The impact of invasive plants on vegetation can vary greatly depending on the characteristics of the invaders and community invasibility. As to the factors that influence a plant's ability to invade, recent studies suggest that arbuscular mycorrhizal fungi (AMF) may be important regulators in plant invasions. To better understand the interactions of Helianthus tuberosus with co-occurring species in its native NorthAmerican and invaded European ranges,we carried out plot-based field surveys to determine whether the cover of H. tuberosus, its stem number and height, bare ground cover and amount of litter differ between ranges and how they interact with numbers of species in the plant community. To provide information aboutAMFcolonization of H. tuberosus,we evaluatedAMFcolonization in both ranges and tested the difference between continents, the effect of cover of H. tuberosus and their interaction with AMF. In the invaded range in Europe, H. tuberosus plants grew taller, had a greater stem density and there were fewer species in the invaded plant communities than in its native range in North America. In contrast, the cover of H. tuberosus and litter cover did not differ between the two continents. Plants of H. tuberosus were colonized by AMF in both ranges, but we found no statistical support for the potential effects of continent, the cover of H. tuberosus and their interaction with AMF. Overall, our study revealed that H. tuberosus exerts a negative impact on co-occurring species in the invaded European range, but not in North America where the species is native. To our knowledge, this is the first evaluation of AMF colonization of H. tuberosus at home and away and the results do not support either the degraded or enhanced mutualism hypotheses.

Weed species loss due to intensive agricultural land use has raised the need to understand how traditional cropland management has sustained a diverse weed flora. We evaluated to what extent cultivation practices and environmental conditions affect the weed species composition of a small-scale farmland mosaic in Central Transylvania (Romania). We recorded the abundance of weed species and 28 environmental, management and site context variables in 299 fields of maize, cereal and stubble. Using redundancy analysis, we revealed 22 variables with significant net effects, which explained 19.2% of the total variation in species composition. Cropland type had the most pronounced effect on weed composition with a clear distinction between cereal crops, cereal stubble and maize crops. Beyond these differences, the environmental context of croplands was a major driver of weed composition, with significant effects of geographic position, altitude, soil parameters (soil pH, texture, salt and humus content, CaCO3, P2O5, K2O, Na and Mg), as well as plot location (edge vs. core position) and surrounding habitat types (arable field, road margin, meadow, fallow, ditch). Performing a variation partitioning for the cropland types one by one, the environmental variables explained most of the variance compared with crop management. In contrast, when all sites were combined across different cropland types, the crop-specific factors were more important in explaining variance in weed community composition.