Functional traits modulate plant community responses to alien plant invasion

Abstract

It is well known that alien plant invasion modifies the composition and diversity of resident plant communities, yet our ability to predict patterns of vegetation responses to invasion is hampered by a poor understanding of which functional traits make some resident plants more or less vulnerable to invader impacts. For example, resident species may be more likely to persist and coexist with the invader if they display different strategies of resource use (i.e. high niche differences) or if they share similar highly competitive traits (i.e. competitive hierarchy).Here, we used a trait-based approach to 1) specifically test whether species changes in abundance following plant invasion is random or depends on specific functional trait values, 2) identify which functional strategies allow resident species to coexist with the invader and 3) assess to what extent communities where the invader has been removed return to a reference state. To do so, we applied a semi-experimental approach with the annual vine Humulus japonicus, an invasive species in riparian habitats of Southern France. We monitored plant communities seven times over two years in non-invaded reference plots, invaded plots and removal plots in which the invader had been removed at the seedling stage. We quantified species richness and functional richness as well as mean community trait values of seven traits.The comparison of invaded and removal plots highlighted a strong impact of Humulus japonicus on both species and functional richness of riparian communities. Resident species that had a distinct flowering onset from the invader faced less pronounced declines. Specifically, species that flowered before the invader reached high cover, and species that had a short flowering duration had lower risk of declining. In addition, species coexisting with the invader when it reached its maximum cover displayed high stature and high seed mass. These results suggest that both niche differentiation (in terms of flowering strategy) and competitive hierarchies (in terms of height and regeneration strategy) play a role in explaining plant community responses to plant invasion. Finally, while species richness recovered rapidly in removal plots, functional richness remained lower than in non-invaded plots, pointing to a stateshift that would otherwise go undetected. Overall, our study highlights that the analysis of the functional traits of both invaders and resident species, combined with regular monitoring over time of non-invaded reference plots, invaded and removal plots can greatly improve our understanding of the impact of plant invasion on resident communities.

Inter‐ and intraspecific trait variation shape multidimensional trait overlap between two plant invaders and the invaded communities

“…This is further supported by a positive shift of the community trait space along the plant height axis under invasion of I. glandulifera (Garnier and Navas ). Moreover, these results are in line with the recent study of Fried et al (), that showed that invasion by the highly competitive alien plant Humulus japonicus resulted in a shift of the native community towards competition related trait values for SLA, plant height and growth form. Although observed trait patterns might also be caused by differences in abiotic (soil) conditions, considering that we used a space‐for‐time substitution to compare invaded and uninvaded communities, we expect this to be unlikely.…”

“…This peripheral location of both invader hypervolumes in the trait space suggests the importance of trait distinctiveness for invasion success following the empty niche hypothesis (Elton , Moles et al , MacDougall et al ). Trait differentiation between invader and community was more pronounced for the leaf economics traits, and seemingly absent for plant height, potentially suggesting that niche differences are more important than fitness differences in driving the patterns (MacDougall et al , Drenovsky et al , Fried et al ). However, we cannot fully exclude that fitness differences are also partly involved, since 1) trait differentiation was also found for the leaf area axis and 2) leaf economics traits can also relate to fitness differences (Leishman et al , van Kleunen et al , Kunstler et al ).…”

Invasive dominance and resident diversity: unpacking the impact of plant invasion on biodiversity and ecosystem function

“…Thomsen et al (2016) made a convincing case for increased use of this method in order to better differentiate the ways that invaders alter community structure (i.e., the diversity of the resident community vs. that of the resultant community). Studies that have taken this approach have been able to characterize both plant community response to invasion and the positioning of an invader in the functional structure of invaded communities (Thomsen et al 2016, Loiola et al 2018, Parra‐Tabla et al 2018, Sodhi et al 2019, Fried et al 2019). Generally, the results of these studies vary, showing how resident plant communities can either experience an increase in functional diversity as a result of invasion (Hejda and de Bello 2013, Loiola et al 2018) or a decrease (Thomsen et al 2016).…”

“…Generally, the results of these studies vary, showing how resident plant communities can either experience an increase in functional diversity as a result of invasion (Hejda and de Bello 2013, Loiola et al 2018) or a decrease (Thomsen et al 2016). Yet, the approach consistently sheds light on how invasive plants tend to act as a biotic filter, disassembling native communities (Parra‐Tabla et al 2018, Fried et al 2019, Sodhi et al 2019). However, synthetic interpretation is confounded by the lack of differentiation between naturalized and invasive alien species in some of these studies (Loiola et al 2018, Parra‐Tabla et al 2018).…”

Functional Divergence Drives Invasibility of Plant Communities at the Edges of a Resource Availability Gradient

“…According to biotic resistance, native communities should repel less competitive invaders (for instance through reduced survival rates) or invaders sharing similar functional features; however, it has been reported that IAS overcome biotic resistance by competition-induced trait shifts [79]. In addition, fine-scale experimental studies [79,80] indicated that functionally distinct plant species might have a better chance to succeed in the invasion process being better competitors or avoiding direct competition and co-occurrence with closely related natives, occupying different spatio-temporal niches [17,81]. Perhaps, a great sharing of functional niches between the two subcommunities may hamper community invasibility (e.g., through competition), thus newly introduced species will likely become naturalized rather than invasive [77].…”