Fluctuated water depth with high nutrient concentrations promote the invasiveness of Wedelia trilobata in Wetland

Abstract The distribution of invasive and native species in wetlands is determined by hydrological conditions; whereas conditions such as water depth fluctuations, variations in the nutrient concentrations are expected to affect the growth and physiological traits of plants. For the assessment of such effects, we conduct greenhouse experiment with three factors; 1) water depth of 5 cm and 15 cm (static and fluctuated); 2) three levels of nutrient concentrations (i) full‐strength Hoagland solution (N1), (ii) ¼‐strength Hoagland solution (N2), and (iii) 1/8‐strength Hoagland solution (N3); and 3) species, invasive Wedelia trilobata (L.) and its congener, native Wedelia chinensis (Osbeck.) under mono and mixed culture. Water depth of 5 cm combined with any of the nutrient treatments significantly restrained the photosynthesis, intracellular CO2 concentration and leaf chlorophyll of both W. trilobata and W. chinensis. Increase in the water depth to 15 cm with low‐nutrient treatment N3 did not sustain the physiological traits of W. chinensis under mono and mixed planting. A great loss was noted in the growth of W. chinensis at 15 cm static and fluctuated water depth with low‐nutrient treatment (N3) and under mixed culture. In addition, water depth fluctuations with both low‐ and high‐nutrient treatments significantly affected the root‐shoot ratio, relative growth rate, and interspecific interaction among these two species. W. trilobata benefited more from competitive interaction index (CII) under fluctuated water depth at 15 cm with high nutrients, and the value of CII was clearly positive. Therefore, higher competitive ability may contribute to the invasiveness of W. trilobata in wetlands.

Waterlogging or change in the water depth resulting from hydrological turbulences is a common stress for plants in wetland ecosystems (McGOWAN et al., 2011;Yuan, Yang, Liu, & Wang, 2017). The climate change is a strong reason for the hydrological disturbances such as tidal variation, flooding, and severe drought. It inevitably imposes stress on plant communities and affects the performance of plants in various transitional terrestrial and amphibious ecosystems (Wang & Li, 2016;Wright, Hornbach, McHugh, & Mann, 2015).
Stress gradient hypothesis supported these studies, which used to predict the competition effect among the neighboring plants along the gradients of environmental or hydrological disturbance (Lortie & Callaway, 2006;Smit, Rietkerk, & Wassen, 2009;Yue et al., 2019).
In wetland ecosystem, it is difficult to maintain water depth at constant level because of fluctuations in water levels. However, hydrological disturbances such as variations in the water depth change the nutrients and affect the interspecific interactions among plants.
The increased water depth increases the loss of nutrients and damage the plant tissues (Sasikala, Tanaka, & Jinadasa, 2008). change in water depth and nutrient levels mainly affect the net photosynthetic rate (PN), stomal conductance (gs), chlorophyll (Chl), transpiration rate (E), carbohydrate availability, and growth rate in many wetland plants (Colmer & Voesenek, 2009;Zhou et al., 2018). There are also some conflicting views about the waterlogging-tolerant plants that can maintain their photosynthetic activity better, conserve energy and generate new tissues after reemergence of flooding (Bailey-Serres & Voesenek, 2008;Nielsen, Podnar, Watts, & Wilson, 2013). Chen et al. (2013) found that invasive Alternanthera philoxeroides was more tolerant to fluctuating water depth than their native, congeneric Alternanthera sessilis in wetland and showed better photosynthetic capacity. Similar results were noted by (Xiao et al., 2010)  W. trilobata is native to tropical region of south America (Wang et al., 2012;Weber, Sun, & Li, 2008) and is a clonal evergreen creeping herb which was also found in southern region of China in 1970s on a large scale Dai, Fu, Wan, et al., 2016b;Qi et al., 2014). In China, initially it was introduced as a groundcover species but later it spread rapidly from garden to roadsides and then to the agricultural fields (Song, Chow, Sun, Li, & Peng, 2010;Talukdar & Talukdar, 2013). While, W. chinensis the native congener of W. trilobata is mostly used as medicinal plant. The growth rate of W. chinensis is very slow as compared to W. trilobata (Dai, Fu, Qi, et al., 2016a;Si et al., 2013). According to results of previous studies related to invasion of W. trilobata in different environments, it is hypothesized that W. trilobata invade successfully in highly fluctuating water depth with high nutrient concentrations. Therefore, we selected W. trilobata and its native congener W. chinensis to examine (1) whether the water depth and nutrient fluctuations affect the growth and photosynthetic capacity of invasive species W. trilobata more or less than its native congener W. chinensis in mono and mixed culture? and (2) how the interspecific interactions among these species are affected by water depth and nutrient fluctuation in mixed culture? Results of this study will be helpful in the management of invasive species in wetlands based on competitive interaction, growth, and physiological responses.

