| Summary: | species interactions and effects of simulated climate change. – Oikos 99: 151–160. In patches of co-occurring species in natural plant communities, there is a finely poised balance between species in the ways in which they respond to prevailing moisture and temperature regimes. However, environmental change scenarios, in which temperature, moisture and ultraviolet-B radiation are suggested to increase, may favour one of the species. The imbalance is likely to occur at the levels of interactions between patches of the different species and at the shoot level when neighbouring shoots belong to different species. We increased temperature and UV-B in a two-way factorial experiment and increased water supply independently in two subarctic mire communities dominated by the mosses Sphagnum fuscum and Dicranum elongatum. The effects of simulated increase in UV-B were studied using two separate radiation systems, i.e. a ‘‘square wave’ ’ system and a ‘‘modulated’’ system. When precipitation was enhanced, both species showed an increase in growth but this was not sustained beyond 5 mm per day. S. fuscum showed a 50 % greater response
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