The influence of residence time and geographic extent on the strength of plant–soil feedbacks for naturalised Trifolium

Release from natural enemies is considered an important mechanism underlying the success of plants introduced to new regions, but the degree to which alien plant species benefit from enemy release appears highly variable and context-dependent. Such variation could arise if enemy release is a transient phenomenon, whereby alien plant species initially escape but subsequently accumulate enemies in their new regions. To evaluate this hypothesis in terms of soil biota, we used 11 Trifolium (clover) species introduced to New Zealand from Europe to test whether species resident for longer or with a larger geographic extent in New Zealand were more adversely affected by soil communities in the introduced range, as expected if species have accumulated inhibitory soil biota over time. We used plant–soil feedback (PSF) experiments to compare the effect of soil biota on the growth of the Trifolium species in soil from their introduced (New Zealand) and native (Spain and the United Kingdom) ranges. We applied a novel statistical approach aimed at isolating the impact of antagonistic soil biota by accounting for variation in plant growth due to mutualistic rhizobia bacteria. The between-range differences in PSF varied considerably among the Trifolium species: some species were released from inhibitory PSF in the introduced range, but the majority experienced similar PSF in both ranges. Averaged over all 11 Trifolium species, PSF was less inhibitory in the introduced than in the native range, implying some release from soil-borne enemies. However, neither residence time nor geographic extent in the introduced range was significantly correlated with the strength of release from inhibitory PSF. Synthesis. Our multispecies study provides some evidence that alien plants can escape antagonistic soil biota in their introduced range, but highlights how plant–soil feedback responses can be highly variable among congeneric plant species in the same region. Our results do not support the hypothesis that the release from inhibitory plant–soil feedback is transient, questioning the generality of this phenomenon