Abstract
Eutrophication is thought to promote plant invasion, resulting in high growth performance of invasive plants, thereby greatly promoting growth-induced intraspecific competition for light. Current hypotheses predict how eutrophication can promote plant invasions, but fail to explain how eutrophication conditions maintain great invasiveness. In different native communities, symbiotic plants of different sizes can avoid light competition by using light in complementary ways; however, whether this mechanism applies to intraspecific competition in invasive plant populations is unclear. A two-year field nitrogen enrichment experiment with spartina Terniflora, a global invasive plant, found that the plasticity of photosynthesis reduced intraspecific competition and increased the biomass of matinee. This plasticity effect is enhanced when there are no nutritional restrictions in corrugated rice grass. In the nitrogen enhancement treatment, the height difference between the melaleuca plants increased as the light intensity under the canopy decreased. Compared with environmental nitrogen, under nitrogen-increasing conditions, the light energy utilization efficiency and specific leaf area of dwarf individuals increased in response to the decrease in light intensity under the canopy. However, this ecophysiological plasticity was not found in tall individuals. Our findings suggest that light utilization plasticity in dwarf individuals can be thought of as a novel mechanism by which invasive plants mitigate intraspecific competition, increase invasive capacity, and challenge the mainstream view that alien plant invasion is constrained by intraspecific competition.
Introduction
Human activities exacerbate biological invasion and eutrophication. Current studies have shown that eutrophication changes nutrient acquisition strategies and plasticity in invasive plants, while alien plants are more widely distributed under eutrophication conditions due to the escape of co-evolving predators. However, these mechanisms are not enough to explain the successful invasion of alien plants.
Nutrient enrichment promotes the shift of competition from nutrition to light. Light is a resource supplied by directionality, and tall and fast-growing individuals reduce the light availability of short, slow-growing species. This competition usually occurs between individuals or between plants. Therefore, the increase in the density of plants of the same species leads to increased intraspecific light competition, resulting in a decrease in individual size. This inference has been well validated in native plant communities, but is still unknown for invasive plant populations.
Invasive plants can grow asynchronously due to their high phenotypic plasticity and often exhibit large differences in age and size. In fact, individuals in a group can make a big difference in their use of resources. In addition, invasive plants often have high phenotypic plasticity to cope with changes in light conditions, allowing them to tolerate low light intensities under the forest. Therefore, invasive plant species of different sizes may use light at different heights, at different times and at different amplitudes to make complementary uses of light. However, previous studies evaluating intraspecific interactions have often evaluated individuals of similar size in greenhouses, which makes it impossible to characterize this phenotypic plasticity of light utilization in all-species plants of different sizes of invasive plants in the field.
In this study, the authors conducted a two-year field nitrogen enrichment experiment using the globally invasive C4 plant Spartina alterniflora to assess whether the phenotypic plasticity of light utilization reduced intraspecific competition and thus increased its invasiveness in the context of increased nutrient availability. This clone has some important characteristics of successful invaders, including high light energy utilization efficiency and strong nutrient acquisition, which allows it to form dense monocultures where the size of the branches varies greatly but yields extremely high. However, due to increased intraspecific competition, the invasiveness of interspersed rice grass may be strongly limited. We assessed the height differences between heterophyllum heterophylla clones, as plant height is closely related to light acquisition, and plants of different heights can use light in a complementary way. We speculate that nitrogen fertilization can enhance the photomechanical plasticity of the interfluillet plants, thereby facilitating the coexistence of plants of different sizes, and increasing the invasiveness of interspecific ricegrass by alleviating intraspecific competition and improving growth performance.
Materials and methods
Study site:The study was conducted on Chongming Island in the Y angtze estuary, China.
Experimental design: Hybrid rice invasion and nitrogen addition experiments, in the monoculture group of interpolated rice grass, 10 4 m*4 m plots were set up, and nitrogen treatment was applied alternately (control and nitrogen addition, 5 replicates).
Quantification of light asymmetry and leaf functional traits: Light asymmetry refers to the decrease in light intensity from the canopy to the soil surface of all unforested samples in each plot, calculated as the slope of the linear regression model between photosynthetic effective radiation (PAR) and height. The light intensity of the unforested square was measured at five altitudes (20, 50, 100, 150, 200 cm) at noon (11:00 to 13:00) using five PAR sensors. The PAR value for a given altitude is the average of 120 data points recorded once per second by the Onset HOBO U30-NRC weather station for 2 minutes || measurment:leaf gas exchange(2nutrient levels × 2 plant sizes × 5 replicates); leaf photosynthetic responses; specific leaf area (SLA).
