Rapid phenotypic differentiation and local adaptation in Japanese knotweed s.l. (Reynoutria japonica and R. x bohemica, Polygonaceae) invading novel habitats

PREMISEMany plant invaders like the Japanese knotweeds are thought to colonize new habitats with low genetic diversity. Such species provide an opportunity to study rapid adaptation to complex environmental conditions. METHODSUsing replicate reciprocal transplants of clones across three habitats, we described patterns of phenotypic response and assessed degree of local adaptation. KEY RESULTSWe found plants from beach habitats had decreased height, number of leaves, leaf area, and biomass allocation to roots and shoots compared to plants from marsh and roadside habitats when grown in their home habitat. In the marsh habitat, marsh plants were generally larger than beach plants, but not different from roadside plants. There were no differences among plants from different habitats grown in the roadside habitat. Despite this evidence of differentiation in beach and marsh habitats, we found mixed evidence for local adaptation. In their "home site" plants from the marsh habitat had greater biomass than plants from the beaches but not compared to plants from roadsides. Biomass comparisons in other habitats were either maladaptive or not significant. However, plants from the roadside had greater survival in their "home site" compared to foreign plants. There were no differences in survival in the other habitats. CONCLUSIONSWe found phenotypic differentiation associated with habitats despite the low reported genetic diversity for these populations. Our results partially support the hypothesis of local adaptation in marsh and roadside habitats. Identifying whether these patterns of differentiation result from genetic or heritable non-genetic mechanisms will require further work.