Revealing Historic Invasion Patterns and Potential Invasion Sites for Two Non-Native Plant Species

The historical spatio-temporal distribution of invasive species is rarely documented, hampering efforts to understand invasion dynamics, especially at regional scales. Reconstructing historical invasions through use of herbarium records combined with spatial trend analysis and modeling can elucidate spreading patterns and identify susceptible habitats before invasion occurs. Two perennial species were chosen to contrast historic and potential phytogeographies: Japanese knotweed (Polygonum cuspidatum), introduced intentionally across the US; and mugwort (Artemisia vulgaris), introduced largely accidentally to coastal areas. Spatial analysis revealed that early in the invasion, both species have a stochastic distribution across the contiguous US, but east of the 90th meridian, which approximates the Mississippi River, quickly spread to adjacent counties in subsequent decades. In contrast, in locations west of the 90th meridian, many populations never spread outside the founding county, probably a result of encountering unfavorable environmental conditions. Regression analysis using variables categorized as environmental or anthropogenic accounted for 24% (Japanese knotweed) and 30% (mugwort) of the variation in the current distribution of each species. Results show very few counties with high habitat suitability (≥80%) remain un-invaded (5 for Japanese knotweed and 6 for mugwort), suggesting these perennials are reaching the limits of large-scale expansion. Despite differences in initial introduction loci and pathways, Japanese knotweed and mugwort demonstrate similar historic patterns of spread and show declining rates of regional expansion. Invasion mitigation efforts should be concentrated on areas identified as highly susceptible that border invaded regions, as both species demonstrate secondary expansion from introduction loci

The response of plant community diversity to alien invasion: evidence from a sand dune time series

This study examines the process of invasion of coastal dunes in north-eastern Italy along a 60-year time series considering alien attributes (origin, residence time, invasive status, and growth form strategy) and habitat properties (species richness, diversity and evenness, proportion of aliens, and proportion of focal species). Vegetation changes through time were investigated in four sandy coastal habitats, using a fine-scale diachronic approach that compared vegetation data collected by use of the same procedure, in four time periods, from the 1950s to 2011. Our analysis revealed an overall significant decline of species richness over the last six decades. Further, both the average number of species per plot and the mean focal species proportion were proved to be negatively affected by the increasing proportion of alien species at plot level. The severity of the impact, however, was found to be determined by a combination of species attributes, habitat properties, and human disturbance suggesting that alien species should be referred to as ‘‘passengers’’ and not as ‘‘drivers’’ of ecosystem change. Passenger alien species are those which take advantage of disturbances or other changes to which they are adapted but that lead to a decline in native biodiversity. Their spread is facilitated by widespread anthropogenic environmental alterations, which create new, suitable habitats, and ensure human-assisted dispersal, reducing the distinctiveness of plant communities and inducing a process of biotic homogenization

Does temperature limit the invasion of Impatiens glandulifera and Heracleum mantegazzianum in the UK?

1. Impatiens glandulifera Royle and Heracleum mantegazzianum Sommier et Levier are widespread, non-indigenous plant species in the UK. A variety of correlational analyses suggest that their spatial extent is limited by climate, although no experimental studies have tested this hypothesis. This paper reports the first detailed experimental examination of the impact of climate on the performance of the two species. 2. Seeds of each species were sown, in each of 2 years, in replicated plots along an elevational gradient (10-600 m a.s.l.) in north-east England. Both species germinated readily at all elevations, even in areas well above their current limits within the study area. The plants were, however, smaller at higher altitudes. Impatiens glandulifera also produced fewer seeds with increasing elevation. 3. Plant performance was assessed in relation to actual and interpolated climate data along the elevational transect. For H. mantegazzianum , the timing of germination was correlated most strongly with the pre-emergence heat sum; for I. glandulifera this relationship was significant in one year only. Maximum height of both species was correlated with increasing post-emergence heat sum, as was pod production by I. glandulifera . The biomass of second-year H. mantegazzianum plants varied non-linearly with post-emergence heat sum. For both species, overwinter survival of seeds was not related to winter temperature or frost days. Overwinter survival of first-year H. mantegazzianum plants declined with increasing frost incidence. 4. The results suggest that, of the two species, only I. glandulifera is currently most limited by temperature, although this is not the only factor determining the distribution of the species

