Drought alters carbon fluxes in alpine snowbed ecosystems through contrasting impacts on graminoids and forbs

Climate change is predicted to increase the frequency of drought events in alpine ecosystems with the potential to affect carbon turnover.

We removed intact turfs from a Nardus stricta alpine snowbed community and subjected half of them to two drought events of 8 d duration under controlled conditions. Leachate dissolved organic carbon (DOC) was measured throughout the 6 wk study period, and a ¹³CO₂ pulse enabled quantification of fluxes of recent assimilate into shoots, roots and leachate and ecosystem CO₂ exchange.

The amount of DOC in leachate from droughted cores was 62% less than in controls. Drought reduced graminoid biomass, increased forb biomass, had no effect on bryophytes, and led to an overall decrease in total above-ground biomass compared with controls. Net CO₂ exchange, gross photosynthesis and the amount of ¹³CO₂ fixed were all significantly less in droughted turfs. These turfs also retained proportionally more ¹³C in shoots, allocated less ¹³C to roots, and the amount of dissolved organic ¹³C recovered in leachate was 57% less than in controls.

Our data show that drought events can have significant impacts on ecosystem carbon fluxes, and that the principal mechanism behind this is probably changes in the relative abundance of forbs and grasses.