Seasonal Water Use Efficiency of a Subalpine Wetland and the impact of Horizon Shade
When considering the full data set, from Green Up to Late Growing Season , Bonsai had an average WUE of 2.9 gC (kg H2O)-1 day (± 0.80) and WUE was highest during periods of peak productivity (Peak Growing Season ) and lowest during shoulder seasons (Green Up &Late Growing Season ) (Table 1). Maximum WUE (5.3 gC (kg H2O)-1) occurred on 20 August and minimum WUE (1.4 gC (kg H2O)-1) occurred on 29 June. During the seasonal maximum, vegetation was highly productive, but had less water available, while during the seasonal minimum vegetation was not yet fully grown and productive despite having a large amount of available water. Periods surrounding the maximum and minimum illustrate the seasonal trends in WUE; during Green Upand Late Growing Season Bonsai had low daily averages (2.4 and 2.9 gC (kg H2O)-1, respectively), while in Peak Growing Season there were high daily averages (3.35 gC (kg H2O)-1). Increased ET duringGreen Up led to a lower WUE and a greater GPP in Peak Growing Season resulted in a higher WUE.
To better understand the impact of shade on WUE at Bonsai, we focused our attention on the Peak Growing Season and daytime data only. We found that the mean GPP was higher for data from SteadyShade period compared to Dynamic Shade (0.179 gC/m2/hh vs 0.173 gC/m2/hh); however, this difference was not statistically significant (Table 1). Mean ET on the other hand was lower during the DynamicShade period compared to Steady Shade (0.070 mm/hh vs 0.077 mm/hh). Again, this difference was also not statistically significant (Table 1). However, these differences in GPP and ET resulted in a statistically significant difference in the mean WUE values between the two shaded periods, with Dynamic Shade mean WUE being higher compared to that of Stable Shade (3.10 vs 2.72 gC/mm, Table 1). Since both ET and GPP were most dependent on the temporal variability in Rg (and that Rg was reduced by shade), shade had an impact on WUE, increasing it via reductions in ET and potential increases in GPP. The binned data suggests improved photosynthetic rates for lower Rg-bins compared to higher Rg-bins during Dynamic shade (Appendix 3).