Provenance-by-environment interaction of reproductive traits in the invasion of Spartina alterniflora in China

Limited life-history plasticity in marginal population of an invasive foundation species: Unraveling the genetic underpinnings and ecological implications

Plant's life history can evolve in response to variation in climate spatio-temporally, but numerous multiple-species studies overlook species-specific (especially a foundation species) ecological effects and genetic underpinnings. For a species to successfully invade a region, likely to become a foundation species, life-history variation of invasive plants exerts considerable ecological and evolutionary impacts on invaded ecosystems. We examined how an invasive foundation plant, Spartina alterniflora, varied in its life history along latitudinal gradient using a common gardens experiment. Two common gardens were located at range boundary in tropical zone and main distribution area of S. alterniflora in temperate zone in China. Within each population/garden, we measured the onset time of three successive phenological stages constituting the reproductive phase and a fitness trait. In the low-latitude garden with higher temperature, we found that reproductive phase was advanced and its length prolonged compared to the high-latitude garden. This could possibly due to lower plasticity of maturity time. Additionally, plasticity in the length of the reproductive phase positively related with fitness in the low-latitude garden. Marginal population from tropic had the lowest plasticity and fitness, and the poor capacity to cope with changing environment may result in reduction of this population. These results reflected genetic divergence in life history of S. alterniflora in China. Our study provided a novel view to test the center-periphery hypothesis by integration across a plant's life history and highlighted the significance in considering evolution. Such insights can help us to understand long-term ecological consequences of life-history variation, with implications for plant fitness, species interaction, and ecosystem functions under climate change.

Latitudinal patterns and their climate drivers of the δ 13C, δ 15N, δ 34S isotope signatures of Spartina alterniflora across plant life-death status: a global analysis

Isotopic signatures offer new methods, approaches, and perspectives for exploring the ecological adaptability and functions of plants. We examined pattern differences in the isotopic signatures (δ 13C, δ 15N, δ 34S) of Spartina alterniflora across varying plant life-death status along geographic clines. We extracted 539 sets of isotopic data from 57 publications covering 267 sites across a latitude range of over 23.8° along coastal wetlands. Responses of isotopic signatures to climate drivers (MAT and MAP) and the internal relationships between isotopic signatures were also detected. Results showed that the δ 13C, δ 15N, and δ 34S of S. alterniflora were -13.52 ± 0.83‰, 6.16 ± 0.14‰, and 4.01 ± 6.96‰, with a range of -17.44‰ to -11.00‰, -2.40‰ to 15.30‰, and -9.60‰ to 15.80‰, respectively. The latitudinal patterns of δ 13C, δ 15N, and δ 34S in S. alterniflora were shaped as a convex curve, a concave curve, and an increasing straight line, respectively. A decreasing straight line for δ 13C within the ranges of MAT was identified under plant life status. Plant life-death status shaped two nearly parallel decreasing straight lines for δ 34S in response to MAT, resulting in a concave curve of δ 34S for live S. alterniflora in response to MAP. The δ 15N of S. alterniflora significantly decreased with increasing δ 13C of S. alterniflora, except for plant death status. The δ 13C, δ 15N, and δ 34S of S. alterniflora are consistent with plant height, stem diameter, leaf traits, etc, showing general latitudinal patterns closely related to MAT. Plant life-death status altered the δ 15N (live: 6.55 ± 2.23‰; dead: -2.76 ± 2.72‰), latitudinal patterns of S. alterniflora and their responses to MAT, demonstrating strong ecological plasticity and adaptability across the geographic clines. The findings help in understanding the responses of latitudinal patterns of the δ 13C, δ 15N, and δ 34S isotope signatures of S. alterniflora in response plant life-death status, and provide evidence of robust ecological plasticity and adaptability across geographic clines.

