Plasticity in six species of native and exotic Asteraceae

Background/Question/Methods

Phenotypic plasticity is the ability of an organism to alter its physiology or morphology in response to the environment.  If exotic, invasive plants exhibit a greater degree of phenotypic plasticity than native species, this may explain why these species are able to invade new habitats. We conducted a greenhouse trial to experimentally test this hypothesis by using a soil nitrogen (N) gradient to create a range of environmental conditions across which plasticity could be expressed.  We limited our study to functionally similar plants with a common life-history strategy: perennial Asteraceae.  Specifically, we selected three native (Eriophyllum lanatum, Solidago spathulata, Microseris laciniata) and three exotic (Lactuca serriola, Hypochaeris radicata, Senecio jacobea) species.  Plants were grown under five levels of soil N (0, 3.9, 7.8, 11.7, and 15.6 g/m²).  The experiment had a fully randomized design with ten plants per combination of species and N, for a total of 300 plants. Survival, height, and relative chlorophyll content were measured every two weeks.  After 150 days, plants were destructively harvested, and below- and aboveground biomass, leaf area and specific leaf area (SLA) were quantified. 

Results/Conclusions

Native species had significantly higher rates of mortality across all N treatments (56% vs 20% for exotics).  Mean mortality rates ranged from 12% for Lactuca to 100% for Microseris; temporal mortality patterns also differed among species.  We calculated the relative distance plasticity index (RDPI), which quantifies the phenotypic distance between individuals grown in different environments, for five plant traits (height, total biomass, SLA, root:shoot ratio, and relative chlorophyll content) on the 150^th day.  There were significant differences in plasticity between species for all traits except SLA.  For example, Hypochaeris had lower plasticity for root:shoot ratio and higher plasticity in height when compared to other species, while Eriophyllum had higher plasticity in chlorophyll content when compared to other species.  Overall, there was no consistent trend in plasticity for native or exotic species.  These results suggest that the degree of plasticity differs greatly among species and is not a mechanism that clearly distinguishes invasive from native Asteraceae.  This study also highlights the importance of selecting appropriate variables and methods for analyses of plasticity.