| Site and plant material
The ramets of W. trilobata and W. chinensis were collected from the single location of Jiangsu University (32.20°N, 119.45°E), Zhenjiang, Jiangsu, P. R. China in early March 2019. For each species, 450 ramets were collected and then cultured in a sand medium. These were watered daily with tap water in order to adapt them to the greenhouse conditions (located at Jiangsu University, Zhenjiang, China). Seven-days later, 360 ramets of each species with strong vivacity were selected for the following experiment.
Before transplanting of the ramets, the average ramet height and weight were recorded which were 11.21 cm and 1.77 g, respectively, for W. chinensis, and 13.76 cm and 2.46 g, respectively, for W. trilobata. The plants were divided into two groups as monoculture and mixed culture. In monoculture, two-ramets of each species per pot, while in mixed culture total two-ramets of W. trilobata and W. chinensis were planted (one ramet for each species) in an internal pot (17.7 cm diameter × 12 cm height). These pots were filled with clean washed sand and then placed to outer pots of 28 cm diameter and 35 cm height.

| Relative growth rate
The relative growth rate (RGR) within the species under treatments was calculated by following (Pérez-Harguindeguy et al., 2013).
Here; Ws2 is the weight of seedling at harvest, Ws1 is the weight of seedling at start the of stress treatments, Ts2 is harvest time and Ts1 is time of start of the experiment.

| Root-shoot ratios
It was computed by the following equation; where RSR is the root-shoot ratio, DWR is the dry weight of root, and DWS is the dry weight of shoot.

| Competitive interaction index
Competitive interaction index (CII) responses of both invasive W. trilobata and its native W. chinensis under mono and mixed culture were calculated based on the total dry weight of a plant. The CII is appropriate for evaluating interactions among two species either positive or negative. Through CII, the performance of each species can be compared when grown in mixed culture to its performance in monoculture. The CII was calculated by using the following equations (Armas, Ordiales, & Pugnaire, 2004;Liu, Yang, & Zhu, 2018): where A is the total dry weight of a plant, while x and y represent to both the species separately, Ax shows the total dry weight of species x (invasive W. trilobata) when grown alone, and Ay is the total dry weight of species y (native W. chinensis) when is grown alone. Axy is the total dry weight of species x when grown with species y, and Ayx is the total dry of species y when is grown with species x.

| Physiological parameters
Physiological parameters selected for measurements were as follows: PN: net photosynthetic rate; g S : stomatal conductance; and Ci: intercellular CO 2 concentration. PN, g S , and Ci were measured by using a portable LI-6400XT, Lincoln, USA photosynthesis measurement system. All these data were recorded during full-sunshine at 9:30-11:30 a.m. once a week during treatments. The following settings were noted during data collection: Photosynthetic active radiation (PAR) was 800 μmol/m 2 s −1 , temperature 28°C and CO 2 concentration were 500 μmol/mol. Leaves were selected from five plants per group of treatment for the measurements.

| Leaf chlorophyll measurement
Plant chlorophyll meter, Oakoch OK-Y104, made in China, was used to measure the leaf chlorophyll (Chl) contents (SPAD). The Chl was noted from the same leaves which were used for photosynthetic measurements. (1)

| Statistical analysis
All measurements were examined statistically through SPSS 17 software (SPSS Inc., IL, USA). Three-way analysis of variance (ANOVA) was used for the data analysis to discriminate the effects of species, water depth fluctuations, different nutrient treatments and their interaction on physiological characteristics and plant growth properties under mono and mixed culture. The Tukey test was applied to determine the differences at 5% significance level (p ≤ .05) between means (n = 5).

