We found a clear variation in ramet length in response to different light conditions, indicating a high phenotypic plasticity of H. coronarium. The effect of light levels in this aspect of ramet morphology was intensified by the intraspecific competition created by rhizome density treatments over time. The number of ramets, when controlled for the initial number of rhizomes, initially suffered a negative effect of intraspecific competition, but this difference was no longer evident in the phase 2. Moreover, the number of ramets was similar regardless of light levels, which indicates that environmental conditions had a secondary role in predicting H. coronarium sprouting. Our results also showed that the proportion of aerial and subterraneous biomass is roughly constant despite variations in light and rhizome density, probably owe to the adjustments in ramet morphology mediated by phenotypic plasticity mechanisms. Together, these results suggest that H. coronarium maintains similar performances under different environmental conditions, being little affected by the negative effects of intraspecific competition resulting from population growth.
Ramet length showed a fast adaptation to the differences between light treatments, being higher in shade treatment at both density treatments since the early stages of invasion, represented by phase 1. In fact, the length of H. coronarium ramets can be positively affected by shading levels (Santos et al. 2005). Accordingly, some clonal species can exhibit higher growth rates in shaded environments, increasing investment on height under low-light conditions (Huber and Wiggerman 1997; Ikegami et al. 2007). Furthermore, higher rhizome density in the shade treatment was related to higher ramet length during phase 2, reinforcing that ramet elongation occurs due to the aggravation of resource scarcity by intraspecific competition. Considering the invasiveness of H. coronarium, as registered in Brazil and other American countries (Kissmann and Groth 1999), our results indicate that closed canopy forests, as riparian forests, can facilitate the growth of ramets, being as susceptible as disturbed, deforested water stream margins to the spatial expansion of this invasive plant.
Contradicting our initial hypothesis, rhizome biomass at the end of the experiment was similar between high and low density treatments, although this result may have accrued from the wide variation between replicates. This result indicates that stochasticity had a considerable effect on the levels of rhizome growth reached during the experiment, which probably masked the consequences of intraspecific competition. On the other hand, our prediction regarding the effect of light treatments on rhizome dry weight was corroborated. Considering that rhizomes are an important propagule (Lange and Marshall 2016), the higher rhizome biomass under shade may allow a greater subterraneous propagation and the effort to colonize new areas as an attempt to escape competition for light.
We expected to find a negative effect of rhizome density on aerial biomass resulting from higher intraspecific competition (Müller et al. 2016). However, although the morphology of ramets have differed between the treatments, the proportion of aerial/belowground biomass (i.e., ramet/rhizome ratio) did not vary between them. The number of ramets was similar regardless of environmental conditions, indicating that light did not increase sprouting of H. coronarium, unlike other invasive clonal plants (Ikegami et al. 2007; Martin et al. 2020; Chen et al. 2019). Our hypothesis on the negative effect of rhizome density on the number of ramets was corroborated in both phases of the experiment, which suggests an important effect of belowground competition on sprouting in both the initial and later stages of H. coronarium invasion. This is in line with other studies reporting that the neighborhood density affects the growth, survival and fitness of clonal species (De Kroon and Kwant 1991; Kleunen et al. 2001; Van Kleunen et al. 2005). This negative effect of rhizome competition, however, seems to be attenuated by other mechanisms in field conditions given the typical formation of dense monospecific stands of H. coronarium, and the comprehension of the multiple factors underlying the population dynamics of this invasive species deserves further studies.
In conclusion, our results suggest a large phenotypic plasticity in H. coronarium pronounced in ramet length. The capacity of such adjustment is analogous to the mechanism of division of labor (Ikegami et al. 2008), and probably is an important compensation strategy to maintain the relation of aerial/subterraneous biomass through variations in environmental conditions and population densities (Pinheiro et al. 2021). These alternative ways of energy allocation may have a key role in the colonization of different habitat types, since Hedychium coronarium can keep biomass production constant on varied environmental conditions and intraspecific competition levels, establishing a compensation mechanism of energy investment in above and belowground biomass. These traits may confer an important competitive advantage over native species and enhance spatial expansion of H. coronarium to different areas, such as opened water lands and closed, forested sites.