Do invasive alien plants differ from non-invasives in dominance and nitrogen uptake in response to variation of abiotic and biotic environments under global anthropogenic change?

Increased invasion of submerged macrophytes makes native species more susceptible to eutrophication in freshwater ecosystems

Invasion and eutrophication are considered to pose serious threats to freshwater biodiversity and ecosystem function. However, little is known about the synergistic effects of invasion density and nutrient concentration on native submerged macrophytes. Here, we selected a common invasive species (Elodea nuttallii) and two native plants (Hydrilla verticillata and Potamogeton maackianus) to elucidate the effects of invasion density and eutrophication on native submerged plants. We found that (1) high nutrient concentrations inhibited the growth of both invasive and native species, but E. nuttallii, with a wide ecological niche, was more tolerant to eutrophication than the two native species. (2) High invasion density had a remarkable negative effect on the growth of the two native species under the medium and high nutrient concentrations. (3) Medium and high invasion densities of E. nuttallii made native macrophytes more susceptible to eutrophication. (4) The two native macrophytes had species-specific responses to medium and high invasion densities under medium and high nutrient concentrations. Specifically, a high invasion density of E. nuttallii significantly delayed the growth of H. verticillata rather than P. maackianus. Thus, it is necessary to consider the synergistic effects of invasion with eutrophication when assessing invasion in freshwater ecosystems. And our results implied that invasion with eutrophication was a powerful factor determining the results of interspecific competition among submerged macrophytes, which could change the biodiversity, community structure and functions of freshwater ecosystems.

Soil nutrient limitation and natural enemies promote the establishment of alien species in native communities

The invasion of alien plant species threatens the composition and diversity of native communities. However, the invasiveness of alien plants and the resilience of native communities are dependent on the interactions between biotic and abiotic factors, such as natural enemies and nutrient availability. In our study, we simulated the invasion of nine invasive plant species into native plant communities using two levels of nutrient availability and suppression of natural enemies. We evaluated the effect of biotic and abiotic factors on the response of alien target species and the resistance of native communities to invasion. The results showed that the presence of enemies (enemy release) increased the biomass proportion of alien plants while decreasing that of native communities in the absence of nutrient addition. Furthermore, we also found that the negative effect of enemy suppression on the evenness of the native community and the root-to-shoot ratio of alien target species was greatest under nutrient addition. Therefore, nutrient-poor and natural enemies might promote the invasive success of alien species in native communities, whereas nutrient addition and enemy suppression can better enhance the resistance of native plant communities to invasion.

Clonal functional traits favor the invasive success of alien plants into native communities

Functional traits are frequently proposed to determine the invasiveness of alien species. However, few empirical studies have directly manipulated functional traits and tested their importance in the invasion success of alien species into native plant communities, particularly under global change. We manipulated clonal integration (a key clonal functional trait) of four alien clonal plants by severing inter-ramet connections or keeping them intact and simulated their invasion into native plant communities with two levels of species diversity, population density and nutrient availability. High community diversity and density impeded the invasion success of the alien clonal plants. Clonal integration of the alien plants promoted their invasion success, particularly in the low-density communities associated with low species diversity or nutrient addition, which resulted in a negative correlation between the performance of alien plants and native communities, as expected under global change. Thus, clonal integration can favor the invasion success of alien clonal plants into degraded resident communities with a high degree of disturbance and eutrophication. Our findings confirm the role of clonal functional traits in facilitating alien plant invasions into native plant communities and suggest that clonal functional traits should be considered to efficiently restore degraded communities heavily invaded by alien clonal plants.

