Rapid evolution of latitudinal clines in growth and defence of an invasive weed

Exploring the complex pre-adaptations of invasive plants to anthropogenic disturbance: a call for integration of archaeobotanical approaches

Pre-adaptation to anthropogenic disturbance is broadly considered key for plant invasion success. Nevertheless, empirical evidence remains scarce and fragmentary, given the multifaceted nature of anthropogenic disturbance itself and the complexity of other evolutionary forces shaping the (epi)-genomes of recent native and invasive plant populations. Here, we review and critically revisit the existing theory and empirical evidence in the field of evolutionary ecology and highlight novel integrative research avenues that work at the interface with archaeology to solve open questions. The approaches suggested so far focus on contemporary plant populations, although their genomes have rapidly changed since their initial introduction in response to numerous selective and stochastic forces. We elaborate that a role of pre-adaptation to anthropogenic disturbance in plant invasion success should thus additionally be validated based on the analyses of archaeobotanical remains. Such materials, in the light of detailed knowledge on past human societies could highlight fine-scale differences in the type and timing of past disturbances. We propose a combination of archaeobotanical, ancient DNA and morphometric analyses of plant macro- and microremains to assess past community composition, and species' functional traits to unravel the timing of adaptation processes, their drivers and their long-term consequences for invasive species. Although such methodologies have proven to be feasible for numerous crop plants, they have not been yet applied to wild invasive species, which opens a wide array of insights into their evolution.

Population genetic differentiation and phenotypic plasticity of Ambrosia artemisiifolia under different nitrogen levels

Rapid adaptive evolution and phenotypic plasticity are two mechanisms that often underlie invasiveness of alien plant species, but whether they can co-occur within invasive plant populations under altered environmental conditions such as nitrogen (N) enrichment has seldom been explored. Latitudinal clines in plant trait responses to variation in environmental factors may provide evidence of local adaptation. Here, we inferred the relative contributions of phenotypic plasticity and local adaptation to the performance of the invasive plant Ambrosia artemisiifolia under different soil N levels, using a common garden approach. We grew A. artemisiifolia individuals raised from seeds that were sampled from six invasive populations along a wide latitudinal cline in China (23°42' N to 45°43' N) under three N (0, 5, and 10 g N m-2 ) levels in a common garden. Results show significant interpopulation genetic differentiation in plant height, number of branches, total biomass, and transpiration rate of the invader A. artemisiifolia across the N treatments. The populations also expressed genetic differentiation in basal diameter, growth rate, leaf area, seed width, root biomass, aboveground biomass, stomatal conductance, and intercellular CO2 concentration regardless of N treatments. Moreover, plants from different populations of the invader displayed plastic responses in time to first flower, hundred-grain weight, net photosynthetic rate, and relative biomass allocation to roots and shoots and seed length under different N treatments. Additionally, individuals of A. artemisiifolia from higher latitudes grew shorter and allocated less biomass to the roots regardless of N treatment, while latitudinal cline (or lack thereof) in other traits depended on the level of N in which the plants were grown. Overall, these results suggest that rapid adaptive evolution and phenotypic plasticity in the various traits that we quantified may jointly contribute to invasiveness of A. artemisiifolia under different levels of N availability. More broadly, the results support the idea that phenotypic plasticity and rapid adaptive evolution can jointly enable invasive plants to colonize a wide range of environmental conditions.

Directional and stabilizing selection shaped morphological, reproductive, and physiological traits of the invader Solidago canadensis

