Effects Of Increasing Deciduous Shrub Litter On Ecosystem Functioning In Alpine Tundra

Rising temperatures due to global warming have resulted in rapid environmental changes in northern environments, resulting in an increase in deciduous shrub growth which has the potential to alter soil ecosystem properties and processes. In order to address some potential mechanisms by which shrubs alter ecosystem properties and processes, we designed an experiment which separated the effects of physical shrub presence and the effects of litter quantity in a fully factorial experiment on a north facing alpine plateau in the southwest Yukon, Canada. Specifically, we asked the following questions: What are effects of shrub litter, independent of shrub presence, on (1) physical ecosystem properties, (2) soil nutrient content and availability and (3) microbial processes. I measured multiple response variables including various physical soil properties (soil moisture and temperature), measures of soil nutrient content (total C and N, available nutrients) and rates of primarily microbially controlled properties and processes (microbial biomass, litter decomposition and extracellular enzyme activity). We found shrubs increased soil temperatures in winter months and decreased them during the summer. Shrubs had few effects on soil nutrients, but their presence increased microbial biomass, and respiration rates, and reduced rates of decomposition. Litter manipulations had a stronger effect than shrubs on many variables, resulting in increased soil moisture, available nutrients, %C and %N, microbial biomass, and extracellular enzyme activity. Our results indicate shrub encroachment will influence available nutrients directly by changing litter inputs, with subsequent effects on microbial processes. Predicted longer-term impacts on carbon mineralization were not evident after 2 years of manipulations, and may require a longer time before effects are shown in the soil carbon pool