3.3. Reproduction effort
Culture, NTC, and CompType(Culture) had significant effects on the number of capitula per invasive plant, and the interactions among CompType(Culture) and PopType were also significant (Table 2). Compared to that in the single-culture, G. quadriradiata when grown together with competitors had significantly lower number of capitula per plant. This effect was stronger when grown together with old competitors. In general, central populations had significantly higher number of capitula per plant than that of the edge population when grown with new competitors (Figs S3, 4). However, when the invader was mixed with the old competitor, there was no statistical difference between the number of capitula per plant of central vs. edge populations. The number of capitula per plant ofG. quadriradiata significantly increased with the nitrogen addition. In the ambient nitrogen treatment, the central population obtained lower number of capitula per plant when it was mixed with the old competitors than that mixed with the new competitors (Fig. 4).
3.4. Root mass ratio (RMR)
The result of the GLMM revealed that PopType, Culture, and CompType(Culture) had significant effects on the RMR of G. quadriradiata , and the interactions among CompType(Culture) and PopType were also significant (Table 1). The RMR of the central population was significantly lower than that of the edge population when grown alone (Fig. 5). In general, the RMR of G. quadriradiata in the mixed-culture was higher than that in the single-culture (Fig. 5, Table 1). The RMR of G. quadriradiata was significantly lower when the invader was grown with old than with new competitors. For the central population of G. quadriradiata , the RMR was higher when grown with new competitors (Fig. 5). Both new and old competitors had similar effect on the RMR on the edge populations.
3.5. Leaf nutrient (nitrogen and phosphorus) concentration
The nitrogen addition treatments had a significant positive effect on the leaf N content (LNC) of G. quadriradiata (Fig. 6a). The interactions among CompType(Culture) and PopType on the LNC were also significant (Table S2). The LNC of the central population was significantly higher than that of the edge population when the invader was grown with old competitors (Fig. 6b).
The GLMM revealed that Culture, NTC, and PopType had significant effects on the leaf P content (LPC) of G. quadriradiata (Table S2). In general, the invader LPC was higher in the single-culture than in the mixed-culture; and it was significantly lower under nitrogen addition treatments than when in ambient nitrogen treatment (Fig. 6c). The LPC of the central population was higher than that of the edge population which was further significantly reduced in the presence of the competitors (Fig. 6d).
3.6. Leaf construction costper unit of mass ( CCmass)
Competitor and population types (separately) had significant effects on the leaf CCmass, and the interactions among PopType and NTC were also significant (Table 2). The leaf CCmass was significantly lower when G. quadriradiata was grown with old competitors than when mixed with new competitors (Fig. 7). In general, the leaf CCmass of the edge population was significantly lower than that of the central population (Fig. 7), especially under high nitrogen addition treatment (Fig. S4).
DISCUSSION
In this study, we aimed to reveal how population differentiation due time since introduction and shared co-evolutionary history with competitors could affect the invader’s response to increased nitrogen deposition. The results of our experiment suggest that nitrogen addition improves the growth and reproductive performance of the invader G. quadriradiata , and enhances its relative competitive advantage on competitors. The central population had both higher growth and reproductive performance when compared to the edge population, and the magnitude of response to nitrogen addition treatments was larger in the central population when in single-culture. In the presence of the competitors, the positive effects of nitrogen addition on invasive plants was diminished. Old competitors with longer shared co-evolutionary history had a stronger competitive inhibition on G. quadriradiata performance than the new competitors. These results suggest that the shared co-evolutionary history between competitors and the invader plays an important role in the plant invasion process.
Response to increased nitrogen deposition
The total mass and number of capitula per plant of G. quadriradiata were higher under nitrogen addition treatment than in ambient conditions (Figs 1-2). Our results then suggest that increased nitrogen deposition improves the growth and reproduction performance ofG. quadriradiata , result which align with a number of other invasive plants (Dawson et al., 2012; Parepa et al., 2019; Yu et al., 2020). Invasive species generally tend to be fast-growing plants with high nitrogen use-efficiency (Feng et al., 2009; Liu et al., 2017a). Fast-growing plants often struggle to get enough nitrogen under natural conditions (Bajpai & Inderjit, 2013; Liu et al., 2018). Therefore, under increasing nitrogen deposition, the colonization and expansion ofG. quadriradiata may be further strengthened, especially in farmlands with frequent artificial fertilization.