Trait evolution in invasive plant species is important because it can impact demographic parameters key to invasion success. Invasive plant species often show phenotypic clines along geographic and climatic gradients. However, the relative contributions of natural selection and neutral evolutionary processes to phenotypic trait variation among populations of invasive plants remain unclear. A common method to assess whether a trait has been shaped by natural selection or neutral evolutionary processes is to compare the geographical pattern for the trait of interest to the divergence in neutral genetic loci (i.e., Q ST -F ST comparisons). Subsequently, a redundancy analysis (RDA) can facilitate identification of putative agents of natural selection on the trait. Here, we employed both a Q ST -F ST comparisons approach and RDA to infer whether natural selection shaped traits of invasive populations of Solidago canadensis in China and identify the potential environmental drivers of natural selection. We addressed two questions: (1) Did natural selection drive phenotypic trait variation among S. canadensis populations? (2) Did climatic, latitudinal, longitudinal, and altitudinal gradients drive patterns of genetic variation among S. canadensis populations? We found significant directional selection for several morphological and reproductive traits (i.e., Q ST  > F ST) and stabilizing selection for physiological traits (i.e., Q ST  < F ST). The RDA showed that stem biomass of S. canadensis was strongly positively correlated with longitude, while leaf width ratio and specific leaf area were significantly positively correlated with the mean diurnal range. Stem biomass had a strong negative correlation with annual precipitation. Moreover, height of S. canadensis individuals was strongly positively correlated with altitude and precipitation of the wettest quarter. A longitudinal shift in precipitation seasonality likely selected for larger stem biomass in S. canadensis. Overall, these results suggest that longitudinal and altitudinal clines in climate exerted strong selection pressures that shaped the phenotypic traits of S. canadensis.

Aboveground herbivory can promote exotic plant invasion through intra- and interspecific aboveground-belowground interactions

Aboveground herbivores and soil biota profoundly affect plant invasions. However, how they interactively affect plant invasions through plant-soil feedbacks (PSFs) remains unclear. To explore how herbivory by the introduced beetle Agasicles hygrophila affects Alternanthera philoxeroides invasions in China, we integrated multiyear field surveys and a 2-yr PSF experiment, in which we examined how herbivory affects PSFs on the performance of native and invasive plants and the introduced beetles. Despite increased herbivory from A. hygrophila, A. philoxeroides dominance over co-occurring congeneric native Alternanthera sessilis remained constant from 2014 to 2019. While occurring at lower abundances, A. sessilis experienced similar herbivore damage, suggesting apparent competitive effects. Our experiments revealed that herbivory on A. philoxeroides altered soil microbial communities, prolonged its negative PSF on A. sessilis, and decreased A. hygrophila larvae performance on the next-generation invasive plants. Consequently, A. hygrophila larvae performed better on leaves of natives than those of invasives when grown in soils conditioned by invasive plants defoliated by the introduced beetles. Our findings suggest that aboveground herbivory might promote rather than suppress A. philoxeroides invasion by enhancing its soil-mediated self-reinforcement, providing a novel mechanistic understanding of plant invasions. These findings highlight the need to incorporate an aboveground-belowground perspective during the assessment of potential biocontrol agents.

Latitudinal trends in the structure, similarity and beta diversity of plant communities invaded by Alternanthera philoxeroides in heterogeneous habitats

Variations in latitudinal gradients could lead to changes in the performance and ecological effects of invasive plants and thus may affect the species composition, distribution and interspecific substitution of native plant communities. However, variations in structure, similarity and beta (β) diversity within invaded communities across latitudinal gradients in heterogeneous habitats remain unclear. In this study, we conducted a two-year field survey along 21°N to 37°N in China, to examine the differential effects of the amphibious invasive plant Alternanthera philoxeroides on native plant communities in terrestrial and aquatic habitats. We compared the differences in the invasion importance value (IV), species distribution, community similarity (Jaccard index and Sorenson index) and β diversity (Bray-Curtis index and β_sim index) between terrestrial and aquatic communities invaded by A. philoxeroides, as well as analyzed their latitudinal trends. We found that the IV of A. philoxeroides and β diversity in aquatic habitats were all significantly higher than that of terrestrial, while the terrestrial habitat had a higher community similarity values. The aquatic A. philoxeroides IV increased with increasing latitude, while the terrestrial IV had no significant latitudinal trend. With increasing latitude, the component proportion of cold- and drought-tolerant species in the terrestrial communities increased, and the dominant accompanying species in the aquatic communities gradually changed from hygrophytes and floating plants to emerged and submerged plants. In addition, the aquatic communities had lower community similarity values and higher β diversity in higher latitudinal regions, while terrestrial communities had the opposite parameters in these regions. Our study indicates that the bioresistance capacities of the native communities to invasive A. philoxeroides in heterogeneous habitats are different; A. philoxeroides invasion leads to higher community homogenization in terrestrial habitats than in aquatic habitats, and terrestrial communities experience more severe homogenization in higher latitudinal regions. These findings are crucial for predicting the dynamics of invasive plant communities under rapid global change.

