Response of Sphagnum species mixtures to increased temperature and nitrogen availability.

To predict the role of ombrotrophic bogs as carbon sinks in the future, it is crucial to understand how Sphagnum vegetation in bogs will respond to global change. We performed a greenhouse experi- ment to study the effects of two temperature treatments (17.5 and 21.7C) and two N addition treatments (0 and 4gNm -2 year -1 ) on the growth of four Sphagnum species from three geographically interspersed regions: S. fuscum, S. balticum (northern and central Sweden), S. magellanicum and S. cuspidatum (south- ern Sweden). We studied the growth and cover change in four combinations of these Sphagnum species during two growing seasons. Sphagnum height increment and production were affected negatively by high temper- ature and high N addition. However, the northern species were more affected by temperature, while the southern species were more affected by N addition. High temperature depressed the cover of the 'wet' species, S. balticum and S. cuspidatum. Nitrogen concentrations increased with high N addition. N:P and N:K ratios indicated P-limited growth in all treatments and co-limitation of P and K in the high N treatments. In the second year of the experiment, several containers suffered from a severe fungal infection, particularly affecting the 'wet' species and the high N treatment. Our findings suggest that global change can have negative consequences for the production of Sphagnum species in bogs, with impor- tant implications for the carbon sequestration in these ecosystems.