Oliver Bossdorf

The ecological and evolutionary processes determining species distributions and 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 its introduced range in Europe, we analyzed biomass production in a two-phase transplant experiment: Plants sampled along a latitudinal gradient were planted in three sites located throughout the range, and then re-transplanted among all three sites after two years. Biomass production and allocation in the second transplant phase were generally not associated with latitude of origin, and previous growth at the same site did not promote performance. We therefore found no evidence for adaptive differentiation or adaptive transgenerational effects. However, at the northern site, with a 25% shorter season, knotweed plants invested much less biomass below-ground, even though overwintering below-ground rhizomes are essential for the species’ survival and spread. The pattern was further strengthened in plants that had previously grown in the northern site. 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 growing seasons may pose a limit to the broad environmental tolerance of R. japonica and restrict its northward spread by reducing below-ground biomass accumulation.

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.

Invasive plants are dynamic eco-evolutionary systems characterized by rapid spread and evolutionary change. Herbarium genomics offers a powerful way to study these processes across broad spatial and temporal scales. We generated low-coverage shotgun-sequencing data from 152 herbarium specimens of the invasive Japanese knotweed species complex ( Reynoutria ), collected across its native range in Japan and China, and its introduced ranges in Europe and North America, spanning 200 years of global spread. Introduced populations of R. japonica , R. japonica var. compacta and R. sachalinensis showed reduced genetic diversity compared to native populations and were genetically most similar to Japanese specimens, indicating Japan as the source of all three introductions. European and North American R. japonica were genetically highly similar and closely related to local R. × bohemica hybrids, suggesting that these hybrids originated post-introduction, through hybridization and subsequent introgression from the R. japonica parent. A few R. × bohemica hybrids in the UK and France shared a multilocus lineage with R. sachalinensis , indicating an independent but ecologically less successful hybridization event involving introgression from R. sachalinensis . A genetically distinct R. × bohemica from Japan was closely related to a Japanese R. sachalinensis specimen, suggesting that hybridization also occurs in the native range. Introduced R . japonica has remained genetically uniform for ∼200 years reflecting long-term founder effects. The dominance of a single R. japonica lineage supports the ‘general-purpose genotype’ hypothesis and the importance of vegetative reproduction in its spread. Herbarium genomics thus uncovers the origin and global spread of Japanese knotweed, providing direct molecular evidence of long-term plant invasions.

Climate change forces species to adapt rapidly to avoid extinction. To directly observe rapid adaptation and extinction, we conducted synchronized evolution experiments with Arabidopsis thaliana in 30 locations across Western Europe, the Mediterranean, the Levant, and North America. Whole-genome pooled sequencing of ~70,000 surviving plants revealed repeatable allele frequency shifts in similar climates but divergent shifts across contrasting ones, indicating evolutionary adaptation. We identified genetic variants linked to climate adaptation, including genes involved in processes ranging from thermal-stress sensing to spring-flowering timing. Evolutionary trends were often predictable, but variable, across environments. In warmer climates, evolutionary predictability correlated with population survival over 5 years, whereas erratic changes preceded extinction. These results show that rapid climate adaptation is possible, but understanding its limits will be crucial for biodiversity forecasting.

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.

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.

Seed harvesting from wild plant populations is key for ecological restoration, but it may threaten the persistence of the source populations. Consequently, several countries have set guidelines limiting the proportions of harvestable seeds. However, these guidelines are inconsistent and lack a solid empirical basis. We used matrix population models based on 280 wild plant species, stored in he COMPADRE Plant Matrix Database, to model the demographic consequences of seed harvesting. Current guidelines do not protect populations of annuals and short‐lived perennials because maximal allowed harvest drew all annual species included in our study to extinction. In contrast, current guidelines are overly restrictive for long‐lived plants because these plants could tolerate even higher seed removal than currently allowed. The maximum possible fraction of seed production that can be harvested without compromising the long‐term persistence of populations was strongly related to generation time of the target species. When harvesting seeds every year, the fraction of seeds that was safe to harvest (safe seed fraction) ranged from 80% in long‐lived species to 2% in most annuals. Less frequent seed harvesting substantially increased the safe seed fraction. In the most vulnerable annual species, it was safe to harvest 5%, 10%, or 30% of a population's seed production when harvesting every 2, 5, or 10 years, respectively. Our results provide a quantitative basis for seed harvesting legislation, based on the generation times of species and harvesting regime.

This article explores the close relationship between dissolved inorganic carbonate ion concentration (DIC) and the calcification state of Phacotus lenticularis , a globally abundant freshwater phytoplankton that is responsible for a significant part of modern lake carbonate production during bloom formation. We cultured 12 freshly isolated P. lenticularis strains under an ecologically relevant range of DIC (0.2 to 12 mmol l ^-1 total scale) for 14 days. From this experiment we gained information on responses in shell formation and growth that highlight strong lower boundaries in morphometric calcite shell variables with regards to DIC. All P. lenticularis strains showed reduced shell thickness by up to 60 % and dissolved calcite crystals structures at declining DIC < 4 mmol l ^-1 . Increasing DIC > 4 mmol l ^-1 had no significant effect on shell thickness and crystal length in the culture experiments. We found a significant preadaptation of all 12 strains to ambient DIC concentrations measured in their lake of origin, but no dependence of growth rates up to a lethal DIC of > 10 mmol l ^-1 . The simulation experiments illustrate the close relationship between shell function and dissolved inorganic carbonate ion concentration in lakes and highlight the need of continued research of important roles in biogenic carbon transformation and storage in a future world.

In natural populations, plants are associated with a huge diversity of bacteria, fungi and other microbes. There is usually substantial microbiome variation between different plant individuals and populations, and the drivers of this variation are still poorly understood, particularly in wild plants. Here, we were interested in the potential of plant genotype and plant age to explain intraspecific variation in the plant microbiome of Lotus corniculatus . In seven natural populations, we genotyped a total of 168 individuals over four years, determined their ages through growth ring analysis, and then sequenced their root, shoot, flower and seed microbiomes. We found that plant genotypes differed both in the diversity and composition of microbes, and that some microbial taxa were associated with particular plant genotypes. The genotype effects tended to be strongest and most consistent for plant-associated bacteria, with the largest plant genotype differences in the microbiome diversity of flowers and seeds. We found less evidence for an effect of plant age on microbiome diversity: the age of plants explained variation in fungi diversity, and it was associated with the abundance of several microbial taxa. Our study indicates that the genotype of a plant and - to a lesser degree its age - influences the diversity and composition of plant-associated microbiota, even in complex natural environments. Importance: The plant microbiome plays a key role in important plant functions such as pathogen resistance, nutrient uptake, and stress tolerance. To fully understand these processes, it is essential to identify the factors that drive microbiome variation. Most research to date has focused on model or crop species under controlled conditions, leaving open questions about the drivers of microbiome diversity and composition in natural populations. In this multi-year field study, we examined how plant genotype and age shape the microbiomes of Lotus corniculatus . We show that the plant genotype strongly influences microbial diversity and composition, while plant age has subtler but still important effects, particularly on fungal communities. To our knowledge, this is the first study to link plant age with microbiome variation in natural populations. These findings demonstrate that plant traits can shape the microbiome even in complex natural environments.