No divergence in Cassiope tetragona (L.) D. Don: persistence of growth response along a latitudinal temperature gradient and under long-term experimental warming

The dwarf shrub Cassiope tetragona is increasingly used for arctic climate reconstructions, the reliability of which depends on a linear climate-growth relationship. Here, we examine the stability of the relationship between climate and annual growth parameters by using a spatial scale from the high to subarctic and long-term experimental warming at a high arctic site. We created four growth chronologies (stem length, total leaf length, number of leaves, average leaf length) for three sites: 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. Leaf lifespan was also determined. Each growing season from 2004 to 2010, 17 Open Top Chambers (OTCs) were placed at Ny-Ålesund, increasing daily mean temperatures with 1.23 oC. At the end of 2010’s growing season, plants were harvested from OTC and control plots, and growth parameters were measured. All growth parameters except average leaf length exhibited a strong linear positive response (R2 between 0.63 and 0.91) to mean July temperatures over the gradient. Average leaf lifespan was 1.4 years shorter in subarctic Sweden compared to arctic Svalbard. All growth parameters increased in response to experimental warming from the onset of the experiment in 2004, except number of leaves, which increased from 2005 onward. Leaf lifespan was not significantly affected by OTC-warming. The linear mean July temperature-growth relationships as well as the long-term effect of experimental warming on growth, confirm that growth parameters of C. tetragona are a reliable arctic-wide temperature proxy. Introduction The spatial and temporal paucity of long high resolution proxy and instrumental climate records in the Arctic restricts knowledge of past climate change at these high latitudes. To increase understanding of the drivers of natural and regional climatic variability within the Arctic, a more uniform coverage of past climate across the Arctic is needed (ACIA, 2005). Previous studies demonstrate that annual growth parameters of Cassiope tetragona may provide such a high resolution arctic-wide climate proxy record (Aanes et al., 2002,