Genomic and phenotypic signatures provide insights into the wide adaptation of a global plant invader

Invasive alien species are primary drivers of biodiversity loss and species extinction. Smooth cordgrass (Spartina alterniflora) is one of the most aggressive invasive plants in coastal ecosystems around the world. However, the genomic bases and evolutionary mechanisms underlying its invasion success have remained largely unknown. Here, we assembled a chromosome-level reference genome and performed phenotypic and population genomic analyses between native US and introduced Chinese populations. Our phenotypic comparisons showed that introduced Chinese populations have evolved competitive traits, such as early flowering time and greater plant biomass, during secondary introductions along China's coast. Population genomic and transcriptomic inferences revealed distinct evolutionary trajectories of low- and high-latitude Chinese populations. In particular, genetic mixture among different source populations, together with independent natural selection acting on distinct target genes, may have resulted in high genome dynamics of the introduced Chinese populations. Our study provides novel phenotypic and genomic evidence showing how smooth cordgrass rapidly adapts to variable environmental conditions in its introduced ranges. Moreover, candidate genes related to flowering time, fast growth, and stress tolerance (i.e., salinity and submergence) provide valuable genetic resources for future improvement of cereal crops.

Tidal channel meanders serve as stepping-stones to facilitate cordgrass landward spread by creating invasion windows

Understanding the mechanisms by which the geomorphic structures affect habitat invasibility by mediating various abiotic and biotic factors is essential for predicting whether these geomorphic structures may provide spatial windows of opportunity to facilitate range-expansion of invasive species in salt marshes. Many studies have linked geomorphic landscape features such as tidal channels to invasion by exotic plants, but the role of tidal channel meanders (i.e., convex and concave sides) in regulating the Spartina alterniflora invasion remains unclear. Here, we examined the combined effects of tidal channel meander-mediated hydrodynamic variables, soil abiotic stresses, and propagule pressure on the colonization of Spartina in the Yellow River Delta, China, by conducting field observations and experiments. The results showed that lower hydrodynamic disturbance, bed shear stress, and higher propagule pressure triggered by eddies due to the convex structure of channel meanders facilitated Spartina seedling establishment and growth, whereas the concave side considerably inhibited the Spartina invasion. Lower soil abiotic stresses also significantly promoted the invasibility of the channel meanders by Spartina. Based on these findings, we propose a conceptual framework to illustrate the effects of the meandering geomorphology of tidal channels on the mechanisms that might allow the landward spread of Spartina and related processes. Our results demonstrate that the meandering geomorphic structures of tidal channels could act as stepping-stones to significantly facilitate the landward invasion of Spartina along tidal channels. This implies that geomorphic characteristics of tidal channels should be integrated into invasive species control and salt marsh management strategies.

The importance of structural and functional characteristics of tidal channels to smooth cordgrass invasion in the Yellow River Delta, China: Implications for coastal wetland management

Understanding the spatiotemporal landscape dynamics and spread pathways of invasive plants, as well as their interactions with geomorphic landscape features, are of great importance for predicting and managing their future range-expansion in non-native habitats. Although previous studies have linked geomorphic landscape features such as tidal channels to plant invasions, the potential mechanisms and critical characteristics of tidal channels that affect the landward invasion by Spartina alterniflora, an aggressive plant in global coastal wetlands, remain unclear. Here, using high-resolution remote-sensing images of the Yellow River Delta from 2013 to 2020, we first quantified the evolution of tidal channel networks by analyzing the spatiotemporal dynamics of their structural and functional characteristics. The invasion patterns and pathways of S. alterniflora were then identified. Based on the above-mentioned quantification and identification, we finally quantified the influences of tidal channel characteristics on S. alterniflora invasion. The results showed that tidal channel networks presented increasing growth and development over time, and their spatial structure evolved from simple to complex. The external isolated expansion of S. alterniflora played a dominant role during the initial invasion stage, and then they connected the discrete patches into the meadow through marginal expansion. Afterwards, tidal channel-driven expansion gradually increased and became the primary way during the late invasion stage, accounting for about 47.3%. Notably, tidal channel networks with higher drainage efficiency (shorter OPL, higher D and E) attained larger invasion areas. The longer the tidal channels and the more sinuous the channel structure, the greater the invasion potential by S. alterniflora. These findings highlight the importance of structural and functional properties of tidal channel networks in driving plant invasion landward, which should be incorporated into future control and management of invasive plants in coastal wetlands.

