Can frequent precipitation moderate drought impact on peatmoss carbon uptake in northern peatlands?

Northern peatlands represent a large global carbon store that potentially can be destabilised by summer water table drawdown. Precipitation can moderate negative impacts of water table drawdown by rewetting peatmoss (Sphagnum spp.), the ecosystems’ key species. Yet, the frequency for such rewetting to be effective remains unknown. We experimentally assessed the importance of precipitation frequency for Sphagnum water supply and carbon uptake during a stepwise decrease in water tables in a growth chamber. CO2 exchange and the water balance were measured for intact cores of three peatmoss species (Sphagnum majus, S. balticum and S. fuscum) representative of three hydrologically distinct peatland microhabitats (hollow, lawn, hummock) and expected to differ in their water table-precipitation relationships. Precipitation contributed significantly to peatmoss water supply at deep water tables, demonstrating the importance of precipitation during drought. The ability to exploit transient resources was species-specific; S. fuscum carbon uptake increased linearly with precipitation frequency at deep water tables, whereas carbon uptake by S. balticum and S. majus was depressed at intermediate precipitation frequencies. Our results highlight an important role for precipitation on carbon uptake by peatmosses. Yet, the potential to moderate drought impact is species-specific and dependents on the temporal distribution of precipitation