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 to enhanced precipitation than did D. elongatum, as would be expected from their habitat preferences. Under ambient temperature, S. fuscum grew 67% faster than D. elongatum and this relative difference in response was maintained after one year under a temperature enhancement. The response by species over the winter period was moderated by their neighbours. S. fuscum growth was enhanced when it grew next to D. elongatum whereas D. elongatum grew better with neighbours of its own species. Increased temperature and UV-B radiation did not affect the interaction between the species.
Although a balance was maintained between the two species over the short duration of the experiment, potential was shown for an imbalance to occur over longer periods and particularly if winter warming and precipitation are greater than those in summer.
During the peak growing season 20% increased UV-B over ambient had a negative effect on S. fuscum under increased temperature but there were no overall seasonal effects on either species, irrespective of method of UV supplementation.