Genetically Based Trait Differentiation but Lack of Trade-offs between Stress Tolerance and Performance in Introduced Canada Thistle

Abstract

Trade-offs between performance and tolerance of abiotic and biotic stress have been proposed to explain both the success of invasive species and frequently observed size differences between native and introduced populations. Canada thistle seeds collected from across the introduced North American and the native European range were grown in benign and stressful conditions (nutrient stress, shading, simulated herbivory, drought, and mowing), to evaluate whether native and introduced individuals differ in performance or stress tolerance. An additional experiment assessed the strength of maternal effects by comparing plants derived from field-collected seeds with those derived from clones grown in the glasshouse. Introduced populations tended to be larger in size, but no trade-off of stress tolerance with performance was detected; introduced populations had either superior performance or equivalent trait values and survivorship in the treatment common gardens. We also detected evidence of parallel latitudinal clines of some traits in both the native and introduced ranges and associations with climate variables in some treatments, consistent with recent climate adaptation within the introduced range. Our results are consistent with rapid adaptation of introduced populations, but, contrary to predictions, the evolution of invasive traits did not come at the cost of reduced stress tolerance.

Keywords: Canada thistle; EICA; adaptation; biological invasion; plasticity; trait evolution.

Figure 1

Sampling Locations of Canada Thistle in the Introduced North American (Peach) and Native European (Turquoise) Ranges.

Figure 2

Trait Divergence in Response to Range and Latitude in Canada Thistle. Population mean trait responses for final stem diameter (stem diameter _t4) (A), final leaf number (leaves_t4) (B), number of flower heads (flowers) (C), and below-ground biomass (below biomass) (D) in the control common garden with latitude in the introduced North American (peach) and native European (turquoise) ranges of Canada thistle, with model predictions and 95% shaded confidence intervals from stepwise reduced models. Range effect was significant for stem diameter and leaf number (p > 0.05) (see Table 1).

Figure 3

Trait Divergence in Response to Treatment and Range in Canada Thistle. Population least square means trait responses for change in leaf number between time point two to the end of experiment (leaves_t4-t2) (A), final leaf number (leaves_t4) (B), and final stem diameter (mm) (stem diameter_t4) (C) to range and treatment in three common gardens between introduced North American (peach) and native European (turquoise) ranges of Canada thistle, along with 95% confidence intervals. Range and treatment were significant for all traits (p < 0.05) (see Table 2).