A canopy photosynthesis model to predict the dry matter production of cocksfoot pastures under varying temperature, nitrogen and water regimes

Abstract

Daily net canopy photosynthesis (P n ) of cocksfoot (Dactylis glomerata L.) was predicted for combinations of temperature, herbage nitrogen (N) concentration and water status from the integration of models of leaf photosynthesis of the light-saturated photosynthetic rate (P max ), photosynthetic efficiency (a) and the degree of curvature (h) of leaf light-response curves. The effect on P n , maximum P n (P n max ) and the optimum leaf area index (LAI at P n max ) was examined when any one of these factors was limiting. The ranges that gave the optimum values of P n (P n max ¼ 30AE8-33AE5 g CO 2 m )2 d )1 ) for temperature (19-22°C) and N concentration (40-50 g N kg )1 DM) were smaller than those for net leaf photosynthesis. Also, P n fell to 0 at a lower level of water stress (pre-dawn leaf water potential, w lp ¼ )12AE5 bar) than for P max . The canopy photosynthesis model was then used to compare predicted and measured dry matter (DM) production for cocksfoot pastures grown under a diverse range of environmental conditions with field data from New Zealand and Argentina. To predict DM production leaf area index and leaf canopy angle were included from field measurements. The model explained about 0AE85 of the variation in cocksfoot DM production for the range of 6AE5-134 kg DM ha )1 d )1 . The canopy model overestimated the DM production by 0AE10 which indicates that a further P max function for leaves of different ages and a partitioning sub-model may be needed to improve predictions of DM production.

Improving the McCall herbage growth model

“…Grass leaves are able to convert radiation into growth more efficiently at low light intensities (e.g., in winter), as previous research clearly shows (Cooper 1970;Peri et al 2003b). Non-linear functions have been used to model radiation conversion efficiency in the past (Johnson & Thornley 1983;Johnson et al 1995;Peri et al 2003a). in the new version, the parameter a (see eqn 2) increases at low radiation levels:…”

Validation of a canopy photosynthesis model for cocksfoot pastures grown under different light regimes

“…Nitrogen as synthetic sheep urine (0 or 300 kg N/ ha) and irrigation (0 or fully) were applied to 2.47 m 2 areas within each cage as a completely randomised 2 · 2 factorial design with two replicates (Peri et al 2003b). …”

“…There are difficulties in measuring total LAI for grasses due to the optical sensor head of the instrument being 40 mm high. To overcome this, aluminium trenches (40 mm deep · 30 mm wide · 1.2 m long) were set up for all treatments so that the top of the sensor was at the soil surface (Peri et al 2003b). In this study, measurements were taken from one reading above the canopy followed by five readings beneath, along the trench (transect).…”

“…For calibration and validation of the canopy photosynthesis model proposed by Peri et al (2003b), measurements were taken from September 1999 to May 2001 from three separate areas. These provided a range of temperature, water, N and light regimes, within the silvopastoral experiment.…”

“…To date, the influence of these factors on cocksfoot canopy photosynthesis, and pasture production in general, in silvopastoral systems has not been quantified. Therefore, the aim of this study was to predict pasture growth rates and DM production in silvopastoral systems by imposing shade response modules to the canopy photosynthesis model developed for cocksfoot (Peri et al 2003b). Specifically, net canopy photosynthesis for cocksfoot under different light regimes is predicted by integrating the leaf photosynthesis models developed for the light-saturated rate (P max ) (Peri et al , 2003a, the photosynthetic efficiency (a) and the degree of curvature (h) (Peri et al 2005) of the leaf light-response curve.…”