Effect of moss water content
The increase in the D6S /D6R ratio from low to high WT (Fig. 2A, 4A) revealed an increase in the photorespiration/photosynthesis ratio, and thus reduction in net photosynthesis at the high WT for both S. fuscum and S. majus . A concomitant decrease inc c/c a from low to high WT (Fig. 3B) suggests that the increase in the photorespiration/photosynthesis ratio was driven by a decline inc c. In addition, the lowerc c/c a at high WT is indicative of higher CO2 diffusion resistance, resulting from the higher moss water content (Fig. S3). In this respect, our data supports the hypothesis that the higher water content at high WT limits CO2 diffusion and therefore C assimilation. This hypothesis is also supported by earlier reports of reductions in net photosynthesis with increases in water content (Titus et al ., 1983; Titus & Wagner, 1984; Schipperges & Rydin, 1998) and a negative relationship between water content andc c/c a (Rice & Giles, 1996; Williams & Flanagan, 1996, 1998).