Incorporating rhizosphere microbiota from the native and non-native ranges into tests of post-naturalisation performance: New Zealand Trifolium as a model system

A critical goal in the study of plant invasions is to understand the traits and mechanisms that contribute to invader success. One of the most compelling hypotheses is the evolution of increased competitive ability (EICA), which posits that invasive plants escape co-evolved pathogens, pests and herbivores from their native range and adapt by down-regulating defences in favour of fitness, thus becoming superior competitors. However, after two decades of rigorous testing, support for all of EICA’s predictions remains equivocal. This lack of consensus may exist because most tests have centred on aboveground interactions, omitting the interactive effects of rhizosphere microbiota, which play pivotal roles in plant performance, fitness and competitive ability. Furthermore, EICA investigations have focused solely on antagonists, while post-naturalisation trade-offs that result in increased competitive ability can also arise when plants are dislocated from mutualists. In this thesis, I use Trifolium species as a model system and expand the EICA framework by growing plants from native (European) and non-native (New Zealand) provenances in rhizosphere soil cultivated in situ by conspecifics in each range. Using this biogeographical framework, I first compared the performance of plants from each provenance and analysed their association with root endophytes (arbuscular mycorrhizal fungi (AMF) and nitrogen-fixing rhizobia). Second, I compared profiles of root flavonoids (which function in both defence and mutualisms) in plants from each provenance when grown in sterilised or unsterilised rhizosphere soil, allowing me to separate microbe-stimulated versus constitutive production. Lastly, I performed an intra-specific competition experiment to test whether non-native plants have developed superior competitive ability. For each investigation, I predicted that the magnitude of divergence between plants from native and non-native provenances would be positively correlated with the naturalisation success of each species, which for these New Zealand Trifolium species is not correlated with their naturalisation date. Non-native plants had lower rhizobia nodulation compared with native conspecifics, and this difference was significant in New Zealand and UK soils. AMF colonisation varied, but was significantly lower among non-natives in Spanish soil. Flavonoid richness was generally reduced among non-native plants—significantly in sterilised native-range soils (suggesting constitutive down-regulation), and in one of the native-range unsterilised soils (UK). However, there was no evidence for performance trade-offs; instead, non-native plants were significantly smaller than native conspecifics in all soils. Neither was there evidence of physiological compensation for decreased mutualist associations, nor increased competitive ability. Lastly, there were no correlations between the magnitude of trait divergence and species’ distributions in the non-native range. Thus, despite significant post-naturalisation differences in a number of traits that might suggest adaptation, these differences do not appear correlated with increased plant performance nor with the naturalisation success of Trifolium in New Zealand. This thesis contributes four important findings to our knowledge of post-naturalisation performance among invasive plants. First, phenotypic differences may be apparent between native and non-native conspecifics, but these differences do not necessarily equate to improved fitness. Second, mutualist availability and effectiveness may not necessarily be a substantial barrier to naturalisation—even among plants that host multiple highly beneficial symbionts in their native range or those that encounter parasitic mutualists in the introduced range. Third, the standard EICA metrics growth and competitive ability are not always the most relevant factors to indicate invasibility, as this study supports work showing decreased size may be equally common and successful invaders are not always better competitors. Fourth, this work revealed that measures of size are not an appropriate surrogate for measures of competitive ability—a valuable finding for future EICA experimental designs.