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The reliability of Cassiope tetragona as temperature proxy might be restricted by influence on growth of precipitation and amount of Photosynthetically Active Radiation (PAR). Carbon-13 discrimination (Delta) in C(3-)plants is generally influenced by temperature and precipitation and can therefore potentially record important additional climatic information.

We doubled precipitation and reduced PAR (-43%), during four and two growing seasons, respectively, at a high arctic site in Svalbard. Differences in discrimination in leaves from separate light, and thus temperature, regimes (sun-facing and soil-facing leaves) were also assessed. A Delta-chronology (1975-2008) in annual shoot length increments was developed.

C. tetragona growth did not respond to enhanced precipitation and only slightly to PAR-reduction. Discrimination against carbon-13 was stronger in leaves and shoots receiving extra precipitation and weaker in sun-exposed leaves compared to soil-facing leaves. The annual climate signal in the Delta-chronology was strongly smoothed by secondary radial growth.

Our results show that the temperature signal in C. tetragona is hardly disturbed by changes in summer precipitation or PAR, which confirms its suitability as temperature proxy. The experimental evidence for the sensitivity of carbon-13 discrimination in C. tetragona to precipitation and temperature changes shows its potential as proxy for hydrological changes in the polar semi-desert.