Unraveling the Effects of Cold Stratification and Temperature on the Seed Germination of Invasive Spartina alterniflora Across Latitude

Seed germination is critical to the life history of plants, playing an important role in the successful recruitment, colonization, and even invasion of new individuals within and outside population distribution ranges. Cold stratification and temperature are the key factors affecting seed germination traits. Studying how these two factors drive geographical variation in seed germination is essential to analyze and predict the geographical distribution range of alien plants in novel habitats. Spartina alterniflora, native to the United States, was introduced into China in 1979 and has spread over 20° of latitude along the eastern coast of China. Germination plays a crucial role in S. alterniflora's large-scale invasion and diffusion across latitude. To evaluate the effects of cold stratification and temperature on seed germination of S. alterniflora across latitude, we collected seeds at seven locations across latitude in China. We exposed these provenances to cold stratification at 4°C (0, 1, 3, and 5 months) and germination temperature (5°C, 15°C, 25°C, and 35°C) treatments in growth chambers. Seed germination was observed for 98 days, and we calculated germination rate, germination index, and germination time. Results indicated that longer cold stratification significantly promoted germination rate and germination index, but decreased germination time. Similarly, higher germination temperature significantly promoted germination rate and germination index, but decreased germination time. Moreover, there were significant interactive effects on germination traits between cold stratification and temperature. Seed germination traits showed linear relationships with latitude, indicating that S. alterniflora seeds from different provenances germinated at different times and adopted different germination strategies. The stratification and temperature are the most important factors regulating the dormancy and germination seeds, so they can be important drivers of this variation along latitude. Under scenarios of warmer regional temperature, seeds at higher latitudes could germinate earlier and have higher germination rate, which would favor a potential northern expansion of this invasive plant.

Does the effect of flowering time on biomass allocation across latitudes differ between invasive and native salt marsh grass Spartina alterniflora?

Parallel latitudinal clines in flowering time have been documented in both the invasive and native ranges of plants. Furthermore, flowering time has been found to affect biomass at maturity. Therefore, understanding how these flowering times affect biomass accumulation across latitudes is essential to understanding plant adaptations and distributions.We investigated and compared trends in first flowering day (FFD), aboveground biomass (AGB), belowground biomass (BGB), and BGB:AGB ratio of the salt marsh grass Spartina alterniflora along latitudinal gradients from the invasive (China, 19-40°N) and native range (United States, 27-43°N) in a greenhouse common garden experiment, and tested whether FFD would drive these divergences between invasive and native ranges.The invasive populations produced more (~20%, ~19%) AGB and BGB than native populations, but there were no significant differences in the FFD and BGB:AGB ratio. We found significant parallel latitudinal clines in FFD in both invasive and native ranges. In addition, the BGB:AGB ratio was negatively correlated with the FFD in both the invasive and native ranges but nonsignificant in invasive populations. In contrast, AGB and BGB increased with latitude in the invasive range, but declined with latitude in the native range. Most interestingly, we found AGB and BGB positively correlated with the FFD in the native range, but no significant relationships in the invasive range.Our results indirectly support the evolution of increased competitive ability hypothesis (EICA) that S. alterniflora has evolved to produce greater AGB and BGB in China, but the flowering and allocation pattern of native populations is maintained in the invasive range. Our results also suggest that invasive S. alterniflora in China is not constrained by the trade-off of earlier flowering with smaller size, and that flowering time has played an important role in biomass allocation across latitudes.

Fluorescent dissolved organic matter facilitates the phytoavailability of copper in the coastal wetlands influenced by artificial topography

Dissolved organic matter (DOM) is a crucial driver in ecosystem services and a central part of the carbon transport and biological cycle in land-sea interaction. DOM exhibits characteristic environmental behavior in the coastal zone, but its sustainability is affected by expanding artificial topography (AT) construction. It requires combining analyses on AT-induced response of field fluorescent DOM (fDOM) and its quenching pattern under metal-complexation. Herein, we conducted systemic investigations into the spatiotemporal dynamics of fDOM compositions with further in-lab verification to study its Cu-binding capacity. We detected three humid-like fDOM components sensitive to AT. The total fDOM intensity was positively correlated with low molecular weight organic acid (LMWOA) extractable Cu and the Cu pools in above-ground biomass. The enriched fDOM serves as an ecological engineer by increasing the Cu mobility, confirmed by an in-lab fluorescence titration. The application of LMWOA greatly enhanced the intensity of one fDOM component, elevated its conditional stability constant, and decreased its quenched proportion, implying that LMWOA might extract part of Cu from fDOM complexation. The present work provides an "fDOM-LMWOA pump" explanation to suggest that fDOM is a novel ecological regulator on vegetation growth under the AT-induced matter accumulation.

