Mechanisms for guild-based assembly rules in a lawn community

Abstract

Assembly rules for plant communities have been widely investigated and debated. Convincingly demonstrating whether or not there are restrictions on species co-existence has proven difficult. The Botany Lawn at the University of Otago is a special case as here there is strong evidence that such rules are working. The least subjective of these assembly rules found in the Botany Lawn is based on guild proportionality between two intrinsic guilds. How this rule is operating—its mechanism—is unknown. Here I look for a mechanism, or mechanisms, for intrinsic guild based assembly rules already found in the Botany Lawn using deductive and inductive methods to investigate the species’ functional niche and how these differences potentially relate to interactions between the intrinsic guilds. Mowing is a major environmental component which directly affects species through defoliation and indirectly through changes in light. Both these were examined here. Responses to defoliation were investigated in two time-course experiments. Changes in carbon:nitrogen ratio, stored carbohydrates, and above and below ground growth rates during recovery following clipping, were all measured. Responses to light, specifically low PAR and low red:far-red ratio light, were also investigated. Leaf placement ability and sensitivity to light quality and quantity were assessed using novel photogrammetry techniques to record leaf movement. Conventional morphological measures including petiole lengths, leaf area, chlorophyll a/b ratio and biomass were also used to assess longer term, growth responses. These responses were correlated with how a species itself modifies the light environment. The intrinsic guilds differed significantly in only three characters: root growth rate, leaf sugar levels and light foraging and tended to differ in how they modified the light environment. A mechanism is proposed incorporating leaf placement and the light environment within the canopy, defoliation tolerance and competitive ability. As a comparison, the two a priori guild classifications, ‘Taxonomic’ and ‘Stratum’, are also tested for differences in functional niche. The concluding chapter discusses possible connections between the three guild classifications.