Climate drives differences in the germination niche of a globally distributed invasive grass

Aims Exotic invasive species are often exposed to strong selection pressures in their new ranges that can often lead to substantial intraspecific variation.Population differentiation in the timing of life history events in response to climate gradients is thought to be an important mechanism facilitating the range expansion of many invasive species.For seed producing plants,the timing of seed germination determines the first environmental conditions experienced by newly emerged germinates,and can have important implications for the successful colonization,establishment and spread of invasive plants—though the role of germination in the success of invasive plants remains poorly understood.Methods We assessed the variation in seed germination dynamics among 10 populations of the invasive plant Johnsongrass(Sorghum halepense)across its North American distribution,capturing both a temperature and precipitation gradient,and whether that variation is associated with home climate.Seeds were exposed to a wide range of temperatures(11–48℃)and two water availability treatments.Important Findings We found that Johnsongrass seeds germinated across a wide range of temperatures,but there was substantial variation among populations in the proportion of seeds that germinated in response to both temperature and water availability.Evidence indicates that as Johnsongrass expanded its range from warmer climates into cooler climates,there was a concurrent shift in the germination temperature niche to cooler temperatures.Our results suggest that the germination of Johnsongrass seeds has adapted to home climate allowing this invader to maximize germination throughout its range,and that this may be an important contributing factor to its invasion into new environments.

Invasive plant rhizome production and competitiveness vary based on neighbor identity

Aims Within-species genetic and phenotypic variation have well-known effects on evolutionary processes,but less is known about how within-species variation may influence community-level processes.Ecologically meaningful intraspecific variation might be particularly important in the context of anthropogenic impacts on natural systems,such as agriculture and species invasion,because human actions can cause strong selection pressures.Methods In a greenhouse study,we explored intraspecific(30 accessions)and ecotypic variation(representing agricultural and nonagricultural habitats)in biomass and rhizome production in response to inter-and intraspecific competition and soil fertility of Johnsongrass(Sorghum halepense),a widespread invasive species and agricultural weed.Important Findings Contrary to our expectations and previous results,we did not find variation in biomass production among Johnsongrass ecotypes at this early life stage.However,we did find that Johnsongrass biomass varied substantially depending on competitor identity,soil fertility treatments and among accessions.Rhizomes were 11%larger in the agricultural ecotype and up to 3-fold larger in fertilized treatment;while rhizome biomass increased by~50%when fertilized,but did not differ among ecotypes.Interestingly,in competition,Johnsongrass produced 32%less biomass and 20%less rhizome mass with a conspecific than when competing interspecifically with corn.Our results indicate species-specific competitive responses and changes in rhizome allocation in response to neighbor identity;suggesting the possibility of adaptation by Johnsongrass to shift allocation under competition.

Evidence for fine-scale habitat specialisation in an invasive weed

Aims As an exotic species colonises a new continent,it must overcome enormous environmental variation in its introduced range.Local adaptation of introduced species has frequently been observed at the continent scale,particularly in response to latitudinal climatic variation.However,significant environmental heterogeneity can also exist at the landscape scale.A small number of studies have provided evidence that introduced species may also be capable of phenotypic and genetic differentiation at much smaller spatial scales.For example,previously we found US agricultural and non-agricultural populations of Sorghum halepense(Johnsongrass)to be phenotypically and genetically distinct.in this study,we inves-tigated whether this phenotypic differentiation of agricultural and non-agricultural populations of S.halepense is the result of fine-scale local specialisation.Methods We surveyed a nationally collected S.halepense germplasm panel and also collected neighbouring agricultural and non-agricultural sub-populations of S.halepense at four sites throughout Western Virginia,USA,raising seedlings in common conditions mimicking both agricultural and non-agricultural habitats.Important Findings At the national scale,we found evidence of habitat differentiation but not specialisation.However,at the local scale,we found evi-dence of specialisation in two of the four local populations to non-agricultural habitat,but no evidence of specialisation to agricultural habitat.These results show that local specialisation is a possible,but not guaranteed consequence of kilometre-scale habitat heterogen-eity in invasive species.This finding contributes to a growing aware-ness of the importance of fine-scale local adaptation in the ecology and management of introduced and weedy species.