Understanding how organisms respond to environmental variation is a fundamental challenge in ecology and evolutionary biology. This challenge is made more urgent by the need to forecast ecological and evolutionary dynamics in the face of climate change. Linking physiological traits, life history strategies, and their responses to environmental variation can assist in producing a mechanistically-based predictive framework for ecologists and climate change scientists to understand population dynamics. The invasive Sahara mustard has successfully colonized arid lands eastward from California to Texas and northward into Nevada and Utah since its introduction in the early 20th century. This system provides an opportunity to untangle intraspecific responses to environmental variation across a diverse set of habitats and the ability to detect rapid evolutionary responses to selection pressures during colonization. We carried out multiple common garden experiments to test for local adaptation across the species invaded range. Seeds from 10 sites spanning the western US were grown multiple generations and functional traits were used to assess responses to source environments. Overall, growth strategies were compared and linked to environmental drivers using Bioclim data.
We examined differential performance of nearly 500 plants and found evidence for local adaptation of Sahara mustard based on phenological, morphological, and physiological traits. The species appears to have synchronized growth and reproduction to favorable conditions by adjusting phenological events (i.e., germination timing, leaf production, flowering). Further, Sahara mustard has maximized its growth rates by rapidly producing leaf area and biomass primarily in response to variable precipitation in its invaded range. Traits were best explained by environmental variability during the species main growing season (e.g., winter precipitation and temperature). Overall, we present Sahara mustard as an eminently-suited study system to understand the evolutionary ecology of species responses to environmental variation, and discuss implications for the continued spread of this species in North America.