Nitrogen Deposition Effects on Invasive and Native Plant Competition: Implications for Future Invasions

Nitrogen (N) deposition has increased dramatically in recent decades, which is significantly affecting the invasion and growth of exotic plants. Whether N deposition leads to invasive alien species becoming competitively superior to native species remains to be investigated. In the present study, an invasive species (Oenothera biennis L.) and three co-occurring native species (Artemisia argyi Lévl. et Vant., Inula japonica Thunb., and Chenopodium album L.) were grown in a monoculture (two seedlings of the same species) or mixed culture (one seedling of O. biennis and one seedling of a native species) under three levels of N deposition (0, 6, and 12 g∙m^-2∙year^-1). Nitrogen deposition had no effect on soil N and P content. Nitrogen deposition enhanced the crown area, total biomass, leaf chlorophyll content, and leaf N to phosphorus ratio in both invasive and native plants. Oenothera biennis dominated competition with C. album and I. japonica due to its high resource acquisition and absorption capacity (greater height, canopy, leaf chlorophyll a to chlorophyll b ratio, leaf chlorophyll content, leaf N content, leaf mass fraction, and lower root-to-shoot ratio). However, the native species A. argyi exhibited competitive ability similar to O. biennis. Thus, invasive species are not always superior competitors of native species; this depends on the identities of the native species. High N deposition enhanced the competitive dominance of O. biennis over I. japonica by 15.45% but did not alter the competitive dominance of O. biennis over C. album. Furthermore, N deposition did not affect the dominance of O. biennis or A. argyi. Therefore, the species composition of the native community must be considered when preparing to resist future biological invasions. Our study contributes to a better understanding of the invasion mechanisms of alien species under N-loading conditions.

Effects of Warming, Phosphorous Deposition, and Both Treatments on the Growth and Physiology of Invasive Solidago canadensis and Native Artemisia argyi

Anthropogenic climate change and species invasion are two major threats to biodiversity, affecting the survival and distribution of many species around the world. Studying the responses of invasive species under climate change can help better understand the ecological and genetic mechanisms of their invasion. However, the effects of warming and phosphorus deposition on the phenotype of native and invasive plants are unknown. To address the problem, we applied warming (+2.03 °C), phosphorus deposition (4 g m^-2 yr^-1 NaH₂PO₄), and warming × phosphorus deposition to Solidago canadensis and Artemisia argyi to measure the direct effects of environmental changes on growth and physiology at the seedling stage. Our results reveal that the physiology parameters of A. argyi and S. canadensis did not change significantly with the external environment. Under phosphorus deposition, S. canadensis had higher plant height, root length, and total biomass compared to A. argyi. Interestingly, warming has an inhibitory effect on the growth of both A. argyi and S. canadensis, but overall, the reduction in total biomass for S. canadensis (78%) is significantly higher than A. argyi (52%). When the two plants are treated with warming combined with phosphorus deposition, the advantage gained by S. canadensis from phosphorus deposition is offset by the negative effects of warming. Therefore, under elevated phosphorus, warming has a negative effect on the invasive S. canadensis and reduces its growth advantage.

Physiology and transcriptome analysis of the response mechanism of Solidago canadensis to the nitrogen addition environment

Solidago canadensis is an invasive plant that can adapt to variable environmental conditions. To explore the molecular mechanism of the response to nitrogen (N) addition conditions in S. canadensis, physiology and transcriptome analysis were performed with samples that cultured by natural and three N level conditions. Comparative analysis detected many differentially expressed genes (DEGs), including the function of plant growth and development, photosynthesis, antioxidant, sugar metabolism and secondary metabolism pathways. Most genes encoding proteins involved in plant growth, circadian rhythm and photosynthesis were upregulated. Furthermore, secondary metabolism-related genes were specifically expressed among the different groups; for example, most DEGs related to phenol and flavonoid synthesis were downregulated in the N-level environment. Most DEGs related to diterpenoid and monoterpenoid biosynthesis were upregulated. In addition, many physiological responses, such as antioxidant enzyme activities and chlorophyll and soluble sugar contents, were elevated by the N environment, which was consistent with the gene expression levels in each group. Collectively, our observations indicated that S. canadensis may be promoted by N deposition conditions with the alteration of plant growth, secondary metabolism and physiological accumulation.

