Phenotypic plasticity and plant invasions: A comparative study of native and exotic plant congeners

Background/Question/Methods

Introduced plants threaten biodiversity and ecosystem functioning, and are costly to control. Some theory suggests that phenotypic plasticity, i.e. an organism’s ability to alter its phenotype in response to the environment, may contribute to the establishment and proliferation of exotics in their introduced ranges. Experimental evidence for this prediction, especially evidence from field studies, is limited and the results are mixed. We measured the plasticity of seven pairs of plant congeners from six families to test the hypothesis that exotics are more plastic than closely-related natives. Each congeneric pair contained an invasive exotic and an ecologically similar native. The pairs were planted in an old-field where we manipulated nutrients (ambient or elevated levels) via fertilizer addition and competition (natural or reduced levels) with herbicide and regular clipping of the surrounding vegetation in a randomized fully-factorial design. We assessed the species’ response to the environmental manipulations by measuring performance traits (relative growth rate, lifespan, leaf mass, plant size, bud duration, basal diameter), as well as traits important for physiological and defensive functions (leaf toughness, specific leaf area, trichome density, leaf water content).

Results/Conclusions

Multivariate and univariate analyses indicated that across environments and species, exotics differed significantly from natives: on average, exotics were larger, produced greater leaf mass, and had thinner and less tough leaves with higher water content.  Using a within-study factorial meta-analysis to quantify the effect of fertilizer and competition on the multiple traits, we found that exotics were not consistently more plastic as hypothesized. The response of exotics to reduced competition was equal to or marginally less than that of the natives. With fertilizer addition, however, exotics and natives did differ.  Exotics showed a significantly greater increase in trichome density, bud duration, and basal diameter, as well as a marginally greater increase in size and leaf mass. Natives, in contrast, showed a significant decrease in leaf water content and an increase in specific leaf area. Although the difference in plasticity between natives and exotics depends on the trait measured and the environment, exotics across six plant families did display a trend of increased ability to take advantage of elevated resources. This result implies that plasticity may play a role in plant invasions and that plasticity to elevated soil nutrients might be more important than plasticity to competitive release.