Phenotypic plasticity of invasive hawkweeds under drought conditions in New Zealand grasslands

<p dir="ltr">Variation in plant functional traits in response to local environments can be explained either by evolution (adaptation and natural selection) or non-genetic processes, such as phenotypic plasticity, which is where an individual alters its phenotypic expression of a genotype. Plasticity may occur in response to changes in environmental conditions over time or differences in environmental conditions in space. Untangling the role of phenotypic plasticity in the successful establishment of invasive species can assist in understanding how species may colonise and spread in the future under climatic change. Invasion by hawkweeds (Asteraceae), particularly <i>Pilosella officinarum</i> and <i>Hieracium lepidulum</i>, has been a concern in New Zealand’s grasslands since c.1950s. I aimed to assess if phenotypic plasticity plays a role in hawkweed invasion in New Zealand. I collected and measured functional traits of 255 individuals of <i>P. officinarum</i> at 17 populations and 234 individuals of <i>H. lepidulum</i> at 16 populations to see if the time of invasion of the population and historical variation in climatic conditions was related to functional trait values for above ground biomass, plant height, specific leaf area, leaf dry matter content, and leaf shape index. I then experimentally tested for plasticity in functional traits using two watering treatments, watered and drought, in a common garden experiment. Seeds were collected from field populations, and functional traits were measured for 89 seedlings from 11 parents across five populations of <i>P. officinarum</i> and 69 seedlings from 11 parents across six populations of <i>H. lepidulum</i>. This was to determine how functional traits vary under common growing conditions and how drought influences the proportion of roots to total biomass, plant height, specific leaf area, leaf dry matter content, leaf shape index, number of leaves, and specific root length. I employed 24 separate Bayesian hierarchical generalized linear models and used the posterior estimates to determine important 2 predictors of functional trait values. I showed that the time of invasion and historical climatic conditions did not affect functional trait values for <i>P. officinarum</i> and <i>H. lepidulum</i>. This indicated that historical climatic conditions were not important in affecting variation in trait values. However, shading of individual plants in both species was related to higher specific leaf area and leaf shape index, and lower leaf dry matter content, while plant height was higher only in <i>H. lepidulum</i>. I showed convergence of functional traits sourced from different populations and parents when grown under common conditions. Compared to plants in the watered treatment, those in drought conditions had a higher proportion of roots to total biomass, lower plant height, lower number of leaves, lower leaf shape index, and higher leaf dry matter content. I showed that, while some functional traits in hawkweeds may have genetic influences, such as the proportion of roots to total biomass and leaf shape index for both species, and specific root length for <i>P. officinarum</i>, phenotypic plasticity plays a crucial role in determining trait values. This plasticity likely results in resilience of hawkweed populations under climatic change, such as drought.</p>