive and negative trends. Accurate information on ongoing and past climate change in the High Arctic is thus recorded by C. tetragona stem length growth. A more uniform coverage of past arctic climate change can therefore be obtained through the construction of stem length growth chronologies for this species.

Willem Barentsz Poolinstituut

Bundeling van kennis, onderzoek en onderwijs over de Noord- en de Zuidpool

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