| Biomass, biomass allocation, and growth traits
W. trilobata and W. chinensis responded differently to different nutrient concentrations and water depth fluctuations under mono and mixed culture. In this study, based on ANOVA results (Table 1), both under the mono and mixed culture, FWS, DWS, FWR, and DWR were significantly affected by water depth, nutrient levels and the interaction between them (WD × N × S). The differences were significant among water depth and nutrient concentrations (F = 42.09, p < .001), among water depth and species (F = 47.82, p < .001), among nutrients and species (F = 16.45, p < .001), and interaction between all of them (F = 35.64, p < .001), respectively, for TFW of both species. Consequently, TDBm of both species were also significant in the interaction between water depth, nutrients, and species (F = 50.33, p < .001). Therefore, it was reflected from the results that water depth and nutrient levels affected the TFW and TDBm of W. trilobata at 15 cm static water depth with N2 and N3 nutrient concentrations under mixed culture, but it did not affect TFW and TDBm at 5 cm static and fluctuated water depth as compared to W. chinensis under monoculture as well as in mixed culture ( Table 1).
The TDBm was higher at 5FN1 for both the species under mono and mixed culture. In the mono and mixed culture, TDBm of native W. chinensis was decreased by increasing water depth to 15 cm and decreasing the nutrients concentration (N2 and N3) (Figure 1a and b). However, W. trilobata had higher TDBmat 5 cm static and fluctuated water depth (5S and 5F) with all levels of nutrient concentrations (N1, N2, and N3) in monoculture. Followed by mixed culture at 15 cm fluctuated water depth (15F) with nutrient concentrations of N1 and N2, W. trilobata also showed higher TDBmthan W. chinensis.
Moreover, biomass as DWS, DWR, and TDBm of a plant for both species were also affected by water depth and nutrients fluctuations (Table 1; Figure 1).
The tested plants showed a significant difference in growth traits like as RSR under both mono and mixed planting. Significant difference was recorded between water depth and species (F = 3.39, p < .005), while the differences were nonsignificant among water depth and nutrients (F = 2.10, p = .060), between species and nutrients (F = 2.35, p = .036). Afterward, RSR was also noted nonsignificant (F = 0.68, p = .086) in the interaction among water depth, nutrients, and species (Table 1). The values of RSR were decreased with increasing water depth in case of native W. chinensis under mixed culture as well as in monoculture. While, in case of W. trilobata, the RSR was sustained at 5S and 5F with N1, N2, and N3, followed by at 15F fluctuated water depth with N1 and N2 nutrient levels, respectively, under both mono and mixed culture ( Figure 2).
The effect of nutrients and water depth fluctuations of both the species appeared as necrosis and lost in their RGR at high water depth of 15S with low nutrient concentration of N3 (Figure 3). In case of W. trilobata, RGR was noted high at 5S and 5F water depth with N1, N2, and N3 nutrients concentration under both mono and mixed culture. Consequently, W. trilobata also showed survival and high RGR at 15S and 15F water depth fluctuations with N1 and N2 nutrient concentrations. Under low nutrient concentrations N3, the RGR of W. trilobata also decreased under mixed culture (Figure 3).