Older populations of the invader Solidago canadensis exhibit stronger positive plant-soil feedbacks and competitive ability in China

PREMISE

The enemy release hypothesis predicts that release from natural enemies, including soil-borne pathogens, liberates invasive plants from a negative regulating force. Nevertheless, invasive plants may acquire novel enemies and mutualists in the introduced range, which may cause variable effects on invader growth. However, how soil microorganisms may influence competitive ability of invasive plants along invasion chronosequences has been little explored.

METHODS

Using the invasive plant Solidago canadensis, we tested whether longer residence times are associated with stronger negative plant-soil feedbacks and thus weaker competitive abilities at the individual level. We grew S. canadensis individuals from 36 populations with different residence times across southeastern China in competition versus no competition and in three different types of soils: (1) conspecific rhizospheric soils; (2) soils from uninvaded patches; and (3) sterilized soil. For our competitor treatments, we constructed synthetic communities of four native species (Bidens parviflora, Solanum nigrum, Kalimeris indica, and Mosla scabra), which naturally co-occur with Solidago canadensis in the field.

RESULTS

Solidago canadensis populations with longer residence times experienced stronger positive plant-soil feedbacks and had greater competitive responses (i.e., produced greater above-ground biomass and grew taller) in conspecific rhizospheric soils than in sterilized or uninvaded soils. Moreover, S. canadensis from older populations significantly suppressed above-ground biomass of the native communities in rhizospheric and uninvaded soils but not in sterilized soil.

CONCLUSIONS

The present results suggest that older populations of S. canadensis experience stronger positive plant-soil feedbacks, which may enhance their competitive ability against native plant communities.

The rapid evolution of an invasive plant due to increased selection pressures throughout its invasive history

Invasive plants are highly successful because they can quickly adapt to selection pressures imposed by both biotic and abiotic stressors. Since selection pressures may vary across temporal and biogeographical gradients, the growth and fitness of invasive plants varies across time. However, only a few studies have focused on the evolutionary potential of invasive plants following their establishment. In this study, the impacts of cadmium (Cd) on the germination and seedling growth of an invasive plant, Ageratina adenophora, were examined. The seeds were collected from different historical populations at the invasion stage (during the early, middle, and new stages of invasion). Plant performance was tested under both heavy metal and simulated herbivory treatments to examine the evolution of A. adenophora under different selection pressures. It was found that early stage A. adenophora populations have higher germinability and weaker seedling growth than the new stage populations. Compared with new stage populations, early-stage populations are more tolerant to simulated herbivory and their germination potential tends to be higher under high Cd stress. It seems that the adaptive strategy of A. adenophora is to invest more energy in growth during the initial stage of invasion. As selection pressures increases over time, more energy seems to be shifted to the improvement of seed quality as well as to the vegetative growth system which improves its ability to tolerate stressful environments. It is important to consider the invasion history of a species when studying the invasive and evolutionary potential of plant species.

Plant invasion alters latitudinal pattern of plant-defense syndromes

The relationship between herbivory and latitude may differ between native and introduced populations of invasive plants, which can generate latitudinal heterogeneity in the strength of enemy release. However, still little is known about how latitudinal heterogeneity in herbivore pressure influences latitudinal variation in defense phenotypes of invasive plants. We tested how latitudinal patterns in multi-variate defense syndromes differed between native (Argentinian) and introduced (Chinese) populations of the invasive herb Alternanthera philoxeroides. In addition, to better understand the drivers underlying latitudinal patterns, we also tested whether associations of defense syndromes with climate and herbivory differed between native and introduced ranges. We found that native plant populations clustered into three main defense syndromes associated with latitude. In contrast, we only found two defense syndromes in the introduced range. One matched the high-latitude syndrome from the native range, but was distributed at both the northern and southern range limits in the introduced range. The other was unique to the introduced range and occurred at mid-latitudes. Climatic conditions were associated with variation in syndromes in the native range, and climatic conditions and herbivory were associated with variation in syndromes in the introduced range. Together, our results demonstrate that plants may under the new environmental conditions in the introduced range show latitudinal patterns of defense syndromes that are different from those in their native range. This emphasizes that geographical dependence of population differentiation should be explicitly considered in studies on the evolution of defense in invasive plants.