Effects of clonal integration and light availability on the growth and physiology of two invasive herbs

1. Clonal plants benefit from the ability to translocate resources among interconnected ramets to colonize heterogeneous habitats. Clonal integration affects the resource level and morphological traits of ramets, and thus may influence their physiology and general performance. Although leaf gas exchange and its associated physiological adjustments are key traits to assess plant fitness, the effect of clonal integration on these functional traits is insufficiently understood. 2. In a glasshouse experiment, we addressed how clonal integration affects gas exchange properties, leaf characters and growth of ramets in two invasive plants, Alternanthera philoxeroides and Phyla canescens, under full sun and 85% shade. We also used stable-isotope labelling to assess the maternal subsidy to daughter ramets. 3. Similar effects of connection were observed in both species for most gas exchange parameters and leaf characters. Clonal integration did not affect photosynthetic capacity and respiratory rates of ramets. When grown in shade, ramets connected with an unshaded mother plant displayed higher area-based leaf nitrogen and chlorophyll content than severed ramets, but the additional nitrogen and chlorophyll was not translated to increased photosynthetic capability. Overall, severed ramets displayed significant light response for leaf nitrogen (area-based), photosynthetic nitrogen use efficiency, chlorophyll to nitrogen ratio, and nitrogen stable-isotope signature, but the light response was eliminated by clonal integration in connected ramets. 4. Both species displayed substantial maternal carbohydrate subsidy that benefited the growth of daughter ramets, but species-specific patterns were observed in the growth of daughter ramets and the amount of subsidy. The amount of subsidy was independent of ramet growth light levels for P. canescens, but shaded, connected ramets of A. philoxeroides received more subsidy than unshaded controls, facilitating a larger growth improvement relative to severed counterparts than P. canescens. 5. Synthesis. We observed increased leaf nitrogen and chlorophyll in shaded, connected ramets of two clonal invasive plants. Clonal integration may facilitate nitrogen assimilation and allow pre-acclimation to high-light conditions for shaded, connected ramets, thus promoting the opportunistic expansion of these invaders on site scale. The species-specific maternal subsidy pattern demonstrated that clonal plants possess different strategies to subsidize ramets under light-limited conditions.