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1. Elton's diversity-invasibility hypothesis predicts that diverse communities are more resistant against alien invaders. However, observational studies frequently find positive relationships between the numbers of alien and native species. It has been suggested, but rarely tested, that environmental heterogeneity may cause such positive relationships.2. Here, we experimentally tested the effects of soil heterogeneity and diversity (species richness) on the invasibility of native communities. We first filled mesocosm pots with either a heterogeneous soil, consisting of one patch of sand, one patch of peat and one patch of an equal mixture of both substrates, or a homogeneous soil, consisting of the mixture only. Then, we planted those pots with 29 native communities consisting of one, four or eight species, and invaded them with populations of four individuals of one of five alien species.3. In the heterogeneous soils, individual alien plants benefited more strongly from the resource-rich peat soil than the native communities did. Moreover, in the mixture soil of the heterogeneous treatment, individual alien plant over-proportionally produced more biomass than in the mixture soil of the homogeneous treatment. Consequently, the populations of naturalized alien plants in each pot benefited from heterogeneous soil conditions, and this tended to be particularly the case when a native community was present. The native communities did not respond to soil heterogeneity, but they had a negative effect on the naturalized plants, irrespective of their diversity.4. Synthesis. Our results indicate that soil heterogeneity might alleviate the competitive effects of native communities on the alien invaders, as the latter took more advantage of the high resource patches than the natives did. The beneficial effect of heterogeneity on invasion success could thus explain why observational studies usually find positive relationships between the numbers of alien and native species. published

Repeated exposure to submergence and de-submergence may induce acclimation in plants growing in riparian areas. However, the effect of each consecutive submergence and de-submergence event has not been evaluated separately. We subjected a riparian species Alternanthera philoxeroides to two different fluctuation frequencies: low fluctuation frequency (LFF) and high fluctuation frequency (HFF). Consecutive submergence and de-submergence had comparable negative effects on growth of A. philoxeroides, while they respectively down- and up-regulated photosynthetic electron transport in both LFF and HFF. The submergence effects on growth were significantly smaller in the 2nd cycle than in the 1 st cycle of LFF, suggesting reduced tissue loss in the 2nd cycle as a result of acclimation. In HFF, the growth of A. philoxeroides was more strongly suppressed than in LFF. During de-submergence, biomass increased in both control and de-submerged plants in LFF, whereas growth recovery was not always seen in HFF. At the end of the experiment, the treatment plants in HFF had only ∼50% biomass of the corresponding plants in LFF. Although HFF enhances tissue loss during submergence and thus impairs growth recovery more strongly during de-submergence than LFF, both LFF and HFF induced photosynthetic, photoprotective or growth acclimation in A. philoxeroides.

Responses to both submergence and de-submergence are important for evaluating flood tolerance of plants. Clonal integration (resource translocation between connected ramets within clones) has been shown to increase flood tolerance of amphibious clonal plants. However, no study has truly separated the effects of clonal integration during de-submergence from those during submergence. We grew 40 clonal fragments of the amphibious plant species Alternanthera philoxeroides, each consisting of two ramet systems, under half-submerged conditions (one ramet system submerged and the other un-submerged) for 30 d and then de-submerged for 20 d. To evaluate the effects of clonal integration during submergence, stolon connection between the two ramet systems was either severed or not on day 1, and 10 replicates of the fragments of each treatment were harvested on day 30. To evaluate the effects of clonal integration after de-submergence, the remaining 20 clonal fragments with connected stolons during submergence were de-submerged; half of them were subjected to stolon severing on day 31 and the remaining 10 fragments were not. All the 20 fragments for de-submergence were harvested on day 50. After 30 d of submergence, stolon connection between the submerged and the un-submerged ramets significantly increased growth, biomass allocation to roots and photosynthetic capacities of the submerged ramets, and also increased growth and photosynthetic capacities of the un-submerged ramets. Consequently, clonal integration significantly increased growth of the apical ramets, the basal ramets and the whole fragments. However, after 20 d of de-submergence, stolon connection did not significantly affect growth or photosynthetic capacities of either de-submerged or un-submerged ramets. The results suggest that clonal integration plays different roles during submergence and de-submergence. It increases performance of submerged ramets during submergence by translocation of photosynthates from un-submerged ramets, but plays little roles during de-submergence.

Cadmium (Cd) is a hazardous environmental pollutant with high toxicity to plants, which has been detected in many wetlands. Clonal integration (resource translocation) between connected ramets of clonal plants can increase their tolerance to stress. We hypothesised that clonal integration facilitates spread of amphibious clonal plants from terrestrial to Cd-contaminated aquatic habitats. The spread of an amphibious grass Paspalum paspaloides was simulated by growing basal older ramets in uncontaminated soil connected (allowing integration) or not connected (preventing integration) to apical younger ramets of the same fragments in Cd-contaminated water. Cd contamination of apical ramets of P. paspaloides markedly decreased growth and photosynthetic capacity of the apical ramets without connection to the basal ramets, but did not decrease these properties with connection. Cd contamination did not affect growth of the basal ramets without connection to the apical ramets, but Cd contamination of 4 and 12 mg·l-1 significantly increased growth with connection. Consequently, clonal integration increased growth of the apical ramets, basal ramets and whole clones when the apical ramets were grown in Cd-contaminated water of 4 and 12 mg·l-1 . Cd was detected in the basal ramets with connection to the apical ramets, suggesting Cd could be translocated due to clonal integration. Clonal integration, most likely through translocation of photosynthates, can support P. paspaloides to spread from terrestrial to Cd-contaminated aquatic habitats. Amphibious clonal plants with a high ability for clonal integration are particularly useful for re-vegetation of degraded aquatic habitats caused by Cd contamination.

