Biological nitrogen fixation (BNF) is the major natural process of nitrogen (N) input to ecosystems. To understand how plant invasion and N enrichment affect BNF, we compared soil N-fixation rates and N-fixing microbes (NFM) of an invasive Spartina alterniflora community and a native Phragmites australis community in the Yangtze River estuary, with and without N addition. Our results indicated that plant invasion relative to N enrichment had a greater influence on BNF. At each N level, the S. alterniflora community had a higher soil N-fixation rate but a lower diversity of the nifH gene in comparison with the native community. The S. alterniflora community with N addition had the highest soil N-fixation rate and the nifH gene abundance across all treatments. Our results suggest that S. alterniflora invasion can increase soil N fixation in the high N-loading estuarine ecosystem, and thus may further mediate soil N availability.

The world has increasingly relied on protected areas (PAs) to rescue highly valued ecosystems from human activities, but whether PAs will fare well with bioinvasions remains unknown. By analyzing three decades of seven of the largest coastal PAs in China, including World Natural Heritage and/or Wetlands of International Importance sites, we show that, although PAs are achieving success in rescuing iconic wetlands and critical shorebird habitats from once widespread reclamation, this success is counteracted by escalating plant invasions. Plant invasions were not only more extensive in PAs than non-PA controls but also undermined PA performance by, without human intervention, irreversibly replacing expansive native wetlands (primarily mudflats) and precluding successional formation of new native marshes. Exotic species are invading PAs globally. This study across large spatiotemporal scales highlights that the consequences of bioinvasions for humanity's major conservation tool may be more profound, far reaching, and critical for management than currently recognized.

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Flowering synchrony can play an important role in plants' reproductive success, which is essential for the successful establishment and spread of invasive plants. Although flowering synchrony has been found to be closely related to climatic factors, the effects of variation in such factors along latitudinal gradient on flowering synchrony and the role of flowering synchrony in the reproductive success of invading populations remain largely unexplored. In a 2-year field study, we examined the latitudinal variation of flowering phenology, especially flowering synchrony, in an invasive plant, , along coastal China, and its relationship with population seed set across three climatic zones. We found that first flowering date was delayed, and flowering synchrony increased with increasing latitude. Flowering synchrony was negatively related to temperature during flowering season but not to soil properties or precipitation, suggesting that climate has shaped the latitudinal pattern of flowering synchrony. Moreover, a positive correlation between flowering synchrony and seed set across latitudes indicates the possible role of flowering synchrony in the latitudinal pattern of sexual reproduction in These results suggest that, in addition to the effects of climate on the growth of invasive species, climatic factors can play an important role in the invasion success of alien plants by regulating the flowering synchrony and thus the reproductive success of invasive plants.

Whether global changes impact native and exotic species differently is unclear, because the changes may favour both native and exotic species over competitors. Previous studies have mainly focused on the separate effects of different environmental changes, but plant communities are influenced by the changes in multiple environmental factors, and it is still unclear whether native and exotic species respond similar to the combined effects of these factors. We hypothesized that differences in interspecific trade‐offs between native and exotic species can lead to the dominance of exotic species when the species are simultaneously subjected to multiple environmental changes. Using coastal saltmarsh plant communities as the study system, we experimentally manipulated flooding and nutrient enrichment, examined the interspecific trade‐offs between competitiveness and stress tolerance for two native and one exotic species, and explored the combined effect of the two environmental changes on both native–native and native–exotic species interactions. We found that flooding and nutrient enrichment oppositely affected native–native species interactions but additively affected native–exotic species interactions. The two factors together resulted in no net change in the relative advantages between the two native species but enhanced the dominance of the exotic species over the native species. This disparity occurred because the exotic species was not subject to the interspecific trade‐off between competitiveness and stress tolerance that constrained the native species. Synthesis. Our results suggest that changes in multiple environmental factors favour exotic species because of evolutionary novel trade‐off patterns. Mechanisms underlying species coexistence in the invaded community such as interspecific trade‐offs need to be considered when future attempts are made to predict the effects of global changes on biological invasions.

