Deciduous Shrub Encroachment Effects on Tundra Soil Properties

Deciduous shrub abundance is increasing in tundra ecosystems as an effect of rising temperatures which may change tundra physical properties and, in turn, microbial communities and biogeochemical processes. Two mechanisms through which shrub presence may affect tundra ecosystems were examined in this study; the physical presence of the shrubs and effects of increasing shrub litter inputs. In a sub-arctic alpine tundra ecosystem, dominated by the deciduous shrub Betula glandulosa, both shrub presence (shrub present and removed) and litter quantity (no litter/litter removed, ambient litter, and twice ambient litter) were manipulated; multiple ecosystem properties where measured within the treatment plots over four years. In Chapter 2, examining the effects of the treatments on physical properties of the environment, soil nutrients, and microbial processes, shrub presence is described to be the main mechanism by which shrubs encroachment caused changes in tundra ecosystems. Shrub presence increased soil temperatures in winter and decreased them in summer, decreased summer soil moisture, and increased soil C:N ratios. There were many fewer direct effects of increasing quantity of shrub litter, although increasing litter did increase microbial biomass, and soil C:N ratios. In Chapter 3 the effects of shrub removal and litter addition on litter decomposition rates were examined, describing impacts of both changes in the decomposition environment and changing litter quality. When compared to other litter types on the tundra B. glandulosa litter was of higher quality (lower C:N ratios) and decomposed faster. However, the presence of shrubs decreased decomposition rates through their effects on the decomposition environment. In conclusion, shrub encroachment is likely to have large effects on tundra ecosystem properties, although the opposing direction of these effects through different mechanisms hinder the ability to predict the net effect of these changes.