Fluctuation range and frequency are two important components of water level fluctuation, but their effects on wetland plants have not been evaluated separately. We subjected eight wetland species to a control treatment with static water level and fluctuation treatments with different ranges or frequencies to examine their effects on plant growth. Acorus calamus, Butomus umbellatus and Iris wilsonii showed high survival rates in all treatments with various fluctuation ranges and frequencies. Their survival rates were higher at the medium fluctuation frequency than at the low and high frequencies, suggesting beneficial effects of the medium frequency. In the experiment comparing the fluctuation ranges, A. calamus and I. wilsonii could maintain the capacity for asexual propagation and accumulate higher biomass compared with the control plants, while biomass of the other six species dramatically decreased. In the experiment comparing fluctuation frequency, species with relatively high survival rates (≥ 50%) maintained or increased the capacity of asexual propagation, and A. calamus and I. wilsonii allocated relatively more biomass to roots, which may enhance plant growth and survival. In contrast, these species did not show increased biomass allocation to shoots in response to both fluctuation range and frequency, presumably because shoots are prone to mechanical damage caused by streaming floodwater. Taken together, biomass accumulation in roots rather than in shoots and the ability to asexually propagate are important for the survival of these species during water fluctuation.

Plant–plant interactions change in response to environmental conditions, and riparian species are commonly influenced by flooding. This study tested whether flooding affects the intraspecific and interspecific competition of two riparian species and whether such effects depend on the topographic positions where plants have established. Seeds of the riparian species Polygonum hydropiper were collected from both low and high positions within the shoreline of the Three Gorges Reservoir. Groups of P. hydropiper seedlings from each position were either grown alone (i.e., without competition), with another group of P. hydropiper seedlings (i.e., intraspecific competition), or with a group of seedlings of the companion species Xanthium sibiricum (i.e., interspecific competition). Each group comprised six replicates. In total, 288 plants of P. hydropiper and 84 plants of X. sibiricum were selected for the experiment. Seedlings were subjected to control and flooding treatments for 60 days. Irrespective of competition type (i.e., intra- or interspecific), both flooding and competition negatively affected the growth and/or photosynthetic capacities of P. hydropiper. Flooding only interacted with competition to explain total biomass. Flooding reduced total biomass in a larger proportion in the absence of competition, and, to a lesser extent, with intraspecific competition, compared to interspecific competition. However, such interaction effects were independent of the positions where the seeds that originated from the plants were collected from. Interspecific competition significantly decreased the chlorophyll content and photosynthetic efficiency of plants, while intraspecific competition did not. In general, plants from lower positions had higher total chlorophyll content than plants from higher positions. These results suggest that flooding may regulate the population dynamics of P. hydropiper by altering its competitive interactions.

Selection pressure of flooding promotes the development of phenotypic plasticity of riparian plants; however, whether the selection effects of flooding are elevation-dependent still remains unclear. Seeds of the riparian species Polygonum hydropiper were collected from 10 different locations each at low and high elevations in the Three Gorges Reservoir Region. Plants germinated from these seeds were subjected to control and flooding treatments. Flooding significantly increased both specific leaf area (SLA) and adventitious root biomass but decreased other growth traits of all progeny. In response to flooding, plasticity of all leaf and root traits except SLA varied significantly among seed families (seedlings emerged from the same mother plant) from low elevation. However, among seed families from high elevation, plasticity of only leaf number, total leaf area, and adventitious root biomass varied significantly. For low-elevation seed families, benefits of plasticity in leaf width, leaf number, average leaf area, and leaf biomass were detected across control and flooded conditions. However, in response to flooding, significant costs of plasticity in leaf length, leaf width, average and total leaf areas were found. Therefore, selection effects of flooding impose great pressure on plasticity of leaf and root traits of low-elevation seed families, which is elevation-dependent.

Elevation can greatly affect both intensity and duration of flooding in riparian areas, and may thus affect the performance of plants distributed therein. However, few studies have examined the performance of riparian plants over elevation difference. This study compared growth and reproductive traits of populations of the riparian species Polygonum hydropiper distributed at low and high elevations of 10 different riparian locations in the hydro-fluctuation belt of the Three Gorges Reservoir Region, China. Both the location and the interaction between location and elevation had significant effects on almost all growth and reproduction traits of P. hydropiper, suggesting that the effects of elevation varied greatly among different locations. Across all locations, high-elevation plants had significantly higher total biomass, stem biomass, root biomass, and plant height than low-elevation plants. Correspondingly, high-elevation plants also had significantly higher stem starch concentrations. In comparison, low-elevation plants had significantly lower biomass, shorter height, and thinner leaves; while they had significantly higher reproductive allocation, as well as faster germination and higher germination rate at a number of locations. Therefore, low energy consumption and high reproductive capacity could be a survival adaption of low-elevation plants to frequent and deep flooding. Local conditions and/or flooding might impose strong selection pressure on P. hydropiper, thus resulting in intra-specific differentiation. Across all locations, low- and high-elevation populations exhibit different growth and reproduction responses to different flooding regimes.