Simulating Grass-Legume Dynamics: a Phenomenological Submodel

Abstract

A simple recipe for modelling the dynamics of the legume component of a grass-legume pasture simulator is proposed, avoiding some of the difficulties associated with representing the pasture submodel as autonomous but interacting grass and legume components. A target legume content of the sward is assumed to depend on the carbon:nitrogen ratio in the plant substrate carbon and nitrogen pools (these represent the labile and easily mobilizable C and N pools in the plant). The rate at which the actual legume content approaches the target content is proportional to the gross specific growth rate of the pasture. The canopy extinction coefficient and the dinitrogen fixation rate for the pasture are adjusted according to the changing legume content. The method has been incorporated into a generic single-species grassland simulator, the Hurley Pasture Model. Seasonal changes in legume fraction and associated variables are simulated. Next, the response of the legume fraction to step changes in nitrogen fertilizer application, carbon dioxide concentration, rainfall and temperature are predicted for a grazed pasture. Yield from frequent harvesting is also examined for four treatments: ambient and elevated carbon dioxide×low and high nitrogen fertilizer application. Qualitatively, the simulations agree well with experimental findings. This indicates that some important aspects of grass-legume competition could operate primarily through the pasture carbon:nitrogen substrate ratio. This ratio may determine those characteristics of morphology, growth and function that largely define the differential success of the two components of grass-legume swards; however, causation would not be proved (as far as this is scientifically possible) without a detailed mechanistic model. The approach may be useful for the investigation of management and climate-change problems in grassland.