Functional traits explain the consistent resistance of biodiversity to plant invasion under nitrogen enrichment

Elton's biotic resistance hypothesis, which posits that diverse communities should be more resistant to biological invasions, has received considerable experimental support. However, it remains unclear whether such a negative diversity-invasibility relationship would persist under anthropogenic environmental change. By using the common ragweed (Ambrosia artemisiifolia) as a model invader, our 4-year grassland experiment demonstrated consistently negative relationships between resident species diversity and community invasibility, irrespective of nitrogen addition, a result further supported by a meta-analysis. Importantly, our experiment showed that plant diversity consistently resisted invasion simultaneously through increased resident biomass, increased trait dissimilarity among residents, and increased community-weighted means of resource-conservative traits that strongly resist invasion, pointing to the importance of both trait complementarity and sampling effects for invasion resistance even under resource enrichment. Our study provides unique evidence that considering species' functional traits can help further our understanding of biotic resistance to biological invasions in a changing environment.

Competitive trait hierarchies of native communities and invasive propagule pressure consistently predict invasion success during grassland establishment

Invasive non-native plants challenge ecosystems restoration, and understanding the factors that determine the establishment of invasive plants is crucial to improve restoration outcomes. However, the drivers of invasibility of plant communities are not sufficiently clear, and combined effects are not understood. Therefore, we investigated the contribution of the main drivers of invasion success during early phases of restoration, i.e., biotic resistance, invasive propagule pressure, and environmental fluctuations. We compared the contribution of these drivers in a series of mesocosms experiments using designed grasslands as a model system, and Solidago gigantea as invasive model species. Two grassland communities were designed according to competitive trait hierarchies with different sowing patterns, reflecting variation in biotic resistance. We then manipulated invader propagule pressure and applied different scenarios of environmental fluctuation, i.e., flood, heat, and N fertilization. Invasive biomass was considered as proxy for invasion success, while native biomass represented restoration success. There were consistent effects of biotic resistance to S. gigantea invasion via competitive trait hierarchies in the three experiments. Communities dominated by species with high-competition traits were more resistant regardless of environmental fluctuation. Clumped seeding of the native community reduced invasibility, whereas high non-native propagule density increased invasion. The effects of environmental fluctuation were less consistent and context-dependent, thus playing a secondary role when compared to biotic drivers of invasion. Restoration initiatives on grasslands impacted by invasive plants should consider biotic resistance of the restored community as a key driver and the importance of controlling further arrivals of invasive species during community assembly.

Clonal integration and phosphorus management under light heterogeneity facilitate the growth and diversity of understory vegetation and soil fungal communities

The spatial heterogeneity of light and nutrient deficiency occurs in many forest understories. Proper fertilization management of unhealthy forests can benefit forest understory diversity and improve the stability of degraded soil; and clonal integration is a major advantage of resource sharing for many forest understory vegetation, such as pteridophytes. In this study, we tested whether understory soil fertilization and clonal integration under light heterogeneity were able to increase the performance and diversity of understory vegetation and soil microbial communities in nature. Field experiments-with or without phosphorus (P) addition, with intact or severed rhizome, and under homogeneous or heterogeneous light environments-were conducted in the understory of a typical evergreen forest in southeast China. Light heterogeneity, P addition and clonal integration promoted the growth, diversity and evenness of ferns and soil microbial biomass C, N and P (MBC, MBN and MBP) at both experimental plot and patch level. They also increased Chao1 richness and Shannon diversity of soil fungal communities at patch level, especially in the high light patches with P addition. The positive effects of P addition and clonal integration on the growth and diversity of ferns and soil microbial biomass were greatly increased under heterogeneous light. The positive effects of clonal integration on the growth were the greatest in the heterogeneous high light patches. Moreover, the interactive effect of P addition and clonal integration increased soil MBN and MBP. Clonal integration promoted the increased growth and diversity of ferns and soil MBC in the heterogeneous light environment (9.35%-35.19%), and enhanced soil MBN and MBP in the P addition treatment (9.03%-12.96%). The interactive effect of P addition and clonal integration largely led to the transition of fungal groups from slow-growing oligotrophic types to fast-growing copiotrophic types. Our results show that the interactions between clonal integration and/or P addition under light heterogeneity increase the benefits of ferns in light-rich patches, and further promote integrative performance of ferns and soil microbial communities.

