Abstract Environmental changes are predicted to have severe and rapid impacts on polar and alpine regions. At high latitudes/altitudes, cryptogams such as bryophytes and lichens are of great importance in terms of biomass, carbon/nutrient cycling, cover and ecosystem functioning. This seven-year factorial experiment examined the effects of fertilizing and experimental warming on bryophyte and lichen abundance in an alpine meadow and a heath community in subarctic Sweden. The aim was to determine whether short-term responses (five years) are good predictors of longer-term responses (seven years). Fertilizing and warming had significant negative effects on total and relative abundance of bryophytes and lichens, with the largest and most rapid decline caused by fertilizing and combined fertilizing and warming. Bryophytes decreased most in the alpine meadow community, which was bryophyte-dominated, and lichens decreased most in the heath community, which was lichen-dominated. This was surprising, as the most diverse group in each community was expected to be most resistant to perturbation. Warming alone had a delayed negative impact. Of the 16 species included in statistical analyses, seven were significantly negatively affected. Overall, the impacts of simulated warming on bryophytes and lichens as a whole and on individual species differed in time and magnitude between treatments and plant communities (meadow and heath). This will likely cause changes in the dominance structures over time. These results underscore the importance of longer-term studies to improve the quality of data used in climate change models, as models based on short-term data are poor predictors of long-term responses of bryophytes and lichens.

Abstract Plant exposure to enhanced UV-B radiation typically induces changes in leaf secondary metabolite profiles which will be inherited in litter, affecting litter breakdown and the carbon (C) dynamics of sensitive plant communities. A key enzyme in the decomposition process is phenol oxidase which is influenced by litter quality and, hence, a decomposition bioindicator. Here we investigated dwarf shrub litter decomposition following experimental community exposure to enhanced UV-B over two decades in the Swedish sub-Arctic. We examined the hypothesis that foliar UV-B exposure would alter litter quality to elevate phenol oxidase activity. This was tested in the field by measuring phenol oxidase activity in freshly collected mixed-community litter from under our experimental vegetation. A laboratory mesocosm was next used in a decomposition assay to investigate individual species responses over eight weeks, with an emphasis on the quality of leachate outputs from decomposing litter (from Empetrum hermaphroditum, Vaccinium vitis-idaea, Vaccinium uliginosum). In the assay bi-weekly collections of leachate were analysed for phenol oxidase activity, together with total phenolics and dissolved organic C (DOC). At the end of the assay litter mass loss and respired C were also determined. The initial assessment on field mixed-community litter found no enhanced UV-B treatment (henceforth: ‘UV-B treatment’) effect on phenol oxidase activity. However, in the controlled laboratory mesocosm assay, significant species-specific effects of the UV-B treatment were evident, with increased phenol oxidase activity in V. vitis-idaea leachate (P < 0.001) and a significant reduction (P = 0.05) in respired C. Leachate DOC release from the UV-B treatment was greater in both Vaccinium species and the effect on V. uliginosum was significant (P < 0.05). The UV-B treatment had no effect on the total phenolic concentration of litter or leachates for any species, but there were significant differences in leachate total phenolics, both over time and between species. Also the initial phenolic concentration in leachates from the decomposing litter of E. hermaphroditum was greater than both Vaccinum species. Results suggest a species specific role for UV-B in influencing enzyme function and decomposition, dependent on individual traits. This has implications for decomposition dynamics in this system and more widely. Our study highlights the value of using a laboratory assay to gain a mechanistic understanding the species level impacts of a global change factor (UV-B) on decomposition, which are otherwise obscured by community-level responses and difficult to determine under field conditions.