Madalin Parepa

Successful large‐scale biological invasions require introduced species to either rapidly adapt to a broad range of—partly novel—environmental conditions, or to possess a high level of phenotypic plasticity. Replicated common garden experiments across the introduced range provide a powerful framework to investigate these complementary mechanisms. To better understand population differentiation, adaptation and plasticity of invasive Japanese knotweed ( Reynoutria japonica ) in Europe, we compared the performance of knotweed plants from 46 European populations, collected across a 2000 km latitudinal transect, in three common gardens with contrasting climatic conditions, one at the southern edge, one in the centre and one at the northern edge of the species' European distribution. The plants exhibited strong phenotypic plasticity across the three gardens, with a more acquisitive growth strategy in the southern garden, and a more conservative strategy and change of architecture in the climatically unfavourable north. Although we observed phenotypic selection on several leaf traits, with some differences in selection between the gardens, we found little evidence for population differentiation or local adaptation, that is variation in plant performance was not negatively related to climate or geographic distance. Greater plasticity in leaf thickness across gardens was linked to the production of fewer but larger shoots, indicating a trade‐off between growth and clonal expansion. In addition, populations from higher latitudes and regions with greater inter‐annual temperature variability displayed increased plasticity in shoot volume and shrubbiness, but reduced plasticity in shoot number. Synthesis . Our results suggest that local adaptation has not played a key role in the success of Japanese knotweed in Europe. Instead, its high overall phenotypic plasticity (‘general purpose genotype’), as well as evolutionary fine‐tuning of plasticity, may have contributed to the species' invasion success across a broad range of environments.

Stomata and leaf traits are key regulators of plant water use efficiency and are expected to have changed in response to rising atmospheric CO ₂ concentrations and climate warming over the past centuries. However, long-term data documenting such changes are rare. We leveraged herbarium collections to track changes in stomatal characteristics and leaf traits in 656 individuals of invasive Japanese knotweed and its hybrid Bohemian knotweed collected across their European range and spanning 160 years of invasive spread. We found that several functional traits including stomatal density and maximum anatomical stomatal conductance did not show significant changes over time but that plants adjusted their stomatal size and shape over time, and that these changes were associated with increased atmospheric CO ₂ levels. Interestingly, Reynoutria japonica showed increases in stomatal size and stomatal elongation, while the hybrid R. × bohemica showed a reduction in stomatal size. Traits also varied systematically with climates of origin. Plants from warmer origins with higher evaporative demands during the growing season had thicker leaves, lower SLA, smaller stomata and higher stomatal density, indicating more conservative water-use strategies. Stomatal density and gas exchange capacity co-varied with leaf structural traits, and there was a trade-off between stomatal size and number. Overall, “fast” leaf economic traits were associated with “slow” physiological traits. Our results suggest that stomatal anatomical plasticity may enhance climate resilience by maintaining a stable maximum gas exchange capacity across environmental gradients. Herbarium collections provide a unique resource for reconstructing plant responses to historical environmental changes and understanding intraspecific trait variation.

The ecological and evolutionary processes determining species range limits remain poorly understood. Ultimately, range limits depend on the species’ abilities to persist under heterogeneous conditions, by adaptive differentiation and phenotypic plasticity, including transgenerational effects. To investigate ecological differentiation and transgenerational effects in the clonal invasive knotweed, Reynoutria japonica , in Europe, we conducted a two-phase transplant experiment: plants sampled along the entire latitudinal gradient were planted in three sites located at the northern range margin, mid-range and near the southern range margin, and then re-transplanted among all three sites after two years. Biomass production and allocation were generally not associated with latitude of origin and previous growth at the same site did not promote performance. We therefore find no evidence that adaptive differentiation or transgenerational effects contribute to the wide distribution of R. japonica in Europe. However, at the northern site, with a 25% shorter season, knotweed plants invested much less biomass below-ground, and the pattern was further strengthened in plants that had grown in the northern site in the previous generation. Overwintering below-ground rhizomes are essential for survival and spread. We further explored limiting climate conditions in a species distribution model for the European range and found that mean annual temperature and temperature annual range are the main predictors of the European distribution of R. japonica . Taken together, our study suggests that low temperatures and associated short seasons may pose a limit to the broad environmental tolerance of R. japonica and restrict its northward spread by reducing below-ground biomass accumulation.

