Fig. 6.
Model tests of observed discrimination (Δobs) on 1 June (A), 20 June (B), 19 July (C), and 23 August 2007 (D). Four models were tested against Δobs including the simple model of discrimination (Δsimple; filled circles), the comprehensive model of discrimination using a mean mesophyll conductance (gm) of 1.72 μmol m−2 s−1 Pa−1 (Δc.mean; open triangles), the comprehensive model of discrimination using a gm estimated from the regression between diurnal gm and time of day (TOD) (Δc.TOD; filled squares), and the comprehensive model of discrimination using a gm estimated from the regression describing the relationship between stomatal conductance of CO2 and gm (Δc.gs; open circles). Δpredicted represents discrimination predictions of any of the four models. On two dates Δc.mean or Δc.TOD performed best, but on other dates and across the whole study Δsimple exhibited the lowest model error. These results support the use of Δsimple to predict leaf-level diurnal carbon discrimination of field-grown juniper.