Modelling carbon dynamics in mixed grass-red clover swards

The model most often underestimated above-ground biomass when using proportion of clover and estimates of radiation use efficiency (ε) and root allocation fraction (br) from values in the pure stands (Method 1). This discrepancy suggested that ε might be higher for the mixed stand than the pure stands. Values of ε and br for the mixed stands were therefore derived by calibration to the observed above-ground biomass (Method 2). The radiation use efficiency (ε) of the mixed stand was on average 6% higher than estimates of ε from pure stand values and proportion of clover. Overall, the variation in radiation use efficiency of mixed stands due to cutting and year was similar to that of pure stands of grass and clover, whereas the relationship to N treatment was weak and more similar to that of clover. The explanation that variations in biomass production were due to changes in root allocation seems reasonable. No variations in radiation use efficiency or root allocation fraction could be related to the proportion of clover. The variations due to cutting and year overwhelmed any such effect.