Artificial modification on lateral hydrological connectivity promotes range expansion of invasive Spartina alterniflora in salt marshes of the Yellow River delta, China

'Invasibility', or the extent to which a habitat is prone to being invaded by plants, is a measure of the resistance of that ecosystem to biological invasion: a limited extent represents abiotic conditions unsuitable for invasion by invasive species; however, human activity can change that and make a habitat prone to rapid invasion. Field surveys and greenhouse experiments were carried out to explore, using spatial analysis, how a strong invader, namely Spartina alterniflora, is assisted by such activities as constructing levees and digging trenches, ditches, and pits in a tidal salt marsh. These activities changed the lateral hydrological connectivity of a salt marsh. The invasibility was then estimated based on the probability of seed dispersal and retention using the classical probabilistic method, and the rate of seedling emergence using threshold analysis. Changes in lateral hydrological connectivity led to more seeds of the invading species being retained, especially in high marshes, and promoted the emergence of its seedlings by making the soil more moist and less saline. The results suggest that changes in the lateral hydrological connectivity in a salt marsh can make it more prone to being invaded. The results have important implications for the control of invasive plants by limiting human activity and thereby regulating lateral hydrological connectivity in coastal ecosystems.

Plasticity and selection drive hump-shaped latitudinal patterns of flowering phenology in an invasive intertidal plant

Patterns of flowering phenology can affect the success of plant invasions, especially when introduced species spread across a wide range of latitude into different climatic conditions. We combined a 4-yr field survey and a 3-yr common garden experiment with the invasive grass Spartina alterniflora that is now widespread along the coast of China to document the latitudinal pattern of flowering phenology, determine if phenology was related to climate or oceanographic variables, and determine whether phenology patterns were fixed versus plastic. In the field, first flowering day displayed a hump-shaped relationship with latitude, with low- and high-latitude plants flowering 100 d and 10 d earlier than plants at middle latitudes, respectively. Peak flowering day showed a similar hump-shaped relationship with latitude, with the interval between first and peak flowering day decreasing with increasing latitude. First flowering day had a hump-shaped relationship with annual growing degree days but a linear positive relationship with tidal range. In the common garden, first flowering day decreased linearly with increasing latitude of origin, as did peak flowering day, and the interval between first and peak flowering day increased with increasing latitude. First flowering day in the common garden had weak or no relationships with abiotic variables at the sites of origin. In both the field and common garden, first flowering day was later in site years for which plants were taller. These results indicate a high degree of plasticity in flowering phenology, with plants flowering later in the field at sites with intermediate temperatures and high tide ranges. Common garden results indicate some selection for earlier flowering at sites with low temperatures, consistent with a shorter growing season. Consistent with life-history theory, plants flowered later under conditions favoring vigorous growth. Earlier flowering and smaller size of plants at high and low latitudes suggests that S. alterniflora has already occupied much of the geographic range favorable for it on the East Coast of Asia.

Dramatic change of bacterial assembly process and co-occurrence pattern in Spartina alterniflora salt marsh along an inundation frequency gradient

Exotic Spartina alterniflora has become widely distributed along most of the coastlines in China in a wide range of inundation frequencies. However, the assembly processes and co-occurrence patterns of the bacterial community in S. alterniflora wetlands under different inundation frequencies remain elusive. In this study, an in-situ mesocosm was established to investigate the changes in soil bacterial community. We found that soil water content was the most decisive factor in influencing the bacterial community. Balanced variation, rather than abundance gradients, accounted for the major shifts in bacterial communities and was significantly and positively correlated with the changes in water content, suggesting that species substitution was facilitated by the increased water content. Deterministic processes were dominant in community assembly, and a large degree of change in water content increased variable selection. Co-occurrence network revealed that increasing water content significantly decreased the average degree and the relative abundance of keystone species, resulting in a network with less complexity. Structural equation modelling suggests that increasing inundation frequency has strong impacts on bacterial community, primarily by altering water content, network degree, and the relative abundance of keystone species. Overall, our results illustrate that increasing inundation frequency significantly influences the bacterial community assembly processes and co-occurrence patterns.

Distinct Community Assembly Processes of Abundant and Rare Soil Bacteria in Coastal Wetlands along an Inundation Gradient

Coastal wetlands are one of the important ecosystems that play a crucial role in the regulation of climate change. Rare taxa (RT) exist in one habitat along with abundant taxa (AT).

