Trait differentiation across urbanized environments in invasive Centaurea melitensis is not predicted by the Plant Economic Spectrum

Abstract

Urbanization imposes unique selective pressures on organisms, but connecting specific urban environmental differences to trait adaptation requires detailed characterization of environmental heterogeneity across urbanized landscapes and tests for adaptation to it, rarely done to date. The Plant Economic Spectrum (PES) predicts that plants invest in different life strategies according to resource availability, ranging from an acquisitive strategy favoring fast growth and aboveground biomass where resources are abundant, to a conservative strategy favoring belowground investment where resources are limiting. Here, we leverage previous work describing the mosaic of urbanized environments in California, USA as a set of Environmental Zones, and use this framework to test for trait differentiation predicted by the PES in the invasive annual plant Centaurea melitensis. Seeds from 31 populations across California were grown in greenhouse conditions for two generations. In the second generation, we measured growth, reproductive, and phenology traits to test for genetically-based differentiation and whether variation follows PES predictions. Variation across all genotypes was not structured along the multivariate trait axes predicted by the PES. Individual traits were associated with environmental variables, including impervious surface cover and tree cover, following some PES expectations, but multi-trait strategies were absent. We found evidence that C. melitensis traits have differentiated genetically across environments with different features of urbanization but did not exhibit the multi-trait strategies predicted by the PES. Our study pairs detailed knowledge of urbanized environments with tests of trait-environment relationships to reveal how urbanization may be selecting on the species that inhabit these spaces.