Soil resource heterogeneity competitively favors an invasive clonal plant over a native one

Impacts of Soil Properties on Species Diversity and Structure in Alternanthera philoxeroides-Invaded and Native Plant Communities

Soil properties can affect plant population dynamics and the coexistence of native and invasive plants, thus potentially affecting community structure and invasion trends. However, the different impacts of soil physicochemical properties on species diversity and structure in native and invaded plant communities remain unclear. In this study, we established a total of 30 Alternanthera philoxeroides-invaded plots and 30 control plots in an area at the geographical boundary between North and South China. We compared the differences in species composition between the invaded and native plant communities, and we then used the methods of regression analysis, redundancy analysis (RDA), and canonical correspondence analysis (CCA) to examine the impacts of soil physicochemical properties on four α-diversity indices and the species distribution of these two types of communities. We found that A. philoxeroides invasion increased the difference between the importance values of dominant plant species, and the invasion coverage had a negative relationship with the soil-available potassium (R2 = 0.135; p = 0.046) and Patrick richness index (R2 = 0.322; p < 0.001). In the native communities, the species diversity was determined with soil chemical properties, the Patrick richness index, the Simpson dominance index, and the Shannon-Wiener diversity index, which all decreased with the increase in soil pH value, available potassium, organic matter, and ammonium nitrogen. However, in the invaded communities, the species diversity was determined by soil physical properties; the Pielou evenness index increased with increasing non-capillary porosity but decreased with increasing capillary porosity. The determinants of species distribution in the native communities were soil porosity and nitrate nitrogen, while the determinants in the invaded communities were soil bulk density and available potassium. In addition, compared with the native communities, the clustering degree of species distribution in the invaded communities intensified. Our study indicates that species diversity and distribution have significant heterogeneous responses to soil physicochemical properties between A. philoxeroides-invaded and native plant communities. Thus, we need to intensify the monitoring of soil properties in invaded habitats and conduct biotic replacement strategies based on the heterogeneous responses of native and invaded communities to effectively prevent the biotic homogenization that is caused by plant invasions under environmental changes.

Soil cadmium pollution facilitated the invasion of alligator weed through enhanced herbivore resistance and competitive ability over a congeneric species

A number of invasive plant species, such as Alternanthera philoxeroides, have been documented to be able to accumulate trace metal elements in their tissues. Since metal accumulation in plants can serve as a defence against herbivores, we hypothesized that metal pollution will increase herbivore resistance of metal-accumulating invasive plant species and such a benefit will grant them a competitive advantage over local co-occurring plants. In this study, we compared the differences in plant growth and herbivore feeding preference between A. philoxeroides and its native congener Alternanthera sessilis in single and mixed cultures with and without soil cadmium (Cd) pollution. The results showed that A. philoxeroides plants were more tolerant to Cd stress and accumulated more Cd in the leaves than A. sessilis. Cd exposure increased the resistance of A. philoxeroides against a specialist and a generalist herbivore compared with A. sessilis. Competition experiments indicated that Cd stress largely increased the competitive advantage of A. philoxeroides over A. sessilis with or without herbivore pressures. The differences in herbivore resistance between the two plant species under soil Cd stress are most likely due to the deterring effect of Cd accumulation and Cd-enhanced mechanical defences rather than changes in leaf specialized metabolites.

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.

Polyploidization-enhanced effective clonal reproduction endows the successful invasion of Solidago canadensis

Clonality and ploidy levels are positively associated with plant invasiveness. However, there is still no consensus on whether polyploidization can promote the invasion of alien plants by enhancing clonality. Our recent long-term community succession study found that the more vigorous clone of introduced polyploid Solidago canadensis succeeded into mono-dominant community, which seems to be a positive correlationship between polyploidization and clonal reproduction. However, the formation process of clonal ramet and how polyploidization improves the clonal reproduction of S. canadensis remains unknown. Here, we compared clonal growth ability among diploids and polyploids of S. canadensis from native and introduced ranges in a common garden. Results showed that the rhizomes of S. canadensis originated from axillary buds of dense nodes at the basal stem of seedling and then produced into clonal ramets from the rhizomes. Diploids had denser nodes and more buds, developed more rhizomes per unit mass and produced more clonal propagules at the early growth stage compared with polyploids. However, the number of juvenile and secondary rhizomes, as well as the diameter and length of rhizomes in polyploid populations was significantly higher or greater than those of diploids, and those clonal traits in introduced polyploids were significantly higher than in native polyploids. Moreover, a phalanx growth form was observed in native and introduced diploid populations, which allocated about 3% and 5% of the total biomass to rhizomes, respectively, resulting in short and weak rhizomes. However, native and introduced polyploids allocated about 35% and 40%, respectively, of the total biomass to rhizomes, resulting in long and strong rhizomes, which were guerrilla growth forms. This study firstly shows that polyploidization enhanced the effective clonal reproduction of S. canadensis through pre-adaptation and rapid post-adaptation evolution, and consequently contributed to its successful invasion.

