Habitat-dependent interactive effects of a heatwave and experimental fertilization on the vegetation of an alpine mire

Aims: Our objective was to investigate how the after-effects of a heat wave interacted with nutrient addition in affecting the vegetation of an alpine mire. We also aimed at assessing if the responses of vegetation to the environmental changes differed between mire habitats. Location: A mire in the Dolomites (South-Eastern Alps, Italy, 1800 m). Methods: We carried out an eight-year (2002–2009) experiment of nutrient addition in an alpine mire that experienced, in summer of 2003, an unprecedented heat wave. The cover of vascular plants and mosses was assessed non-destructively and the presence of all vascular plant and moss species was recorded in 60 plots. Results: Interacting effects of the heat wave and experimental fertilization brought about profound changes in the vegetation of the mire. Patterns of change varied considerably between two habitats (hummocks and lawns). In the short term (2003–2005), the 2003 heat wave triggered a change in vegetation structure and PFT cover, with vascular plants expanding at the expense of mosses in both habitats. In the mid-term (2005–2009), moss cover increased in hummocks because Sphagnum mosses recovered and non-Sphagnum mosses expanded, especially under N addition. Conversely, in lawns vascular plants expanded while mosses did not recover. These trends were strengthened when nitrogen was added with no concomitant phosphorus addition. A decline of species richness was observed in lawns, most likely due to stronger interactions among species. Conclusions: Hummocks have a greater ability than lawns to recover from extreme climatic events under higher levels of nitrogen influx. Conversely, a persistent increase in vascular plant cover at the expense of moss cover in lawns may jeopardize the carbon sink function of lawn habitats.