Microbial communities commonly consist of a large number of rare taxa (RT) and few abundant taxa (AT), and it is important to identify the differences of the community assembly processes between RT and AT in response to environmental changes. However, the community assembly processes governing AT and RT in coastal wetland soils along an inundation gradient remain elusive. Here, an in situ mesocosm, with continuous inundation gradients and native mangrove Kandelia obovata or exotic cordgrass Spartina alterniflora, was established to determine the patterns and driving factors of community turnover and assembly processes of AT and RT. We found that RT exhibited a remarkably lower turnover rate than AT, and the niche breadth of RT was significantly narrower than that of AT. In comparison with AT, RT presented stronger phylogenetic signals for ecological preferences across environmental gradients. Null model analyses revealed that RT were more phylogenetically clustered and primarily governed by homogeneous selection, while AT were more overdispersed and dominated by dispersal limitation. Soil water content was the most decisive factor for community turnover and assembly processes of both AT and RT. Structural equation modeling analysis showed that RT were strongly associated with K. obovata biomass rather than S. alterniflora biomass, suggesting a strong relationship between RT and the growth of mangrove K. obovata. Overall, our study revealed distinct assembly processes of soil AT and RT communities in coastal wetlands, which is crucial for mechanistic understanding of the establishment and maintenance of soil microbial diversity in coastal wetlands under conditions of global environmental changes.

IMPORTANCE Coastal wetlands are one of the important ecosystems that play a crucial role in the regulation of climate change. Rare taxa (RT) exist in one habitat along with abundant taxa (AT). In this study, we found that RT exhibited narrower niche breadth and stronger phylogenetic signals than AT. Null model analyses showed that RT were more phylogenetically clustered and primarily governed by homogeneous selection, while AT were more overdispersed and dominated by dispersal limitation. Revealing the differences in the community assembly processes between AT and RT in coastal wetlands is critical to understand the establishment and maintenance of soil microbial diversity in coastal wetlands with regard to environmental changes.

Windows of opportunity for smooth cordgrass landward invasion to tidal channel margins: The importance of hydrodynamic disturbance to seedling establishment

Despite increasing concerns about the global threat of cordgrass (S. alterniflora) expansion and the interest in its invasion mechanisms, there is not yet a general understanding of the mechanistic processes underlying the interaction between cordgrass invasion and geomorphic structures such as tidal channels. This study elucidated the effects of the hydrodynamic disturbance of tidal channels on initial seedling establishment of cordgrass in the margins of two different types of tidal channels (i.e., main tidal channels and secondary tributaries). We performed field experiments that transplanted cordgrass seedlings to above-mentioned tidal channel margins with on-site controlled hydrodynamic conditions. The results showed that high hydrodynamic disturbance intensity (i.e., HDI) on the margin of main tidal channels (i.e., MMC) was not beneficial to cordgrass invasion, whereas low HDI created windows of opportunity for cordgrass invasion to the margin of secondary tributaries (i.e., MST) by facilitating the survival, growth, and stability of cordgrass seedlings. The presence of high HDI predominantly reduced the seedling survival and total biomass of cordgrass, whereas root biomass allocation of cordgrass increased significantly to resist dislodgment and toppling. Moreover, field investigations showed that soil salinity and moisture in the margin of tidal channels were not the limiting factors affecting the establishment of cordgrass seedlings. However, higher propagule pressure combined with suitable soil salinity-moisture conditions (i.e., low salinity and high moisture) laid a firm foundation favoring seedling establishment. Our results highlight the importance of hydrodynamic disturbance as a dominating driver regulating seedling establishment of cordgrass in tidal channel margins and the potential implications for controlling cordgrass landward invasions.

