Abstract
As a consequence of land‐use change and the burning of fossil fuels, atmospheric concentrations of CO 2 are increasing and altering the dynamics of the carbon cycle in forest ecosystems. In a number of studies using single tree species, fine root biomass has been shown to be strongly increased by elevated CO 2 . However, natural forests are often intimate mixtures of a number of co‐occurring species. To investigate the interaction between tree mixture and elevated CO 2 , A lnus glutinosa , B etula pendula and F agus sylvatica were planted in areas of single species and a three species polyculture in a free‐air CO 2 enrichment study ( B angor FACE ). The trees were exposed to ambient or elevated CO 2 (580 μmol mol −1 ) for 4 years. Fine and coarse root biomass, together with fine root turnover and fine root morphological characteristics were measured. Fine root biomass and morphology responded differentially to the elevated CO 2 at different soil depths in the three species when grown in monocultures. In polyculture, a greater response to elevated CO 2 was observed in coarse roots to a depth of 20 cm, and fine root area index to a depth of 30 cm. Total fine root biomass was positively affected by elevated CO 2 at the end of the experiment, but not by species diversity. Our data suggest that existing biogeochemical cycling models parameterized with data from species grown in monoculture may be underestimating the belowground response to global change.