A SPATIALLY EXPLICIT POPULATION-MODEL OF STOLONIFEROUS N-FIXING LEGUMES IN MIXED PASTURE WITH GRASS

Citation
S. Schwinning et Aj. Parsons, A SPATIALLY EXPLICIT POPULATION-MODEL OF STOLONIFEROUS N-FIXING LEGUMES IN MIXED PASTURE WITH GRASS, Journal of Ecology, 84(6), 1996, pp. 815-826
Citations number
40
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Ecology
Journal title
ISSN journal
0022-0477
Volume
84
Issue
6
Year of publication
1996
Pages
815 - 826
Database
ISI
SICI code
0022-0477(1996)84:6<815:ASEPOS>2.0.ZU;2-#
Abstract
1 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 patc h-scale oscillations in legume content, using a cellular automaton wit h variable hierarchy between the two species. 2 We define cell states in the automaton by species content and soil N status. Grass-legume os cillations at the patch scale are represented as an alternation betwee n states of grass dominance (high N) and legume dominance (low N). To this physiologically based population oscillation, we add local extinc tions of legume and state-dependent success in legume invasion. 3 Legu me populations oscillate at the field scale, given arbitrary initial c onditions. However, spatially random perturbations to the soil N statu s (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 pre vious, purely physiological model.4 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 pla ys 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. 5 Seasonal disturbances that act uniformly across the field, such as winter (legume) mortality and/ or springtime fertilizer application, can lead to sustained field scal e variations in legume content that are only partly explained by the l evel of seasonal disturbance itself. Another large part is explained b y previous years' legume contents. Pastures may therefore exhibit a 'm emory' for legume performance which helps to explain the perceived 'un predictability' of some grass-legume associations. 6 We argue that leg ume dynamics in mixed pastures cannot be fully understood without comb ining 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-sca le.