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

Abstract

Summary1. 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 preacclimation 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.

Clonal integration in Ludwigia hexapetala under different light regimes

“…These parent ramets also produced greater belowground biomass, suggesting either a greater resource acquisition demand to support offspring ramets or a shift to resource storage via rhizomes. Offspring ramets, in general, receive considerable parental resources benefitting their growth (Xu et al, 2010(Xu et al, , 2012, and the direction of resource sharing tends to favor younger ramets (Alpert, 1996). Parent ramets may have translocated resources to offspring at the expense of their own shoot growth and development.…”

“…This flexibility in growth form provides many benefits to clonal plants, including improved ability to grow and colonize in low resource environments (Stuefer et al, 1994;Alpert, 1996). Under severe growing conditions or high environmental variability, clonal plants may have increased survivorship (Xu et al, 2010) because clones can respond plastically by dividing labor and sharing resources (Alpert, 1996(Alpert, , 1999aXiao et al, 2007). However, the benefits of clonal integration among ramets can vary among wetland plant species (Pennings and Callaway, 2000).…”

Effects of fragmentation on the survival and growth of the invasive, clonal plant Alternanthera philoxeroides

“…It grows clonally via monopodial stolons up to 10 m long that can bear leaves, roots, and axillary stolons at each node; a node along a stolon with its leaves and roots thus corresponds to a ramet. Clones can form extensive networks of connected ramets within one growing season (Geng et al 2006), and physiological integration between connected ramets can modify their individual and combined growth and spread (Wang et al 2008Yu et al 2009;Dong et al 2010a, b;Xu et al 2010). Disturbances such as herbivory, mowing, and application of herbicides frequently fragment clones into pieces of different sizes, and even very small fragments such as ones consisting of a single ramet or highly stressed fragments such as in aquatic mats stranded over the winter on dried sediments can survive and produce new stolons and ramets Dong et al 2010a, b;Dugdale et al 2010).…”

Clonal integration in homogeneous environments increases performance of Alternanthera philoxeroides

“…The species rarely produces viable seeds and has very low genetic diversity in China (Ye et al 2003;Li and Ye 2006;Geng et al 2007), suggesting that it spreads mainly vegetatively. Clonal integration can increase the performance of A. philoxeroides in heterogeneous environments Xu et al 2010).…”

“…Clonal integration, the movement of resources or signals between connected ramets within clonal fragments, has been repeatedly shown to increase the performance of clonal plants in heterogeneous environments where connected ramets experience different levels of resource availability (e.g., Alpert and Mooney 1986;Hutchings and Wijesinghe 1997;Xu et al 2010;Zhang et al 2012;Song et al 2013;Touchette et al 2013). By resource, we mean something whose use by one plant precludes its simultaneous use by another, such as light, water, mineral nutrients, CO 2 , or O 2 taken up by plant organs; or physical space used for attachment.…”