Increased adaptive phenotypic plasticity in the introduced range in alien weeds under drought and flooding

Abstract

Phenotypic plasticity is an essential mechanism by which plants respond to changes in their environment, but our understanding of the evolution of plasticity is still limited. Comparing plasticity of introduced alien species across native and introduced provenances can indicate potential evolution of adaptive plasticity. We examined reaction norms across an experimental soil moisture gradient for native and introduced provenances of two Rumex spp. to ask whether plasticity was (a) adaptive or maladaptive, (b) greater in the more widespread R. obtusifolius, and (c) greater in the introduced range. We cloned genotypes from the United Kingdom (native range) and New Zealand (introduced range) and grew them under drought, mesic or flooded conditions. We measured biomass and functional traits to assess differences in, and fitness implications of, trait means and plasticity, where plasticity was quantified as the slope of the reaction norm. Plasticity to drought was often positively correlated with biomass and likely adaptive, while plasticity to flooding was sometimes negatively correlated with biomass and thus potentially maladaptive. Plasticity to drought was greater in R. obtusifolius than in the less widespread R. conglomeratus, as expected, although no difference was found under flooding. Compared to plants from the native range, introduced provenance R. obtusifolius had greater plasticity in chlorophyll content and water use efficiency under drought, both of which were positively correlated with biomass, suggesting that greater adaptive plasticity may have evolved in New Zealand. This capacity for adaptation could increase their range and exacerbate their impact in the future.