A fluctuating resource supply makes some of the most problematic invasive plants more successful against native plants. However, the evidence so far comes from simplified pot experiments in the greenhouse or garden, and it remains an open question whether these mechanisms are also relevant in natural conditions. Here, we present an experiment that tested the effects of nutrient fluctuations on plant invasion in the field. In eight sites in South-West Germany invaded by Asian knotweed (Reynoutria x bohemica) or Canadian goldenrod (Solidago canadensis), we manipulated the temporal patterns of nutrient availability by applying liquid fertilizer at weekly intervals over a 10-week period. There were three treatments: (1) constant nutrient supply, with equal amounts at every application, (2) variable nutrients supply, with double amount of nutrient as in the constant treatment but added at every second application, totaling the same total amount for the entire experiment, and (3) control with water only. We found that the invaders did not benefit from the additional nutrients when they were supplied in a constant manner but became more dominant after several months of variable nutrient supply. To our knowledge, this is the first field evidence that resource fluctuations can promote invasive plants in natural communities and shows that fluctuating resources can alter invasion success even within a single growing season. The key questions are now how general these effects are across other invasive plants and possibly also strong native ruderals, and what the longer-term effects of the fluctuating resources are on community composition and diversity.

Biological invasions expose populations of introduced species to strong selection forces in abiotic and biotic factors, which is expected to lead to adaptive differentiation of populations, particularly for large-scale invaders. To better understand population differentiation and phenotypic plasticity of invasive Japanese knotweed ( Reynoutria japonica ) in Europe, we compared the performance of 45 European knotweed populations, collected across a 2000 km latitudinal transect, in three common gardens with contrasting climatic conditions, one at the southern edge, one in the center, and one at the northern edge of the species’ European distribution. The plants exhibited strong phenotypic plasticity across the three gardens, with a more acquisitive growth strategy in the southern garden, and a more conservative strategy and change of architecture in the climatically unfavorable north. Although we observed phenotypic selection on several leaf traits, and on the plasticity of leaf thickness, with some selection differences between the gardens, there was little evidence of population differentiation, and none for local adaptation. Only for plant architecture (shrubbiness), we detected heritable population variation in trait means and plasticity. However, variation in plasticity was unrelated to climatic variability of origin, and to plant fitness. Synthesis . Our results suggest that evolutionary processes - local adaptation and evolution of plasticity - did not play a key role in the success of Japanese knotweed in Europe. Instead, its high baseline plasticity appears to be the key factor that makes it such a strong invader across a broad range of environments.

Population trends derived from systematic monitoring programmes are essential to identify species of conservation concern and to evaluate conservation measures. However, monitoring data pose several challenges for statistical analysis, including spatial bias due to an unbalanced sampling of natural regions or habitats, variation in observer expertise, frequent observer changes, and overdispersion or zero-inflation in the raw data. An additional challenge arises from so-called ‘rolling’ survey designs, where each site is only visited once within each multi-year rotation cycle. We developed a GAMM-based workflow that addresses these challenges and exemplify its application with the highly structured data from the Ecological Area Sampling (EAS) in the German federal state North Rhine-Westphalia (NRW). First, we derive a routine that allows informed decisions about the most appropriate combination of distribution family (Poisson or negative binomial), model covariates (e.g., habitat characteristics), and zero-inflation formulations to reflect species-specific data distributions. Second, we develop a correction factor that buffers population trend estimates for variation in observer expertise as reflected in variation in total bird abundance. Third, we integrate region-specific trends that adjust for between-year variation in the representation of habitat or natural regions within the yearly subset of sampled sites. In a consistency check, we found good match between our GAMM-based EAS trends and TRIM-based trends from the German Common Bird Monitoring scheme. The study provides a template script for R statistical software so the workflow can be adapted to other monitoring programmes with comparable survey designs and data structures.