Rapid Genetic Divergence of an Invasive Species, Spartina alterniflora, in China

Hundreds of plants and half a kilogram of seeds of Spartina alterniflora, which were collected from Morehead City in North Carolina, Sapelo Island in Georgia, and Tampa Bay in Florida, were introduced to China in 1979. However, according to documented records, S. alterniflora from different origins were introduced to different areas when the species was first introduced to the coastal areas of China in the 1980s. In order to understand the relationship between the invasive S. alterniflora populations of China and the native S. alterniflora populations of the United States, and whether the genetic structure and genetic diversity of the invasive populations of China were affected by different introductions in the 1980s, molecular markers were used to determine the levels of gene flow and its effect on population differentiation. A total of 715 samples of S. alterniflora were collected from nine invasive populations in China and nine native populations from the United States. The genetic diversity and genetic structure of invasive and native populations were compared using microsatellite markers. The heterozygosity of Chinese invasive populations of S. alterniflora (HO = 0.538, HE = 0.725) were similar with those of native populations (HO = 0.530, HE = 0.744), which may attribute to its multiple introductions with the multisource populations from different geographic areas of the United States. However, the lower allelic diversities of Chinese invasive populations were detected, which may be due to the founder effect, or the bottleneck, which supports the theory that the allelic diversity is more sensitive to population bottlenecks than heterozygosity. The results of the STRUCTURE analysis among all sampling sites showed that the value of ΔK was largest when K = 2, which indicated that the invasive S. alterniflora populations in China had completed differentiated from the native populations of the United States. This may be because of admixture and hybridization of three non-overlapping original populations, or the postintroduction rapid evolution in China, and reproductive isolation under long-term geographic isolation. There was significant differentiation among invasive populations, which was mainly affected by different human-mediated introductions in 1980s. Significant genetic structure (K = 7) and high genetic differentiation (Fst = 0.30193) were detected in Chinese invasive populations, which may due to the low natural gene flow among populations. The genetic structure of the invasive populations in China was still affected by the human-mediated introductions in the 1980s, and the different initial introductions might promote differentiation among the invasive populations. In fact, the human-mediated long-distance dispersal should take the most of responsibility for the rapid spread of S. alterniflora along the coast of China. Multisource introductions of S. alterniflora are perhaps helpful for local adaptation but itself cannot cause rapid spread along the whole coast of China. Meanwhile, we suggest that the prevention of gene exchange among populations of S. alterniflora is the first and most important step in the control of the species on the coast of China, because admixture and hybridization of isolated populations might generate new heterosis and increase the difficulty of managing S. alterniflora in China.

Bacterial Succession in Salt Marsh Soils Along a Short-term Invasion Chronosequence of Spartina alterniflora in the Yellow River Estuary, China

As an exotic plant species, Spartina alterniflora seriously threatens native ecosystem function in Chinese coastal regions. Unveiling the dynamics of soil bacteria community during its invasion is essential for a better understanding of related biogeochemical processes, while the shift in soil bacterial community over invasive time remains unclear. A short-term chronosequence was identified to assess the impacts of Spartina alterniflora invasion on soil nutrients and bacterial community composition and structure (using 16S rRNA gene high-throughput sequencing) over the time of invasion (i.e., (1) at least 10 years, (2) nearly 5 years, (3) less than 2 years, and (4) in native salt marshes or 0 years) in the Yellow River Estuary. The results exhibited an orderly change in the soil physicochemical properties and bacterial community composition over the invasion time. Soil pH showed a significant decrease with the accumulation of soil organic matter (SOM), whereas soil nutrients such as soil dissolved organic carbon (DOC), total nitrogen (TN), nitrate (NO₃^-), ammonium (NH₄⁺), K⁺, and Mg²⁺ were generally increased with the age of the invasion. The number of operational taxonomic units (OTUs, 97% similarity level) exhibited a decreasing trend, which suggested a decline in bacterial diversity with the invasion age. The dominant groups at the phylum level were Proteobacteria, Bacteroidetes, Chloroflexi, Acidobacteria, and Gemmatimonadetes (the sum of relative abundance was > 70% across all samples). The relative abundances of Chloroflexi and Gemmatimonadetes steadily decreased, while the abundance of Bacteroidetes significantly increased with the plant invasion. The distribution pattern of the soil bacteria was clearly separated according to the principal coordinate analysis (PCoA) and canonical correspondence analysis (CCA) in native and invaded salt marshes. The variation in the soil bacterial community was tightly associated with the soil physicochemical properties (Mantel test, P < 0.05). Variance partitioning analysis (VPA) showed that plant traits explained 4.95% of the bacterial community variation, and soil variables explained approximately 26.96% of the variation. Network analysis also revealed that plant invasion strengthens the interaction among soil bacterial communities. Overall, our findings highlight the bacterial community succession during the Spartina alterniflora invasion in coastal salt marsh soils, which can provide insight regarding the association between soil development and invasive plant.

