Biomass density, height increment and water content
For further assessment of possible morphological effects of observed responses to increasing atmospheric CO2 on S. fuscum , we measured its biomass density (total biomass dry weight per volume) and height increment under all the test conditions (Fig. 3E, F). No effect of atmospheric CO2 on biomass density was observed, instead the variation in density (4.1 to 42.4 g dm-3) was mostly explained by WT (45%,P< 0.001), light intensity (34%,P< 0.001) and the interaction between WT and light (14%, P< 0.001, Table 1). Raising the WT decreased biomass density by 41-71%, whereas light intensity increased it 2.1- to 3.5-fold at low WT and 1.6- to 2.6-fold at high WT (Fig. 3E).
Most of the variation in height increment (0.04-0.26 mm d-1) was explained by temperature (51%,P< 0.001) and light intensity (24%,P< 0.001), while the interactions between light and WT or CO2 as well as temperature and WT or CO2 made minor contributions (each 2-3%,P <0.05, Table 1). Increasing the temperature increased the height increment 2.4- to 4.5-fold (Fig. 3F), while increasing the light intensity resulted in ~0.6- and ~0.35-fold reductions in the height increment at low and high WT, respectively. Increasing atmospheric CO2 significantly reduced height increment by 0.19 to 0.35-fold only at the high temperature and low light treatment, at low WT (Fig. 3F, Fig. S2).
S. fuscum ’s water content was consistently 4-9 % lower at the high CO2 level, but CO2 only explained a small amount of the total variation (6.6-23.4 g g-1) in its water content (1%, P =0.006; Table S2, Fig. S3). Most of the variation was explained by WT (83%, P <0.001), temperature (5%,P <0.001), and the interactions between WT and temperature (6%, P <0.001) and between WT and light (3%, P <0.001, Table S2). At low WT, the water content varied between 7.4 and 10.6 g g-1, within the reported optimal range for photosynthesis (6-10 g g-1: Silvola & Aaltonen, 1984; Schipperges & Rydin, 1998). At high WT, the water content was between 15.4 and 22.3 g g-1, far outside the optimal range for photosynthesis (Fig. S3).