The role of residence time and mutualistic interactions on the strength of plant-soil feedbacks in naturalised Trifolium

Alien plant species may benefit from leaving behind specialised natural enemies when initially introduced to new regions, but the strength of this enemy release may subsequently decline as enemies accumulate, leading to a reduction in the performance of alien plants over time. In addition, alien plants can be dislocated from beneficial interactions with mutualists, limiting their performance. In this thesis, I examine whether longer-naturalised and more widespread alien plant species experience a weaker escape from soil-borne enemies, as expected if enemy escape is transient. A comparative biogeographic approach was adopted in which plant-soil feedback (PSF) responses were contrasted between the introduced (New Zealand, NZ) and native (Europe) range for 11 Trifolium species. A statistical approach was developed to remove the effect of mutualistic nitrogen-fixing bacteria (rhizobia) from PSF responses. The biogeographic difference in PSF between NZ and Europe was then used to quantify the strength of escape from inhibitory PSF for each Trifolium species in order to identify whether the strength of escape was correlated with the residence time and geographic spread of the species in NZ. I also identified whether Trifolium have retained soil-borne mutualists in NZ by comparing: a) the richness and community structure of arbuscular mycorrhizal fungi (AMF) associated with three Trifolium species in NZ and the UK using TRFLP; and b) the symbiotic performance, strain richness and genetic relatedness of rhizobia associated with seven Trifolium species in NZ and the UK by conducting a glasshouse experiment, genetic fingerprinting (rep-PCR with ERIC primers) and phylogenetic analysis of the nodD gene. The strength of biogeographic escape from inhibitory PSF varied considerably among the Trifolium species, although most species had similar PSF in both ranges. In contrast to expectations, the strength of escape was not significantly correlated with the residence time of the species. While there was also no overall significant correlation with the geographic spread of the species, less widespread, non-agricultural species experienced a stronger escape from inhibitory PSF, independent of their residence time. The richness and community structure of AMF taxa associated with three Trifolium species was similar in NZ and the UK. Rhizobia strains isolated from NZ Trifolium had similar nodulation ability as strains isolated from UK plants for all seven Trifolium species tested, including agricultural and accidentally introduced species. Genetic fingerprinting indicated that the strain richness of Trifolium rhizobia in NZ soils is comparable to that in UK soils. Phylogenetic analysis showed that strains of Trifolium rhizobia in NZ are not genetically distinct from UK strains, suggesting that NZ Trifolium utilise rhizobia that were co-introduced from Europe. The Trifolium species studied appear to have retained key soil-borne mutualists in NZ that have probably facilitated their successful naturalisation and spread. The findings highlight that the strength of escape from inhibitory PSF is plant species-specific, even among species within a single genus occurring in the same geographic region; this emphasises the need to examine multiple alien species when testing invasion hypotheses. Although the results suggest that the geographic spread and agricultural status of alien plant species may partly influence the strength of escape from inhibitory PSF, a hypothesised decline in the strength of escape with longer residence time of the alien plant species is not supported; the generalisability of this hypothesis is thus questioned