Contrasting plant adaptation strategies to latitude in the native and invasive range of Spartina alterniflora

Biological invasions offer model systems of contemporary evolution. We examined trait differences and evolution across geographic clines among continents of the intertidal grass Spartina alterniflora within its invasive and native ranges. We sampled vegetative and reproductive traits in the field at 20 sites over 20° latitude in China (invasive range) and 28 sites over 17° in the US (native range). We grew both Chinese and US plants in a glasshouse common garden for 3 yr. Chinese plants were c. 15% taller, c. 10% denser, and set up to four times more seed than US plants in both the field and common garden. The common garden experiments showed a striking genetic cline of seven-fold greater seed set at higher latitudes in the introduced but not the native range. By contrast, there was a slight genetic cline in some vegetative traits in the native but not the introduced range. Our results are consistent with others showing that introduced plants can evolve rapidly in the new range. S. alterniflora has evolved different trait clines in the native and introduced ranges, showing the importance of phenotypic plasticity and genetic control of change during the invasion process.

Herbivory may promote a non-native plant invasion at low but not high latitudes

BACKGROUND AND AIMS

The strengths of biotic interactions such as herbivory are expected to decrease with increasing latitude for native species. To what extent this applies to invasive species and what the consequences of this variation are for competition among native and invasive species remain unexplored. Here, herbivore impacts on the invasive plant Alternanthera philoxeroides and its competition with the native congener A. sessilis were estimated across latitudes in China.

METHODS

An common garden experiment spanning ten latitudinal degrees was conducted to test how herbivore impacts on A. philoxeroides and A. sessilis, and competition between them change with latitude. In addition, a field survey was conducted from 21°N to 36.8°N to test whether A. philoxeroides invasiveness changes with latitude in nature as a result of variations in herbivory.

KEY RESULTS

In the experiment, A. sessilis cover was significantly higher than A. philoxeroides cover when they competed in the absence of herbivores, but otherwise their cover was comparable at low latitude. However, A. philoxeroides cover was always higher on average than A. sessilis cover at middle latitude. At high latitude, only A. sessilis emerged in the second year. Herbivore abundance decreased with latitude and A. philoxeroides emerged earlier than A. sessilis at middle latitude. In the field survey, the ratio of A. philoxeroides to A. sessilis cover was hump shaped with latitude.

CONCLUSION

These results indicate that herbivory may promote A. philoxeroides invasion only at low latitude by altering the outcome of competition in favour of the invader and point to the importance of other factors, such as earlier emergence, in A. philoxeroides invasion at higher latitudes. These results suggest that the key factors promoting plant invasions might change with latitude, highlighting the importance of teasing apart the roles of multiple factors in plant invasions within a biogeographic framework.

Low genetic differentiation yet high phenotypic variation in the invasive populations of Spartina alterniflora in Guangxi, China

Genetic variation and population structure may reflect important information for invasion success of exotic plant species and thus help improve management of invasive plants. Spartina alterniflora is an invasive plant that is a major threat to the economy and environment of the coastal regions in China. We analyzed the genetic structure and diversity of six populations of S. alterniflora differing in invasion histories in Guangxi, China. A total of 176 individuals from the six populations produced 348 AFLP fragments. The average heterozygosity was significantly lower than in the native population. And genetic bottlenecks were also detected in most populations. Standardized FST statistics (Φpt = 0.015) and AMOVA results indicated weak genetic differentiation. Genetic admixture and obviously isolation by distance indicated populations in Guangxi come from a pre-admixed population by a single introduction. High phenotypic variations of S. alterniflora in Guangxi influenced by soil salinity and temperature might be an important reason for the successful invasion.

