Contrasting trait responses to latitudinal climate variation in two lineages of an invasive grass

Plants are expected to respond to global environmental change through shifts in functional traits and in their ranges. These shifts could alter productivity and interactions among species or genetic lineages, ultimately leading to changes in distributions and abundance. In particular, cosmopolitan species are predicted to increase growth with decreasing latitude due to differences in climate and temperature. The pattern of changes in growth may vary among genotypes within species, leading to different responses with latitude. To evaluate whether climate can affect geographically distinct genotypes of cosmopolitan invasive species differently, we evaluated the trait responses of two lineages of the common reed, Phragmites australis, to variation in environmental conditions spanning North America’s Atlantic coast. Using three reciprocal transplant common gardens, we tested for the effects of garden location and plant lineage on traits related to biomass production, flowering frequency, leaf morphology, and leaf-level physiology. We found that aboveground biomass, stem density, and flowering frequency responded non-linearly to increasing latitude in one or both lineages. These results suggest that measures of plant traits over narrow latitudinal ranges may not accurately reflect organismal-level responses to global change at broad spatial scales. Given the responses to latitude that we observed in P. australis, we propose that feedbacks between growth and reproductive rate will influence range shifts in these two lineages. Such range shifts could lead to genetic admixtures, subsequently yielding more productive, locally-adapted genotypes.