Temporal and end-of-growing-season measurements of plant performance:plant height and stem height (1-2 times per month);aboveground biomass;the number of ramets.
Calculations and statistical analysis:Log-transformed before analysis to achieve normality; Linear mixed-effect (LME) models were used to assess the effects of N fertilization on stem height, height difference, light asymmetry, and RNE separately for each year, with plot number included as a random effect. Similar models were also used to assess the effects of N fertilization, thinning, and their interaction on aboveground and individual biomass and plant density; Two-way ANOVA was performed to assess the effects of N fertilization, plant size (e.g., tall and short), and their interaction on leaf functional traits; Constructed a structural equation model (SEM) to explore the pathways of how N fertilization affected plant biomass mainly based on four hypotheses.
Results
Phenotypic plasticity of light use in response to N: Nitrogen fertilizer significantly increased light asymmetry (i.e., low light intensity under the forest) during both growing seasons (P<0.05; Figure 1a). Melee improved the light energy utilization plasticity of the short plants (Figure 2) to cope with the low light intensity under nitrogen fertilizer treatment. The gas exchange results showed that the light energy utilization efficiency and SLA of the short strains treated with nitrogen fertilizer were significantly higher than those of the ambient nitrogen treatment (average P<0.05. Figure 2a, b). No similar results were found on higher strains (Figures 2a, b). In addition, under the conditions of nitrogen application, the photosynthetic rate and light saturation point of the short plants were significantly lower than those of the high plants (average P<0.001; Figure 2c, d), illustrating the different needs of light for different asynchronous strains. In addition, nitrogen fertilization significantly increased the photosynthetic rate and light saturation point (p<0.01 of dwarf and tall plants. Figure 2c, d). Therefore, the survival of shorter sub-plants under the forest and the significant growth of higher canopy branches significantly increased the height difference at population level under nitrogen application conditions (P<0.05; Figure 1c), but does not affect the mean plant height (Figure 1b).
The results of Effects of N on plant performance and competition: Growth trajectory analysis (Appendix S1: Figure S6) showed that nitrogen fertilization increased the maximum growth rate of the asynchronous plant of Rice Meadowsgrass and reduced the difference in the maximum growth rate (mean P<0.05; Figures 3a, b) show that the maximum growth rate of each branch is basically the same. The results showed that nitrogen fertilization increased the maximum growth rate of interfluillet rice grass and reduced the difference in the maximum growth rate of the plants (mean P<0.05; Figure 3a, b). Nitrogen fertilizer reduced the ecological niche overlap of asynchronous strains by changing the date when the asynchronous plant reached the maximum growth rate (P<0.05; Figure 3c). This is related to the significant decrease in intraspecific competition intensity under nitrogen fertilizer treatment in the past two years (P<0.05; Figure 3d). Therefore, the density growth under nitrogen application did not reach the asymptotic curve, especially in 2019 (Appendix S1: Figure S7), resulting in a significant increase in the density and aboveground biomass of matinee under nitrogen application (p<0.05; Figure 4a, b). In addition, thinning reduced the total aboveground biomass and stem density of interflaged rice grass, independent of the N treatment (interval: mean P<0.001; Figure 4a, b), but thinning increased the individual biomass of interflorum rice grass (interval felling: average P<0.001; Figure 4c).
Structural equation simulation results show that nitrogen application regulates intraspecific competition intensity through two opposite pathways, thereby affecting the aboveground biomass of materia mathoidens (Figure 5). Nitrogen fertilizer indirectly increases and decreases intraspecific competition intensity by increasing light asymmetry and increasing height difference. The negative effect of the height difference (normalized coefficient = 0.86 × 0.85 = 0.73) is greater than the positive effect of light asymmetry (normalized coefficient = 0.66× 0.40 = 0.24). These results show that the aboveground biomass of melee under nitrogen application conditions is mainly limited by light, and the high difference caused by light utilization plasticity reduces intraspecific competition.
Original link: https://esajournals.onlinelibrary.wiley.com/doi/epdf/10.1002/ecy.3665