Genetic admixture accelerates invasion via provisioning rapid adaptive evolution

Genetic admixture, the intraspecific hybridization among divergent introduced sources, can immediately facilitate colonization via hybrid vigor and profoundly enhance invasion via contributing novel genetic variation to adaption. As hybrid vigor is short-lived, provisioning adaptation is anticipated to be the dominant and long-term profit of genetic admixture, but the evidence for this is rare. We employed the 30 years' geographic-scale invasion of the salt marsh grass, Spartina alterniflora, as an evolutionary experiment and evaluated the consequences of genetic admixture by combining the reciprocal transplant experiment with quantitative and population genetic surveys. Consistent with the documentation, we found that the invasive populations in China had multiple origins from the southern Atlantic coast and the Gulf of Mexico in the US. Interbreeding among these multiple sources generated a "hybrid swarm" that spread throughout the coast of China. In the northern and mid-latitude China, natural selection greatly enhanced fecundity, plant height and shoot regeneration compared to the native populations. Furthermore, genetic admixture appeared to have broken the negative correlation between plant height and shoot regeneration, which was genetically-based in the native range, and have facilitated the evolution of super competitive genotypes in the invasive range. In contrast to the evolved northern and mid-latitude populations, the southern invasive populations showed slight increase of plant height and shoot regeneration compared to the native populations, possibly reflecting the heterotic effect of the intraspecific hybridization. Therefore, our study suggests a critical role of genetic admixture in accelerating the geographic invasion via provisioning rapid adaptive evolution.

Photosynthetic tolerance to non-resource stress influences competition importance and intensity in an invaded estuary

In an attempt to clarify the role of environmental and biotic interactions on plant growth, there has been a long-running ecological debate over whether the intensity and importance of competition stabilizes, increases or decreases across environmental gradients. We conducted an experiment in a Chinese estuary to investigate the effects of a non-resource stress gradient, soil salinity (from 1.4‰ to 19.0‰ salinity), on the competitive interactions between native Phragmites australis and invasive Spartina alterniflora. We linked these effects to measurements of photosynthetic activities to further elucidate the underlying physiological mechanism behind the competitive interactions and the driver of invasion. The experiments revealed that while biomass of both species decreased in the presence of the other, competition did not alter photosynthetic activity of either species over time. P. australis exhibited high photosynthetic activity, including low chlorophyllase activity, high chlorophyll content, high stomatal conductance and high net photosynthetic rate, at low salinity. Under these conditions, P. australis experienced low competitive intensity, leading to high biomass production and competitive exclusion of S. alterniflora. The opposite was observed for S. alterniflora: while competitive intensity experienced by P. australis increased with increasing salinity, and photosynthetic activity, biomass, competitive dominance and the importance of competition for P. australis growth decreased, those of S. alterniflora were stable. These findings demonstrate that S. alterniflora invasion driven by competitive exclusion are likely to occur and expand in high salinity zones. The change in the nature of competition along a non-resource stress gradient differs between competitors likely due to differences in photosynthetic tolerance to salinity. The driver of growth of the less-tolerant species changes from competition to non-resource stress factors with increasing stress levels, whereas competition is constantly important for growth of the more-tolerant species. Incorporating metrics of both competition intensity and importance, as well as linking these competitive outcomes with physiological mechanisms, is crucial to understanding, predicting, and mediating the effects of invasive species in the future.

Low genetic diversity contrasts with high phenotypic variability in heptaploid Spartina densiflora populations invading the Pacific coast of North America

Species can respond to environmental pressures through genetic and epigenetic changes and through phenotypic plasticity, but few studies have evaluated the relationships between genetic differentiation and phenotypic plasticity of plant species along changing environmental conditions throughout wide latitudinal ranges. We studied inter‐ and intrapopulation genetic diversity (using simple sequence repeats and chloroplast DNA sequencing) and inter‐ and intrapopulation phenotypic variability of 33 plant traits (using field and common‐garden measurements) for five populations of the invasive cordgrass Spartina densiflora Brongn. along the Pacific coast of North America from San Francisco Bay to Vancouver Island. Studied populations showed very low genetic diversity, high levels of phenotypic variability when growing in contrasted environments and high intrapopulation phenotypic variability for many plant traits. This intrapopulation phenotypic variability was especially high, irrespective of environmental conditions, for those traits showing also high phenotypic plasticity. Within‐population variation represented 84% of the total genetic variation coinciding with certain individual plants keeping consistent responses for three plant traits (chlorophyll b and carotenoid contents, and dead shoot biomass) in the field and in common‐garden conditions. These populations have most likely undergone genetic bottleneck since their introduction from South America; multiple introductions are unknown but possible as the population from Vancouver Island was the most recent and one of the most genetically diverse. S. densiflora appears as a species that would not be very affected itself by climate change and sea‐level rise as it can disperse, establish, and acclimate to contrasted environments along wide latitudinal ranges.