Potential local adaptation in populations of invasive reed canary grass (Phalaris arundinacea) across an urbanization gradient

Urban stressors represent strong selective gradients that can elicit
evolutionary change, especially in non-native species that may harbor
substantial within-population variability. To test whether urban stressors
drive phenotypic differentiation and influence local adaptation, we
compared stress responses of populations of a ubiquitous invader, reed
canary grass (Phalaris arundinacea). Specifically, we quantified responses
to salt, copper, and zinc additions by reed canary grass collected from
four populations spanning an urbanization gradient (natural, rural,
moderate urban and intense urban). We measured ten phenotypic traits and
trait plasticities, because reed canary grass is known to be highly
plastic and because plasticity may enhance invasion success. We tested the
following hypotheses: 1) source populations vary systematically in their
stress response, with the intense urban population least sensitive and the
natural population most sensitive, and 2) plastic responses are adaptive
under stressful conditions. We found clear trait variation among
populations, with the greatest divergence in traits and trait plasticities
between the natural and intense urban populations. The intense urban
population showed stress tolerator characteristics for resource
acquisition traits including leaf dry matter content and specific root
length. Trait plasticity varied among populations for over half the traits
measured, highlighting that plasticity differences were as common as trait
differences. Plasticity in root mass ratio and specific root length were
adaptive in some contexts, suggesting that natural selection by
anthropogenic stressors may have contributed to root trait differences.
Reed canary grass populations in highly urbanized wetlands may therefore
be evolving enhanced tolerance to urban stressors, suggesting a mechanism
by which invasive species may proliferate across urban wetland systems
generally.