Ecophysiological implications
Our results show that hummocks with characteristic WT levels have profited from CO2fertilization imposed by the 20th centuries CO2 increase. The high WT conditions in this study resulted in abolishing the effect of CO2 for both hummocks and lawns, demonstrating the importance of the water content for photosynthetic C fluxes. Even under optimal moisture conditions for photosynthesis (at WT of -7 cm), lawns did not respond to increased atmospheric CO2 (Fig. 4A). Together with the observed response of S. fuscum , this suggests a species-specific suppression of photorespiration, which may be attributed to differences in leaf-anatomy between hummock and lawn species (Rice & Giles, 1996). Thus, in contrast to hummock-species, certain lawn-species may not be capable of responding to changes in atmospheric CO2.
Predicted increases in precipitation in the northern hemisphere are suggested to be compensated for by concomitant increases in evapotranspiration due to higher temperatures (Frolking et al. , 2011). This suggests that hydrological conditions of northern peatlands will remain relatively stable in a changing climate. Therefore, our data point towards changes in peatland topography in response to climate change, with the competitive advantage of hummocks over lawns and hollows.
Our results further indicate that for hummocks with typical WT conditions, the increase in atmospheric CO2 during the 20th century was the major driver of Sphagnumphotosynthetic C fluxes. During the early and mid-Holocene, atmospheric CO2 was relatively stable, at ~270 ppm (Indermühle et al ., 1999), thus temperature appeared to control northern peatland C fluxes; peat C accumulation followed the increase in temperature during the early Holocene and the cooler and wetter climate during the neoglacial period (Yu et al., 2009; Loisel et al., 2014). Our results suggest that the ongoing increase in atmospheric CO2 today, in marked contrast to its stability in the Holocene, has important consequences for peatland C fluxes.