Reassessing the role of competition in the evolution and expression of adaptive strategies in plants

Abstract

In this thesis, I utilise the vast amount of competition literature that has accumulated to conduct four literature surveys testing the predictions of traditional strategy theory on the role of competition in the evolution of plant strategies and life histories. Traditional theory predicts that seedlings emerging from large seeds have the advantage over those emerging from small seeds in environments of dense vegetation. Through my synthesis I show that large seed size is associated with high competitive ability but only when plants are competing against other seedlings. My findings offer a new interpretation of seed size strategies suggesting that seedlings have little chance of establishing in dense communities but rather, mainly emerge in open spaces. Next I tested whether shade avoidance plasticity was an adaptive response under competition. Shade avoidance responses have often been thought of as a strategy plants use to outcompete their neighbours for light resources. I found that shade avoidance plasticity was not associated with an increase in competitive performance as has been predicted by theory but that instead the adaptive value of shade avoidance lies in gathering resources to allow earlier reproduction under competition. I tested if these findings extended to plasticity in general which theory predicts gives plants a competitive advantage by allowing them to adjust growth to acquire more of the available resources than their competitors. I found that plasticity was not associated with an increase in competitive ability in either short or long-lived species. Instead plasticity was correlated with increased reproductive efficiency in short-lived species whilst this did not occur in long-lived species. Finally, I tested whether functional traits specific leaf area (SLA) and maximum height were good predictors of competitive ability. I found that neither maximum height nor SLA were good predictors of competitive performance. My findings highlighted that the theoretical basis of our understanding of competition needs to be improved before we can effectively use functional traits to predict competitive outcomes. Overall, this thesis challenges some of the key foundational assumptions of strategy theory and illustrates the need for a change in the way we measure competitive ability.