Different mechanisms have been proposed to explain invasion success of alien species, among which genetic differentiation and phenotypic plasticity are extensively studied. In order to explore whether the invasion of Spartina alterniflora is facilitated by enhanced phenotypes or trait plasticity, we conducted a common garden experiment on this clonal invasive plant, in which three native populations (from the US) and eight introduced populations (from China) were included and grown at two water levels. We measured their plant traits related to growth, reproductive phenology, reproductive output, and their plasticity in response to water availability. No difference in plant height was observed between native and introduced populations of S. alterniflora, but introduced populations had greater total biomass than the native ones at high water level. Reproductive performances facilitating invasion success were also found for introduced populations, indicated by the advanced initiation of phenological events, greater total number of ramets, and more investment of total biomass in rhizomes versus stalk and leaf in both water treatments. Finally, a non-significant difference in inflorescence biomass for flowering individuals and phenotypic plasticity of most measured traits was observed between native and introduced populations. In conclusion, enhanced growth and asexual reproduction make greater contributions to the invasion success of S. alterniflora in China.

Eutrophication is believed to promote plant invasion, resulting in high growth performances of invasive plants and, therefore, the great potential for growthinduced intraspecific competition for light. Current hypotheses predict how eutrophication promotes plant invasion but fail to explain how great invasiveness is maintained under eutrophic conditions. In diverse native communities, co-occurring plants of varying sizes can avoid light competition by exploiting light complementarily; however, whether this mechanism applies to intraspecific competition in invasive plant populations remains unknown. Using a 2-year field nitrogen (N)-enrichment experiment on one of the global invasive plants, Spartina alterniflora, we found that the plasticity of light use reduced intraspecific competition and increased biomass production in S. alterniflora. This plasticity effect was enhanced when S. alterniflora had no nutrient limitations. In the N-enrichment treatments, the height difference among S. alterniflora ramets increased as light intensity decreased under the canopy.Compared with ambient N, under N enrichment, shorter individuals increased their light-use efficiency and specific leaf area in response to the reduced light intensity under the canopy. However, such ecophysiological plasticity was not found for taller individuals. Our findings revealed that the light-use plasticity of short individuals can be envisaged as a novel mechanism by which an invasive plant alleviates intraspecific competition and increases its invasiveness, challenging the prevailing perspective that the invasiveness of exotic plants is constrained by intraspecific competition.

Biological invasion represents a global issue of concern due to its large negative impacts on native ecosystems and society. Elucidating the evolutionary history and genetic basis underpinning invasiveness is critical to understanding how alien species invade and adapt to novel environments. Smooth cordgrass (Spartina alterniflora, 2n = 6x = 62) is a notorious invasive species that causes heavily negative effects on native ecosystems worldwide. Here we addressed the evolutionary mechanisms underlying the invasion and dispersal history of this species along the China coast in the past decades. We employed nine microsatellites and three chloroplast fragments to investigate phylogeographic structure and genetic diversity of 11 native US and 11 invasive Chinese S. alterniflora populations. Demographic history simulation was also performed for both the native and invasive populations, respectively. Comparative genetic analyses of these natural populations revealed that although all the Chinese populations were introduced only once, high level of genetic diversity with weak geographic structure was observed. In particular, both the genetic features and mathematical simulation illustrated very recent population expansion in the Chinese populations. We found that genetic variants identified in native US populations were mixed in the Chinese populations, suggesting the recombination of these original variants during the invasion process. These genetic attributes indicate that Chinese populations might not have experienced a genetic bottleneck during the invasion process. High genetic diversity and genetic admixture might have contributed to the success of invasion of S. alterniflora in China. Our study provides a framework of how the smooth cordgrass spreads along the China coast as well as its potential genetic mechanisms underlying the invasion.