Phenotypic plasticity in plant trait expression may facilitate mutual invasibility between close relatives

Background/Question/Methods

Community assembly theory postulates that species will be able to coexist if they can invade one another's populations when rare--the criterion of mutual invasibility.  Phenotypic plasticity in trait expression could facilitate mutual invasibility, potentially contributing to species coexistence.  For example, if plasticity results in trait divergence in competition, then greater niche differentiation could enhance coexistence.  To test this hypothesis, we conducted mutual invasibility experiments with four congeneric pairs of introduced plants that coexist in old-field habitats in Cleveland, Ohio.  Each resident species was allowed to establish, and then it was experimentally invaded by its congener.  Further, to determine how environmental context mediates trait expression, we conducted these mutual invasibility experiments across a factorial set of soil history (conspecific and congener soil) and competition distance (three distances to competitor) treatments. Two resource allocation traits, specific leaf area (SLA) and root:shoot biomass, were measured for each resident and invader to determine whether species were plastic in their trait expression, such that traits differed based on status as a resident or invader, and whether observed trait plasticity might facilitate coexistence between close relatives.  Generalized linear models were used to determine the effects of experimental treatments on plant biomass and trait expression.

Results/Conclusions

Species were able to establish in the presence of their congener, and species identity, soil environment, and competition distance interacted to influence invader biomass and trait values.  We found significant phenotypic plasticity in both SLA and root:shoot.  SLA tended to be higher within a species when it was grown as an invader, compared with when it was grown as a resident.  Invaders also tended to have higher root:shoot than when grown as residents, potentially enhancing competition for soil resources.  The strength of these trait patterns varied across species, suggesting that species differed in their degree of plasticity.  Differences in developmental stage between residents and invaders could influence trait expression, and, consistent with this hypothesis, we found that total biomass was a significant covariate on both SLA and root:shoot.  However, trait means often remained significantly higher in invaders after controlling for plant total biomass, suggesting that ontogenetic differences are not the sole driver of the greater resource acquisition strategy for these plants when rare.  These results are consistent with phenotypic plasticity contributing to coexistence between close relatives.