| Physiological traits
In mono and mixed culture, ANOVA showed significant differences among all the treatments, species and their interactions except for the results of gs (Table 2). Physiological responses including PN and gs were lower in native W. chinensis under both the mono and mixed culture than in its invasive congener W. trilobata (Table 2; Figure 4a and b). Generally, the PN of both W. trilobata and W. chinensis was decreased with increasing water depth and by lowering the nutrients concentrations. However, PN of W. chinensis was more affected at high water depth of 15S with very low level of nutrients N3 under both mono and mixed culture. Consequently, the highest values of PN 7.20 and 7.03 μmol (CO 2 ) m −2 s −1 were recorded for W. trilobata in monoculture, and 6.88 and 6.81 μmol (CO 2 ) m −2 s −1 at 5FN1 and 5SN1 were noted in mixed culture, respectively, than in its competitor W. chinensis at same treatment levels (Figure 4a and b).
When considering the different concentration of nutrients and water depth fluctuations, the similar pattern was noted for gs as found in PN in both W. trilobata and W. chinensis. There were significance differences in gs among water depth (F = 6.82, p < .001), nutrients (F = 5.43, p < .001), but nonsignificant among species and their interaction (Table 2). Upon treatments, in case of W. chinensis, a substantial decrease in gs was observed at 15S static water depth with low concentration of nutrients N3 under both mono and mixed culture than in its invasive competitor W. trilobata (Figure 4c (Figure 4c and d).
The intercellular CO 2 concentration (Ci) is another parameter that has been used to estimate the effects of hydrological variations on the physiology of W. trilobata and W. chinensis. The values of Ci became lower with increasing water depth and by lowering the nutrient concentrations (Figure 5a and b). Significant effect noted on Ci due to water depth (F = 1,770.56, p < .001), nutrients (F = 2,201.18, p < .001), and species (F = 272.63, p < .001). It was also recorded significant in their interaction as water depth and nutrients (WD × N), water depth and species (WD × S), nutrients and species (N × S), and water depth, nutrients and species (WD × N × S) ( Table 2). Ci was found significantly higher in W. trilobata than W. chinensis under both mono and mixed culture (Figure 5a and b).
Leaf Chl is another physiological parameter which is affected by nutrients concentrations and water depth variations. Based on the ANOVA results, the values of Chl were noted significant in water depth (F = 3,826.50, p < .001), nutrients (F = 5,514.84, p < .001), and species (F = 514.16, p < .001). It was also recorded significant in their interaction as water depth and nutrients (WD × N), water depth and species (WD × S), nutrients and species (N × S), and water depth, nutrients and species (WD × N × S) ( Table 2). W. trilobata competed well with their native competitor W. chinensis and showed higher values of Chl from low to high water depth with different nutrient concentrations, that is, at N1, N2, and N3 under both mono and mixed culture (Figure 5c and d).

| Competitive interaction index
The values of CII of W. trilobata were higher and positive than CII values of W. chinensis (Figure 6). The average values of CII for W. trilobata were TA B L E 1 Three-way ANOVA results with interactions among water depth, nutrients levels, and species for the effects of water depth fluctuations with different nutrients treatments on growth-related data for W. trilobata and W. chinensis

F I G U R E 1
Effects of water depth fluctuations with different nutrient treatments on the (a) total dry biomass in monoculture, (b) total dry biomass in mixed culture of W. trilobata and W. chinensis. Mean + SE with different letters indicate a significant difference among mono and mixed culture treatments (at p < .05) positive when grown at 5 cm water depth with low (N3) and high (N1) nutrient concentrations either the condition was static or fluctuated and were negative at the 15 cm water depth with low and high nutrient concentrations except for the treatment of 15FN1 (Table 3; Figure 6)., The mean values for W. chinensis were decreased with increasing water depth from 5 to 15 cm under both the conditions, that is, static or fluctuated; the values in all treatments were noted negative. The value at 15SN3 treatment was noted more negative, showed the sensitivity of W. chinensis in mixed planting than its competitor W. trilobata ( Figure 6).
However, the effect of water depth and nutrients on CII was noted significant. The differences were significant in CII values for both the species among the water depth (F = 0.132, p < .001), nutrients (F = 0.136, p < .001), species (F = 1.003, p < .001), and their interaction (p < .001) except the interaction between water depth and nutrients which was nonsignificant (p = 0.168) ( Table 3).