Effects of clonal integration and nutrient availability on the growth of Glechoma longituba under heterogenous light conditions

Introduction

Clonal integration of connected ramets within clones is an important ecological advantage. In this study, we tested the hypothesis that the effects of clonal integration on performance of donor and recipient ramets when one resource is heterogeneous can be influenced by the availability of another resource of donor ramets.

Methods

We conducted a greenhouse experiment on the widespread, perennial herb Glechoma longituba. Clonal fragments consisting of pairs of connected ramets were grown for seven weeks. The younger, apical ramets were exposed under 30% or 100% light condition and the older, basal ramets were treated with three levels of nutrients. The connections between ramets were either severed or left intact. 30% light condition negatively affected the growth of apical ramets, basal ramets and the whole fragments.

Results

Clonal integration significantly increased the growth of apical ramets, but decreased the growth of the basal ramets. Medium and high level nutrient availability of basal ramets significantly increased the growth of apical ramets, basal ramets and the whole fragments. At the high nutrient level, the reduction in growth of basal ramets from clonal integration was decreased, but the growth responses of apical ramets and the whole fragments to clonal integration were not influenced by nutrient availability.

Conclusion

The results suggested that clonal integration was benefit to the growth of apical ramets of Glechoma longituba but at the cost of reducing the growth of basal ramets. Although the high nutrient level could reduce the cost that clonal integration brought to the unshaded basal ramets, but could not increase the benefit that clonal integration brought to the shaded apical ramets and whole fragment.

Clonal integration promotes the growth of Phragmites australis populations in saline wetlands of the Yellow River Delta

Estuarine wetlands are highly heterogeneous due to strong interactions between freshwater input and seawater intrusion. However, little is known about how clonal plant populations adapt to heterogeneous salinity in soil environments. In the present study, the effects of clonal integration on Phragmites australis populations under salinity heterogeneity were studied using field experiments with 10 treatments in the Yellow River Delta. Clonal integration significantly increased plant height, aboveground biomass, underground biomass, root-shoot ratio, intercellular CO₂ concentration, net photosynthetic rate, stomatal conductance, transpiration rate, and stem Na⁺ content under homogeneous treatment. Under the heterogeneous salt treatment, clonal integration significantly affected total aboveground and underground biomass, photosynthetic traits, and stem Na⁺ content under different salt gradients. The increase in salt concentration inhibited the physiological activity and growth of P. australis to varying degrees. Compared with the heterogeneous saline environment, clonal integration was more beneficial to P. australis populations in the homogeneous saline habitat. The results of the present study suggest that P. australis prefers homogeneous saline habitats; however, plants can adapt to heterogeneous salinity conditions via clonal integration.

Advantages of growth and competitive ability of the invasive plant Solanum rostratum over two co-occurring natives and the effects of nitrogen levels and forms

Introduction

Atmospheric nitrogen (N) deposition has often been considered as a driver of exotic plant invasions. However, most related studies focused on the effects of soil N levels, and few on those of N forms, and few related studies were conducted in the fields.

Methods

In this study, we grew Solanum rostratum, a notorious invader in arid/semi-arid and barren habitats, and two coexisting native plants Leymus chinensis and Agropyron cristatum in mono- and mixed cultures in the fields in Baicheng, northeast China, and investigated the effects of N levels and forms on the invasiveness of S. rostratum.

Results

Compared with the two native plants, S. rostratum had higher aboveground and total biomass in both mono- and mixed monocultures under all N treatments, and higher competitive ability under almost all N treatments. N addition enhanced the growth and competitive advantage of the invader under most conditions, and facilitated invasion success of S. rostratum. The growth and competitive ability of the invader were higher under low nitrate relative to low ammonium treatment. The advantages of the invader were associated with its higher total leaf area and lower root to shoot ratio compared with the two native plants. The invader also had a higher light-saturated photosynthetic rate than the two native plants in mixed culture (not significant under high nitrate condition), but not in monoculture.

Discussion

Our results indicated that N (especially nitrate) deposition may also promote invasion of exotic plants in arid/semi-arid and barren habitats, and the effects of N forms and interspecific competition need to be taken into consideration when studying the effects of N deposition on invasion of exotic plants.

Effects of Clonal Integration on Foraging Behavior of Three Clonal Plants in Heterogeneous Soil Environments

Environments are ubiquitously heterogeneous in nature, and clonal plants commonly benefit from both clonal integration and foraging responses in heterogeneous environments. While many studies have examined clonal integration and foraging responses separately, few have tested the effect of clonal integration on the foraging response of clonal plants to environmental heterogeneity. We grew offspring ramets of each of three clonal plants (Hydrocotyle vulgaris, Duchesnea indica, and Glechoma longituba) in both homogeneous and heterogenous soil environments and severed their stem connection to a mother ramet (to prevent clonal integration from the mother ramet) or kept it intact (to allow clonal integration). Without clonal integration from the mother ramet, soil heterogeneity had no effect on biomass or number of ramets for any of the three species. With clonal integration, soil heterogeneity also had no effect on biomass or number of ramets of D. indica and G. longituba, but significantly decreased biomass and marginally significantly decreased number of ramets of H. vulgaris. Without clonal integration, offspring ramets did not demonstrate either shoot or root foraging responses in terms of total, shoot and root biomass and ramet number in the heterogeneous soil environment in any of the three species. With integration, offspring ramets of H. vulgaris also did not demonstrate either root or shoot foraging responses, but offspring ramets of G. longituba demonstrated both root and shoot foraging responses, and those of D. indica demonstrated a root foraging response when they grew in the heterogeneous soil environment. We conclude that clonal integration can alter the foraging response of clonal plants, but this effect is species-specific. Our results also suggest that foraging responses of clonal plants in heterogeneous soil environments may not necessarily benefit the growth of clonal plants.

