Abstract
Many of the most invasive plant species in the world can propagate clonally, suggesting clonality offers advantages that facilitate invasion. Gaining insights into the clonal growth dynamics of invasive plants should thus improve understanding of the mechanisms of their dominance, resilience and expansion. Belonging to the shortlist of the most problematic terrestrial invaders, Reynoutria japonica var. japonica Houtt. (Japanese knotweed) has colonized all five continents, likely facilitated by its impressive ability to propagate vegetatively. However, its clonal growth patterns are surprisingly understudied; we still do not know how individuals respond to key environmental conditions, including light availability and disturbance. To contribute to filling this knowledge gap, we designed a mesocosm experiment to observe the morphological variation inR. japonicagrowth in homogeneous or heterogeneous conditions of light stress (shade) and disturbance (mowing). Rhizome fragments were planted in the middle of large pots between two habitat patches that consisted of either one or a combination of the following three environmental conditions: full light without mowing, full light with frequent mowing, or shade without mowing. At the end of the experiment, biomass and traits related to clonal growth (spacer and rhizome lengths, number of rhizome branches, and number of ramets) were measured. After 14 months, all individuals had survived, even those frequently mowed or growing under heavy shade. We showed thatR. japonicaadopts a ‘phalanx’ growth form when growing in full light and a ‘guerrilla’ form when entirely shaded. The former is characteristic of a space-occupancy strategy while the latter is more associated with a foraging strategy. In heterogeneous conditions, we also showed that clones seemed to invest preferentially more in favorable habitat patches rather than in unfavorable ones (mowed or shaded), possibly exhibiting an escape strategy. These observations could improve the management of this species, specifically by illustrating how aggressive early control measures must be, by highlighting the importance of repeated mowing of entire stands, as this plant appears to compensate readily to partial mowing, and by informing on its potential responses towards the restoration of a cover of competitive native plants.
Highlights
Clonality is an attribute frequently associated with plant invasiveness (Lloret et al 2005), and many of the most invasive plants in the world are clonal (Liu et al 2016; Lowe et al 2000; Pyšek and Richardson 2007)
This is not surprising, as the highly plastic modular growth form of clonal plants releases them from many constraints related to being sessile
Some habitat patches harbored no ramets for months until their number quickly increased after a first ramet colonization
Summary
Clonality is an attribute frequently associated with plant invasiveness (Lloret et al 2005), and many of the most invasive plants in the world are clonal (Liu et al 2016; Lowe et al 2000; Pyšek and Richardson 2007). Clonality enables survival and persistence of populations in absence of sexual reproduction (Eriksson 1997), rapid cover of and dominance of invaded sites (Herben and Hara 1997; Pyšek 1997) and, through clonal integration, the exchange of water, nutrients, carbon and information between connected ramets to reduce resource shortages and mitigate the effects of stress and disturbance (Jónsdóttir and Watson 1997; Liu et al 2016) Out of these many advantages, some have been shown to be associated with invasiveness such as high root foraging abilities or clonal integration (Keser et al 2014; Song et al 2013).
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