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.