Plant Performance and Soil Nitrogen Mineralization in Response to Simulated Climate Change in Subarctic Dwarf Shrub Heath

To simulate a future, warmer climate, we subjected subarctic dwarf shrub heath to 5°C direct soil warming for five consecutive growing seasons (1993-1997). Supplemental air warming treatments were imposed on warmed soil by plastic tents in 1994 and open-top chambers in 1995. Plant responses to warming were assessed by changes in: 1) shrub phenology, 2) current-year aboveground biomass in the dominant shrubs (Empetrum hermaphroditum, Vaccinium myrtillus, V. uliginosum and V. vitis-idaea), and 3) vascular and nonvascular plant cover. We estimated warming effects on soil nitrogen (N) availability by in situ buried bag incubation of soils. Soil warming stimulated soil N cycling and shrub growth and development in the short term (2-3 yr). In the second year, net N mineralization rates doubled in warmed soil (4.3 kg N ha-1 season-1 in untreated soil vs 9.2 kg ha-1 season-1). Greater N availability likely contributed to the observed 62% increase in current-year growth of V. myrtillus, the dominant deciduous shrub. In the third year, soil and air warming increased shoot production by > 80% in the evergreen shrubs V. vitis-idaea and E. hermaphroditum. Soil warming had no detectable effects on plant growth or soil N cycling in the fifth year, suggesting that the long-term response may be less dramatic than short-term changes. Past fertilization studies in arctic and subarctic tundra reported an increase in the abundance of graminoids. Despite enhanced soil N mineralization in the second year, we found that warming had little effect on plant community composition after five years. Even in an extreme climate warming scenario, it appears that subarctic soils mineralize an order of magnitude less N than was applied in fertilization experiments. High-dose fertilization studies provide insight into controls on plant communities, but do not accurately simulate increases in N availability predicted for a warmer climate.