A spatially explicit population model of stoloniferous N-fixing legumes in mixed pasture with grass

In a previous paper, we outlined the physiological prerequisites for population oscillations between a grass and a nitrogen-fixing legume, such as clover. Here, we examine the field-scale consequences of patch-scale oscillations in legume content, using a cellular automaton with variable hierarchy between the two species. We define cell states in the automaton by species content and soil N status. Grass-legume oscillations at the patch scale are represented as an alternation between states of grass dominance (high N) and legume dominance (low N). To this physiologically based population oscillation, we add local extinctions of legume and state-dependent success in legume invasion. Legume populations oscillate at the field scale, given arbitrary initial conditions. However, spatially random perturbations to the soil N status (e.g. urine) establishes a pasture structure that dampens the field scale oscillation. The stabilizing pasture structure comprises moving patches of legume dominance. This pattern was not predicted by our previous, purely physiological model. The model highlights that a patchy species distribution does not in itself mean the species is dispersal limited. In this model, changing the dispersal ability of legumes plays only a limited role in determining their proportion in the mixture. Legume abundance depends as much on the rate at which favourable (low N) sites become available for invasion. Seasonal disturbances that act uniformly across the field, such as winter (legume) mortality and/or springtime fertilizer application, can lead to sustained field scale variations in legume content that are only partly explained by the level of seasonal disturbance itself. Another large part is explained by previous years' legume contents. Pastures may therefore exhibit a 'memory' for legume performance which helps to explain the perceived 'unpredictability' of some grass-legume associations. We argue that legume dynamics in mixed pastures cannot be fully understood without combining ecological and physiological concepts of species interactions at three different scales: competitive interactions at the patch-scale, dispersal at the between-patch scale, and seasonality at the field-scale.