| Summary: | This research examines the responses of the circumpolar, evergreen, dwarf-shrub, Cassiope tetragona (L.) D. Don (Ericaceae) (arctic White heather) to experimental warming and natural climatic variability through dendrochronological techniques. The study's goals were: (1) to quantify C. tetragona growth and reproductive responses prior to and during experimental temperature enhancement, (2) to reconstruct 20th century climate for Alexandra Fiord and central Ellesmere Island based on C. tetragona chronologies, (3) to conduct a comparative analysis of my climate reconstruction with other climate proxies from northern North America. In 1992, permanent open-top chambers (OTCs) were established in heath tundra plant communities at Alexandra Fiord, Ellesmere Island. The OTCs raised the growing season temperature by 1.3°C. Using dendrochronological techniques, the responses of C. tetragona prior to (1986-1991) and during (1992-1998) the experimental warming were investigated Cassiope tetragona exhibited a strong reproductive response, but weak growth response to the experimental wanning. The results confirm that reproductive development in high arctic plants is more sensitive to the thermal environment than vegetative growth, and, summer temperatures are critical to tundra ecosystems for successful reproduction. Secondly, dendroclimatological analysis of C. tetragona was used to reconstruct past summer temperature for Alexandra Fiord and central Ellesmere Island. Three C. tetragona populations were sampled at Alexandra Fiord (78° 52'N, 75° 47'W) and one at Hot Weather Creek (79° 58'N, 84° 28'W). Two growth and two reproduction chronologies were developed for each sampling site. The 95-year long reconstruction (1899-1994) of August-September average air temperature for Alexandra Fiord was the longest C. tetragona-based proxy record and explained 51% of the dependent climate variance. A second reconstruction of August-September average air temperature based upon chronologies from Alexandra Fiord and Hot Weather Creek explained 66% of the dependent climate variance. The reconstructions revealed an increase in summer temperature from the 1920s-1960s, a cooling period from the 1960s-1970s, and finally, a second increase in summer temperature beginning in the 1980s. Other arctic climate proxies support this study's conclusions. Finally, correlation analysis revealed that high summer Arctic Oscillation index values are associated with unfavorable growing season conditions, resulting in reduced growth and reproduction in C. tetragona populations on Ellesmere Arts, Faculty of Geography, Department of Graduate
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