Rachel M. Mitchell and Jonathan D. Bakker. University of Washington
Background/Question/Methods Phenotypic plasticity is the ability of an organism to alter its physiology or morphology in response to the environment. Soil nitrogen (N) limits plant growth in many temperate ecosystems and can be highly variable both spatially and temporally, suggesting that phenotypic plasticity in response to soil N may explain the success of invasive exotic species compared to native species. We used a greenhouse trial to experimentally test this hypothesis. We limited our study to functionally similar plants with a common life-history strategy. Specifically, we selected three native (Eriophyllum lanatum, Solidago spathulata, Microseris laciniata) and three exotic (Lactuca serriola, Hypochaeris radicata, Senecio jacobea) perennial Asteraceae 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 of each combination of species and N, for a total of 300 plants. We were particularly interested in species x N interactions as evidence of plasticity. Height, leaf number, leaf length and relative chlorophyll content, and reproductive output 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. The fresh roots were also digitally scanned and analyzed to quantify specific root length. 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 L. serriola to 100% for M. laciniata; temporal mortality patterns also differed among species. Species x N interactions were significant for root and shoot biomass, indicating that species differed in how they altered tissue allocation strategies under varying soil N levels. S. spathulata, S. jacobea and H. radicata demonstrated plasticity for both root and shoot biomass, while E. lanatum only demonstrated plasticity in shoot mass and L. serriola did not demonstrate plasticity in either root or shoot biomass. Variables such as SLA and relative leaf chlorophyll content were highly species specific but did not exhibit significant phenotypic plasticity, possibly because consistency in these traits is critical to plant competition, reproduction and survival. 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 for analyses of plasticity.