Effect of soil spatial configuration on Trifolium repens varies with resource amount

Soil spatial heterogeneity involves nutrients being patchily distributed at a range of scales and is prevalent in natural habitats. However, little is known about the effect of soil spatial configurations at the small scale on plant foraging behavior and plant growth under different resource amounts. Here, we experimentally investigated how a stoloniferous species, Trifolium repens, responded to varied resource amounts and spatial configuration combinations. Plant foraging behavior (i.e., the orientation of the primary stolon, mean length of the primary stolon, foraging precision, and foraging scale) and plant growth (i.e., total biomass, root biomass, shoot biomass, and root/shoot) were compared among differently designed configurations of soil resources in different amounts. The relationships of foraging behavior and plant biomass were analyzed. The results showed that the effect of the spatial configuration of soil resources on Trifolium repens depended on the resource amount. Specifically, when the total resource amount was low, fragmented soil patches promoted root foraging and increased Trifolium repens plant biomass; however, when the total resource amount was high, the soil spatial configuration did not affect foraging behavior or plant growth. Our results also showed that plant growth was facilitated by root foraging scale to adapt to low resource amounts. We conclude that the spatial configuration of soil resources at small scales affects whole plant growth, which is mediated by a distinct foraging strategy. These findings contribute to a better understanding of how the growth strategy of clonal plants responds to heterogeneous environments caused by different resource amounts and its spatial configurations.

Soil heterogeneity in the horizontal distribution of microplastics influences productivity and species composition of plant communities

Contamination of soils by microplastics can have profound ecological impacts on terrestrial ecosystems and has received increasing attention. However, few studies have considered the impacts of soil microplastics on plant communities and none has tested the impacts of spatial heterogeneity in the horizontal distribution of microplastics in the soil on plant communities. We grew experimental plant communities in soils with either a homogeneous or a heterogeneous distribution of each of six common microplastics, i.e., polystyrene foam (EPS), polyethylene fiber (PET), polyethylene bead (HDPE), polypropylene fiber (PP), polylactic bead (PLA) and polyamide bead (PA6). The heterogeneous treatment consisted of two soil patches without microplastics and two with a higher (0.2%) concentration of microplastics, and the homogeneous treatment consisted of four patches all with a lower (0.1%) concentration of microplastics. Thus, the total amounts of microplastics in the soils were exactly the same in the two treatments. Total and root biomass of the plant communities were significantly higher in the homogeneous than in the heterogeneous treatment when the microplastic was PET and PP, smaller when it was PLA, but not different when it was EPS, HDPE or PA6. In the heterogeneous treatment, total and root biomass were significantly smaller in the patches with than without microplastics when the microplastic was EPS, but greater when the microplastic was PET or PP. Additionally, in the heterogeneous treatment, root biomass was significantly smaller in the patches with than without microplastics when the microplastic was HDPE, and shoot biomass was also significantly smaller when the microplastic was EPS or PET. The heterogeneous distribution of EPS in the soil significantly decreased community evenness, but the heterogeneous distribution of PET increased it. We conclude that soil heterogeneity in the horizontal distribution of microplastics can influence productivity and species composition of plant communities, but such an effect varies depending on microplastic chemical composition (types) and morphology (shapes).

Effects of Soil Nutrient Heterogeneity on the Growth and Invasion Success of Alien Plants: A Multi-Species Study

Spatial heterogeneity in soil nutrient availability can influence performance of invasive plant species under competition-free environments. However, little was known about whether invasive plants perform better under heterogeneous than under homogeneous soil nutrient conditions in competition with native plant communities. We conducted a multi-species greenhouse experiment to test the effect of soil nutrient heterogeneity on the growth and invasion success of alien plants in a native plant community. We grew ten alien invasive plant species that are common in China under a homogeneous or heterogeneous environment alone or together with a community consisting of six native plant species from China. Compared with the homogeneous soil condition, the heterogeneous soil condition significantly increased aboveground biomass of the invasive plants. However, soil nutrient heterogeneity did not affect the relative abundance of the invasive species, as measured by the ratio of aboveground biomass of the invasive species to total aboveground biomass of the whole community. There were no significant interactive effects of soil nutrient heterogeneity and competition from the native community on aboveground biomass of the invasive plants and also no significant effects of soil nutrient heterogeneity on its relative abundance. Our results indicate that soil nutrient heterogeneity has a positive effect on the growth of invasive plants in general, but do not support the idea that soil nutrient heterogeneity favors the invasion success of exotic plant species in native plant communities.