Plant Height Dynamics and Competition: From Local Interactions to Global Patterns

Abstract

Competition is widely recognized as a major force influencing plant community structure, with functional traits predicted to mediate competitive interactions between species. Despite theoretical predictions that taller plant species should exert stronger competitive effects on neighbours, plant height distributions in nature are generally right-skewed for species within neighbourhoods and communities. My thesis explores this contradiction between theoretical predictions and observed patterns by examining how plant height mediates species interactions across environmental gradients in grassland ecosystems. First, I investigated how differences in maximum height influence species co-occurrence patterns along a competition intensity gradient measured using neighbour removal experiments. I found that negatively co-occurring species pairs differ more in maximum height than positively co-occurring pairs; these patterns varied with competition intensity. Second, I tested for limiting similarity in an old-field community by measuring height for all reproductive individuals (n = 19,587) across 100 focal plots along an estimated competition intensity gradient. Results show that height spacing between coexisting species increased significantly with competition intensity, providing strong support for limiting similarity predictions, which we did not detect with a conventional community-scale null modeling analysis that assumed uniform competition. Third, I investigated the impacts of fertilization and herbivore exclusion on community potential maximum height distributions using data from a global nutrient enrichment experiment (the Nutrient Network) across 13 grassland communities. I found that species which persisted in or colonized plots following treatment application had greater maximum potential heights on average (25% and 34%, respectively) than species lost following treatment application. Additionally, local climate influenced how height distributions responded to fertilization and herbivore exclusion treatments. Collectively, this research advances our understanding of size-mediated competition in plant communities by: (1) empirically linking competition with species co-occurrence patterns and maximum height differences; (2) demonstrating that accounting for variation in competition intensity reveals trait-based assembly processes obscured by traditional methods; and (3) showing that environmental context limits the competitive advantage of height. These findings highlight the importance of considering local competitive environments and broader climatic conditions in community assembly studies, providing a framework for reconciling theory regarding height-mediated competition with observed patterns in natural communities.