Soil Warming and Fertilization Effects on Growth Ring Widths of Arctic Shrubs - Application of a Novel Dendroecological Approach.

Abstract:

Climate warming is suggested as the main driver of shrub expansion in arctic tundra regions. Shrub expansion may have consequences on biodiversity and climate, especially through its feedbacks with the energy budget. A better understanding of shrub expansion mechanisms, including growth rate patterns and stem anatomy changes, and their sensitivity to climate is needed in order to quantify related feedbacks. We present a novel dendroecological approach to determine the response of three arctic shrub species to increased soil temperature and nutrients. A full factorial block-design experiment was run for four years with a total of thirty plots. Six individuals of each species were sampled from each plot to test for treatment effects on growth rate and stem anatomy. We compared the ring width of the four years of experiment with the one of the four previous years. The preliminary results for Betula nana and Salix pulchra suggest a significant effect of the treatments on the growth ring width. The response is stronger in Salix pulchra than in Betula nana individuals. And, while Salix pulchra is more sensitive to the combined soil warming and fertilization treatment, Betula nana is to the fertilization treatment. We could not observe an effect of treatment on the stem anatomy, likely because bark thickness co-varies with age. We found significant positive correlations of cork, cortex and phloem thickness with xylem thickness (used as a proxy of age), and a significant difference in stem anatomy between species. The results suggest species-specific growth sensitivity to soil warming and nutrient enhancement. The use of experimental dendroecology by manipulating environmental conditions according to future climate scenarios and testing effects on shrub anatomy and annual growth will increase our understanding on shrub expansion mechanisms. Ongoing plant trait analysis and consecutive application in a 3D radiative transfer model will allow to quantify the feedback of increased shrub growth on the land surface energy budget.