Desempenho germinativo da invasora Leucaena leucocephala (Lam.) de Wit. e comparação com Caesalpinia ferrea Mart. ex Tul. e Caesalpinia pulcherrima (L.) Sw. (Fabaceae) Germination performance of the invader Leucaena leucocephala (Lam.) de Wit. compared to Caesalpinia ferrea Mart. ex Tul. and C. pulcherrima (L.) Sw. (Fabaceae)

O sucesso das plantas na colonização de novas áreas está fortemente relacionado ao comportamento germinativo das sementes nas condições ambientais locais. O objetivo deste trabalho foi comparar o desempenho germinativo da exótica invasora Leucaena leucocephala ao das leguminosas (Caesalpinia ferrea, nativa e C. pulcherrima, exótica) não consideradas invasoras. Sementes não escarificadas foram expostas às temperaturas de 15, 20, 25, 30, 35 e 40 °C, sob um fotoperíodo de 12 horas, durante 100 dias. Tanto a porcentagem final quanto a velocidade de germinação apresentaram diferenças significativas entre as três espécies. C. ferrea teve baixo desempenho, com temperatura ótima para a germinação a 25°C, porcentagem de germinação de 59% e IVG de 0,68 sementes d-1. Já C. pulcherrima, apresentou altas porcentagens de germinação em todas as temperaturas testadas, com faixa ótima entre 20-40 °C. As sementes de L. leucocephala apresentaram as menores porcentagens de germinação e temperatura ótima de 35 °C. O desempenho germinativo desta exótica não se sobressai ao das demais espécies estudadas e fatores como a pressão de propágulos, escarificação natural e bom desempenho em outras fases da vida devem contribuir para seu perfil de invasora.<br>Success in establishing and colonizing new areas is directly related to germination behaviour of seeds under local environmental conditions. This work aimed to compare germination performance of the exotic invader Leucaena leucocephala to two legumes (Caesalpinia ferrea, native, and C. pulcherrima, exotic) which are not considered invasive. Non-scarified seeds were exposed to constant temperatures of 15, 20, 25, 30, 35 and 40ºC, within a photoperiod of 12 hours, for 100 days. Both final germination percentage and germination speed showed significant differences among the three species. C. ferrea had low germination percentage, with optimal germination temperature at 25ºC, reaching germination percentage of 59% and germination speed of 0.68 seeds d-1. In contrast, C. pulcherrima had high germination percentages at all temperatures tested, with highest germinability from 20 to 40ºC. Seeds of L. leucocephala had the lowest germination percentage and optimal germination temperature was 35ºC. The germination performance of this exotic is not outstanding, and factors such as propagule pressure, natural scarification and good performance in other life stages must contribute to its invasiveness

Clinal differentiation during invasion: Senecio inaequidens (Asteraceae) along altitudinal gradients in Europe.

Plant population differentiation may play a role in decreasing the ability to predict whether, where, and when an introduced species will invade. However, few studies have addressed the level of genetic change an alien species may undergo during range expansion, e.g. in response to climatic variation with altitude. In the present study, we tested the hypothesis that invasive populations of Senecio inaequidens (Asteraceae) differentiated during migration from two independent introduction sites into divergent altitudinal and climatic zones. We carried out two years of common garden experiments with eight populations from a Belgian altitudinal transect and ten populations from similar French transect. Climatic analysis revealed that the Belgian transect followed a temperature and precipitation gradient. A temperature and summer drought gradient characterized the French study site. We evaluated differentiation and clinal variation in the following characters: days to germination, days to flowering, height at maturity, final plant height and aboveground biomass. Results showed that S. inaequidens populations differentiated in growth traits during invasion. First year of experiment, regressing population growth trait means against source population altitude indicated the presence of clinal variation along both transects. Second year, similar results were found along the French transect, i.e. a reduction in height at maturity, plant height and aboveground biomass with increasing altitude. Including seed mass as a covariate did not change the outcome of the analysis. The possible evolutionary causes for the differentiation observed are discussed.FRFC 2.4605.0