The effects of Bidens alba invasion on soil bacterial communities across different coastal ecosystem land-use types in southern China

Environments in both biotic and abiotic ecosystems have been affected by the colonization of non-native flora. In this study, we examined the effect of Bidens alba invasion on different land-use types along a coastline in southern China. Bacterial communities in each site were determined using 16S rDNA sequencing, and soil physicochemical properties were analyzed using standard methods. Although our results indicated that B. alba invasion did not have a significant effect on the alpha diversity of bacteria, it caused significant differences in soil bacterial community composition between invaded and uninvaded soil across different land-use types. Beta diversity and several physicochemical properties in forest, orchard and waterfront environments were recorded to be more susceptible to B. alba invasion. A high proportion of the variation of bacterial communities can be explained by a combination of environmental variables, indicating that environmental selection rather than plant invasion is a more effective process in coastal microbial assemblages. By comparing topological roles of shared OTUs among invaded and uninvaded soil, keystone taxa in invaded soil were identified. Acidobacteria was the major phyla involved in the invasive process which could be driven by environmental selection. How key phyla react in our experiment should be verified by further studies.

Effects of soil nutrient variability and competitor identify on growth and co-existence among invasive alien and native clonal plants

Changes in soil nutrients variability could significantly interact with other global change processes (such as community dynamics, biological invasion). Global exchange and accumulation of alien species caused environmental and economic threats in the introduced ranges. Their invasion success or not in local plant communities is largely depended on the interactions and competitive outcomes with other species and environmental conditions. Here, we tested whether the interactions of nutrient variability and competitor identity influence plant performance, potential invasion success of invasive species and their co-existence with native species. In both greenhouse and field experiment, we subjected three congeneric and naturally co-occurring pairs of invasive alien and native clonal plants in China to different nutrient variability (constant high, multiple pulses and/or single pulse) and competitor identity (intra-specific competitors, native competitors, invasive competitors and both native & invasive competitors). Our results showed that total biomass or the increase of cover of invasive species was significantly larger than those of the native species regardless of competitor identity. Native competitors significantly decreased biomass proportion of native species, but did not affect that of invasive species. The whole community with invasive target species accumulated more total biomass than with native species under multiple pulses nutrient when with the native competitors. Invasive species produced significantly higher biomass proportion than natives under all competitor identity treatments except for native & invasive competitors. Multiple mixed competitors (i.e. native & invasive competitors) decreased the plant performance and dominance of invasive target species, to some extent, thus construction of multi-species competition might facilitate coexistence of native and invasive species in communities. Interactions between native competitors or native & invasive competitors, and nutrient variability play important roles in plant performance and potential invasion success in communities. Multiple invasional interference may have significant implications for the co-existence of invasive and native species, and for management of invasive species.

Warming alone increased exotic snail reproduction and together with eutrophication influenced snail growth in native wetlands but did not impact plants

Warming and eutrophication can have varying effects on exotic species performance and their interactions. These effects can vary with trophic level, but are rarely investigated simultaneously on exotic species from multiple trophic levels. To address this, we manipulated temperature, nutrients, and plant origin (native vs. exotic) in snail invaded wetland communities. Warming increased exotic apple snail (Pomacea maculata) reproduction (4-fold increase in egg mass) and also number of egg clutches produced while warming slowed exotic snail growth, suggesting a trade-off between reproduction and growth in exotic snails influenced by warming and nutrients. However, exotic snail size varied with warming and nutrients. Additionally, warming reduced native plant mass with no effect on exotic plants while nutrients had greater positive effects on exotic plants biomass. In combination warming and nutrient enrichment will likely increase exotic snail growth, while nutrient enrichment alone will contribute to exotic plant dominance. In conclusion, the individual and interactive effects of warming and eutrophication vary with the trophic level of exotic species with trade-offs in exotic herbivores depending on environmental conditions, making it difficult to predict effects of multiple anthropogenic factors on co-occurring exotic plants and their effects on native communities.