While successful plant invasions often occur in novel environments, invasive species usually occupy broad niches within their native and introduced ranges. A better understanding of the process of invasion therefore requires a wide sampling of ranges, and a good knowledge of introduction history. We tested for differentiation in herbivore resistance among 128 introduced (European, North American) and native (Chinese, Japanese) populations of the invasive Japanese knotweed Reynoutria japonica in two common gardens in the native range: one in Shanghai and the other in Yunnan. In both common gardens, we found that herbivore resistance of plants from introduced populations differed from that from native populations in China but not from native populations in Japan, the putative source of introduction. Compared to native Chinese populations, plants from native Japanese populations and introduced European and North American populations had thicker leaves in both common gardens, and a lower C:N ratio but higher flavonoids content in the Shanghai garden. Variation in herbivore resistance was more strongly associated with climate of collecting sites for populations from the native range than for those from introduced ranges. Our results support the hypothesis that introduction of particularly resistant plants from Japan may have played a key role in driving biogeographic variation in herbivore resistance. Our study highlights the importance of understanding introduction history to interpret the biogeographic divergence of global plant invaders.

Summary: Many widespread invasive plant species express high phenotypic variation across novel environments, providing a unique opportunity to examine ecological and evolutionary dynamics under global change. However, studies often lack information about the origin of introduced populations, limiting our understanding of post‐introduction evolution. We assessed the responses of Reynoutria japonica from 128 populations spanning latitudinal transects in the native (China and Japan), and introduced (North America and Europe) ranges when grown in two common gardens. Plants from introduced populations differed in almost all traits from those from Chinese populations, but were similar to plants from the putative origin in Japan. Compared to Chinese populations, North American, European and Japanese populations expressed lower trait values and plasticity in most traits. However, plants from both introduced and Japanese populations expressed higher clonality and plasticity in clonality than plants from Chinese populations. Further, introduced populations expressed higher plasticity in clonality but lower plasticity in basal diameter compared to Japanese populations. Our findings emphasize the potential role of clonality and plasticity in clonality for invasion success. In addition, our study highlights the importance of comparisons to source populations within the native range to identify evolutionary responses of introduced plants to novel environments.

Aim: Plant invasions are a global problem that requires studying plants and their environmental associations across native and introduced ranges.Location: 2000 km transects in China, Europe and North America.Time Period: June 2019–July 2020.Major Taxa Studied: Japanese knotweed (Reynoutria japonica).Methods: We surveyed 150 populations of Japanese knotweed, a noxious invader of the temperate zone, along 2000 km transects in native China and the introduced ranges of Europe and North America.Results: We found that larger plants and denser populations in the introduced ranges were associated with shifts in leaf economy and chemical defences. Introduced knotweed populations had higher SLA but reduced leaf chlorophyll, lignin, C:N ratio and leaf toughness along with altered leaf tannins, flavonoids and alkaloids. We found three distinct multivariate knotweed phenotypes primarily in the introduced ranges, and two multivariate knotweed phenotypes mainly in native populations.Main Conclusions: Decreased herbivore and pathogen impacts in introduced populations and changes in environmental associations indicate that enemy release and novel habitat conditions might have driven the emergence of novel ecological strategies in this global plant invader.

Biological invasions pose a global challenge, affecting ecosystems worldwide and human societies. Knowledge of the evolutionary history of invasive species is critical to understanding their current invasion success and projecting their future spread. However, to date, few studies have addressed the evolutionary history and potential future spread of invaders simultaneously. In this study, we explored both evolutionary history and spatiotemporal dynamic patterns of the distribution of Reynoutria japonica, known as one of the world's worst plant invaders. We analysed 265 R. japonica samples from its current geographical ranges across three continents, using seven chloroplast DNA (cpDNA) markers to establish the phylogenetic relationships among extant populations. We combined these with ecological niche modelling to infer historical and more recent migration patterns and predict potential future distribution changes under climate change. Our results indicate that climate fluctuations and sea level changes likely facilitated the expansion of R. japonica from southern Japan to continental East Asia in the Pliocene, followed by a contraction in East Asian populations. In the recent Holocene, human activities have then enabled a linage of this species to spread from Japan to Europe and North America, resulting in three major global clades. Future climate scenarios suggest a northward expansion of R. japonica in Europe and North America, but shrinking habitat in China. Our study, thus, demonstrates the complex influences of historical climate‐driven migrations, human activities and future climate changes on the global distribution of an invasive species.