Shrub abundance and seasonal variation in ecosystem properties across a fertilization chronosequence in moist acidic tundra

Background/Question/Methods

Changes in global climate have resulted in a ‘greening’ of the Arctic as the abundance of deciduous shrub species increases. Shrub cover may result in higher winter soil temperatures through insulating effects of increased snow accumulation, but shrubs may produce thicker litter layers and increased shading which could result in lower summer soil temperatures. We examined the responses of moist acidic tundra to increased soil nutrient availability and the accompanying increase in shrub abundance at the Arctic Long Term Ecological Research (LTER) site at Toolik Lake, Alaska. We examined a chrono-sequence of long-term fertilization experiments, composed of experiments fertilized for 4, 14 and 21 years, which has resulted in increasing shrub density with time since fertilization. The fertilized plots receive both nitrogen (N, 10 g/m²/yr) and phosphorus (5 g/m²/yr) annually following snowmelt. Measurements of plant species composition and relative cover are taken annually. In the 2011 growing season we measured weekly variation in soil available N, depth-to-permafost, and soil temperature and seasonal litter production in fertilized and control plots across the chrono-sequence.

Results/Conclusions

With fertilization, plant communities shift from relatively equal abundance of sedges and deciduous and evergreen shrubs towards dominance by Betula nana (dwarf birch). Differences between fertilized and control plots depend strongly on the seasonal timing of measurements. Both soil ammonium and nitrate concentrations were higher in fertilized plots for the two oldest sets of treatments, with differences most pronounced early in the season. The newest plots, however, showed little difference in available nitrogen between fertilized and control treatments – high N demand resulting in immediate uptake of available N likely quickly reduced concentrations in fertilized plots. Thaw depths were greater in control than fertilized plots. Differences between treatments in soil temperature, and correspondingly  thaw depth, are likely partially due to the increased insulation of higher quantities of leaf litter. Our results suggest that pronounced differences in soil properties caused by altered soil nutrients and vegetation occur early in the growing season and may be missed if peak season measurements are used to summarize treatment effects. The relative contributions of increased soil nutrients, litter and shrubs caused by fertilization must be examined in concert with changes in soil temperature to better understand how shifts towards greater shrub abundance are affecting arctic ecosystems.