| D ISCUSS I ON
The growth of W. trilobata and W. chinensis was significantly affected by hydrological variations either by increasing the water depth or by varying the nutrient concentrations (Table 1 and 2). The RGR of W.
chinensis was decreased in mono and in mixed culture at 15S and 15F water depths with N1, N2, and N3 nutrient concentrations (Figure 4) because the 15 cm depth of water was high than the initial height of both W. trilobata and W. chinensis. Oxygen supply and carbohydrate use efficiency may reduce in this condition and in return nutrient consumption and an aerobic respiration increase. It led to an overall reduction in TFW and TDBm of both the species. Normally, under stress, the mother ramets provide nutrients to the water-stressed and nutrient-deficient seedlings through clonal integration (Gibbs & Greenway, 2003). However, in our study, at start of the experiment,  1 and 2) due to its high phenotypic plasticity. Many invasive species have an ability to survive due to their morphological adaptation to submergence stress through their elongating of shoots and roots (Wang, Chen, Liu, & Li, 2014). Previous studies specify that W. trilobata spreads rapidly through the elongation of clonal ramets (Dai, Fu, Qi, et al., 2016a;Qi et al., 2014;Si et al., 2014), and their fast dispersion through vegetative propagation is one of the fundamental factors for the survival and successful invasion of W. trilobata in worst conditions (Song et al., 2010;Wu, Hu, & Chen, 2005).
Hydrological fluctuations cause plants to suffer from oxygen deficiency, resultantly acetaldehyde is produced and this damages the plant growth (Luo et al., 2012;Steffens, Steffen-Heins, & Sauter, 2013). However, the hydrological fluctuations such as water depth variation and different nutrient concentrations have great effect on the growth of the studied invasive species in mono and mixed culture and its native congener W. chinensis (Table 1 and 2). In this, the fluctuation up to 5 cm range was low to affect the growth rate of W. trilobata and W. chinensis. Additional possible description for this effect may be that both species have capability to tolerate such fluctuations of water depth and show the compensatory growth. In some studies, it has been found that some wetland plants can stand against fluctuations up to 30 cm water depth without significant loss in the biomass (Wang, Zhang, et al., 2016a;Zhou et al., 2018). Luo, Jiang, Li, and Yu (2015) demonstrated that small variation in hydrological condition did not affect the plant's growth in wetland.
With the water depth fluctuation, there are other sources of stress that could affect the growth of target plants, such as nitrogen pulses, changed water content and low nutrients (Sun, Ding, & Ren, 2009;Wang et al., 2015). Zhang et al. (2016) suggested that increasing concentration of nutrients promoted growth of invasive Alternanthera philoxeroides and alleviated the stress of submergence. In our study, the reduction in the RSR indicated that increased water depth with low nutrients availability inhibited the root growth of the W. chinensis (Figure 2). However, the change in water depth at 5 cm did not affect the RSR and RGR under any nutrient conditions except N3, which suggested that nutrient levels also had an impact on growth of the native W. chinensis even at small water depth variations (Figures 2 and 3 Hydrological fluctuation are also has a great impact on other factors like interspecific and intraspecific interactions in the wetland ecosystem (Wang, Shi, et al., 2016b). During the interaction, there is a trade-off among plants between facilitation and competition, which means that in a suitable environment, plants show better competition and in a stressful environment exhibit more facilitation (Zhou et al., 2017). Fluctuation of water depth with nutrient treatments significantly affected the competitive interaction index between W. trilobata and W. chinensis (Table 3; Figure 6). W. trilobata benefited more from the competitive inter- regeneration of ribulose-1,5-disphosphate to promote the growth.
These results agree with the preceding findings that invasive plants have significantly higher values of photosynthetic traits than native plants (Godoy et al., 2011;Kleunen, Weber, et al., 2010b). Similarly, Chen et al. (2013) found that invasive plants have high growth rate than native plants because of its better photosynthetic ability, and (Geng et al., 2006) noticed that invasive plants can showed higher competitive ability in both low water and waterlogged conditions. Therefore, flexibility, more competitive ability and high photosynthetic capacity may contribute to the invasiveness of W. trilobata under high water depth with high nutrient concentration in wetland ecosystem.

| CON CLUS IONS
Invasive W. trilobata exhibited an escape strategy in order to adjust in high water depth with nutrient poor environment. While, W. chinensis presented lower values of physiological traits, with greater loss in the biomass. W. chinensis showed sensitivity because of its lower plasticity and poor competitive ability to water depth at 15 cm with all levels of nutrients except N1 under both mono and mixed culture. Higher tolerance of W. trilobata to water depth at 15 cm even with low nutrients N3 exhibited its higher photosynthetic capacity.
High nutrients at N1 promoted the growth of W. trilobata and led it to its survival under high water depth, which may partially describe the ability of W. trilobata to invade waterlogged habitats in wetland.
Moreover, these consequences may also contribute to our understanding about wetland invasive communities that how invasive species respond to the natural fluctuations and hydrological conditions modified by humans in the wetland. However, further studies based on growth and physiological traits will be needed to evaluate the invasions of invasive species at wetland in response to multiple environmental stresses.

CO N FLI C T O F I NTE R E S T
We have no conflicts of interest to declare.

AUTH O R CO NTR I B UTI O N
For this research articles, the individual contributions is listed as