Evidence of local adaptation to aridity but not nitrogen deposition in invasive annuals

Rapid adaptation of traits and trait plasticity may contribute to plant
invasiveness and confer fitness advantages in novel environments resulting
from global change. The importance of trait differentiation in invasive
plant populations is well-recognized, particularly in response to climate.
However, it is largely unknown how invasive plant populations will respond
evolutionarily to atmospheric nitrogen (N) deposition resulting from air
pollution, which is a major contributor to invasion success in many
ecosystems. Using a common garden experiment, a widely used method for
testing local adaptation, we assessed potential differences in plant
functional traits and nitrogen plasticity across populations of two
widespread invasive annuals from sites spanning a range of N deposition
and aridity throughout southern California. These species exhibited clear
population-level differences in traits and N responses, but these were
unrelated to N deposition. Instead, we detected significant relationships
between several traits and aridity, and populations from more arid sites
exhibited reduced N plasticity for multiple traits. Multivariate
plasticity indices also showed a strong negative relationship with aridity
across populations for both species. However, trait responses to N
addition also appeared be influenced by species’ drought-coping
strategies. In Bromus diandrus, a drought-escaping early-season annual
grass, populations from less arid sites showed increased plasticity in
shoot growth and more rapid flowering in response to N addition. In
contrast, Centaurea melitensis, a drought-tolerant late-season forb,
showed climate-driven shifts in biomass allocation in response to N;
populations from more arid sites invested more in roots, while populations
from less arid sites allocated more to leaves. These contrasting N
responses strongly suggest distinct growth strategies and ecophysiological
tradeoffs shaped by adaptation to local climate conditions. While elevated
N availability may indeed promote invasion, climate stress might exert an
overriding influence on local adaptation of plant invaders in dryland
ecosystems subject to N deposition.