Germination characteristics and frost tolerance of seedlings are crucial parameters for establishment and invasion success of plants. The characterization of differences between populations in native and invasive ranges may improve our understanding of range expansion and adaptation. Here, we investigated germination characteristics of Ambrosia artemisiifolia L., a successful invader in Europe, under a temperature gradient between 5 and 25 °C. Besides rate and speed of germination we determined optimal, minimal and maximal temperature for germination of ten North American and 17 European populations that were sampled along major latitudinal and longitudinal gradients. We furthermore investigated the frost tolerance of seedlings. Germination rate was highest at 15 °C and germination speed was highest at 25 °C. Germination rate, germination speed, frost tolerance of seedlings, and the temperature niche width for germination were significantly higher and broader, respectively, for European populations. This was partly due to a higher seed mass of these populations. Germination traits lacked evidence for adaptation to climatic variables at the point of origin for both provenances. Instead, in the native range, seedling frost tolerance was positively correlated with the risk of frosts which supports the assumption of local adaptation. The increased frost tolerance of European populations may allow germination earlier in the year which may subsequently lead to higher biomass allocation—due to a longer growing period—and result in higher pollen and seed production. The increase in germination rates, germination speed and seedling frost tolerance might result in a higher fitness of the European populations which may facilitate further successful invasion and enhance the existing public health problems associated with this species.Electronic supplementary materialThe online version of this article (doi:10.1007/s00442-013-2813-6) contains supplementary material, which is available to authorized users.

The knowledge of phenotypic variation in the European range of the highly allergenic Ambrosia artemisiifolia L. (common ragweed) is not entirely complete, even though it is an invasive species of utmost concern. We hypothesized the prevalence of phenotypic differentiations between common ragweed populations in the introduced range, and we assumed that those differentiations were related to environmental conditions at the points of origin. Using a common garden experiment, we investigated biomass allocation, growth rates, and flowering phenology of 38 European common ragweed populations originating from a major geographical gradient. We observed considerable phenotypic variation in growth parameters and flowering phenology, e.g. mean aboveground biomass varied from 23.3 to 47.3 g between the populations. We were able to relate most measured traits with environmental parameters prevailing at the points of origin. For example, early growth of ruderal populations was highly correlated with temperature and precipitation at the point of origin. Late growth and flowering phenology were highly correlated with latitude, i.e. individuals from northern populations grew smaller and flowered and dispersed their pollen and seeds up to 5 weeks earlier than individuals from southern populations. We also found a longitudinal gradient in flowering phenology which has not yet been described. The existence of such a high variability in the introduced range may facilitate further range expansion. We suggest that the correlation with environmental variables rests upon genetic variation possibly due to adaptations to the respective environment. To clarify if such adaptation results from multiple events of introduction or as evolutionary response after introduction, genetic investigations are needed.

Aim Common ragweed (Ambrosia artemisiifolia L.) is a medically relevant invasive species of great public interest due to its highly allergenic pollen. We aimed at modelling its potential range, its net primary production (NPP) and important phenological stages.Location Europe and North America. MethodsWe developed a new physiological model for common ragweed and applied it to simulate the species 0 potential distribution (calibrated with the native range), NPP and phenology in North America and Europe. Based on this model, we investigated which regions are suitable for ragweed growth in Europe and simulated the timing of phenological stages that determine pollen release. ResultsThe model predicted the observed distribution of ragweed in North America well. The application to Europe suggests that large parts of Europe are climatically suitable for ragweed growth and reproduction. The highest potential NPP was predicted in southern-central and south-eastern Europe and southern France, roughly corresponding with hotspots of atmospheric pollen load, but also indicating a higher potential than currently achieved in western Europe and along parts of the northern edge of its distribution. The predicted time of pollen releases in Europe corresponded well with measurements from pollen traps. Main conclusionsThe results suggest that our mechanistic model adequately represents physiological and ecological characteristics that determine the potential distribution, productivity and phenology of common ragweed. The model could be used for predicting the potential distribution and performance of ragweed in the future under climate change and might thus contribute to improved longer term predictions of exposure to allergenic pollen.

No abstract