While nitrogen addition promoted growth and reproductive performance of G. quadriradiata in single-culture, in the presence of competitors this effect was weakened (Figs 1-2). Moreover, the strength of interspecific competition ofG. quadriradiata increased with nitrogen addition (Fig. 3), suggesting that competitors might also take advantage of the extra soil nitrogen available and compete more intensively with co-occurring invasive species. The competition with species might decrease the positive effect of increased nitrogen deposition on G. quadriradiata directly impacting its expansion rate.
Population differentiation
The growth and reproductive performance of the central population ofG. quadriradiata were higher than the edge population (Figs 2-4), which may mainly be due to the stronger nutrient accumulation capacity (especially in mixed culture) of the central population (Fig. 6). The lower RCI values under ambient nitrogen treatment indicated that the central population was also subjected to less competitive pressure when planted with competitors (Fig. 3). These results suggest a significant population differentiation between the central and edge population ofG. quadriradiata on growth and competitiveness. Many invasive plants experience population differentiation in the process of range expansion, which helps them to adapt to different environments (Dematteis et al., 2020; Helsen et al., 2020). Generally speaking, the central population of invasive plants may face higher competitive pressure and for longer periods of time (Miller et al., 2020; Phillips et al., 2010; Shine et al., 2011), and subsequently may evolve to be a stronger competitor compared to the edge populations. Our results indicate that, in our study area, the higher competitive ability of the central population of G. quadriradiata will potentially help it to better occupy the existing habitats.
When planted alone, the total mass of the central population was significantly higher than that of the edge population only in treatments with nitrogen addition, with no significant differences under ambient nitrogen (Fig. 2). This reveals that the magnitude of response of the central population to elevated nitrogen was larger than that of the edge population, suggesting that populations of invasive plant species with longer time since introduction could have a larger advantage in the context of future increased nitrogen deposition. Additionally, our results indicate that the central population allocated more nitrogen to growth under increased nitrogen deposition, especially when competing with the old competitors (Fig. 6b). The literature shows that native and invasive populations of invasive plant probably have different nitrogen use strategies, with invasive populations generally showing a quicker return in nitrogen use (Feng et al., 2011). What is still less explored, and we show evidence here, is that different populations of invaders respond uniquely to increases in nitrogen during the range expansion. Therefore, future work should be careful to not overestimate the effects of elevated nitrogen deposition in the range expansion of invasive plants by sampling only the central populations or underestimate them by studying only the edge population.
The effect of competitors with different shared co-evolutionary history
Our results show that the extent of co-evolutionary history betweenG. quadriradiata and its competitors influenced the outcomes of competition in our experimental setup. When G. quadriradiata was planted with old competitors, its growth, competitive, and reproductive performances were worse than when growing with the new competitors (Figs 2-4). These results contradict our proposed hypothesis and other studies that have shown better performance of invaders over natives (Alexander et al., 2015; Sun & He, 2018). This may be due to the overlooked adaptations of other species over invasive species with a longer history of co-evolution (Huang et al., 2018; Oduor, 2013; Strauss et al., 2006). It has been suggested that intense competition from invasive plants is a selective factor that eliminates native plant genotypes that cannot tolerate such competition, leading to the accumulation of native plant genotypes that can tolerate intense competition in native plant populations (Leger & Espeland, 2010; Strauss et al., 2006). Genetic variation in traits that are resistant to strong competition from invasive plant species may enable native plants to evolve to adapt to invasive plant species (Oduor, 2013; Strauss et al., 2006). For example, a study on the African savannas suggests that Parthenium hysterophorus invasions may have exerted selective pressure on native plants, leading to the differentiation of growth and reproductive traits between invasive and adjacent non-invasive habitats (Oduor, 2022). And it has been found that native plants may evolve a tolerance reducing the negative effects of invasive plants allelopathic compounds (Huang et al., 2018). We speculate that a probable cause to this pattern in our study is due to the lower photosynthetic capacity of G. quadriradiata when mixed with the old competitors (Fig. S5).
With respect to the range expansion of G. quadriradiata, the old competitors had a stronger competitive inhibition on the central population when compared to the new competitors (Fig. 2). This result was mainly attributed to the lower root allocation of plants from the central population when planted with old competitors than when planted with new competitors (Fig. 5). One potential explanation is that due to intense aboveground competition with the old competitor, invasive plants needed to allocate more biomass aboveground instead of belowground (McCarthy & Enquist, 2007; Puglielli et al., 2021). In contrast, the performance of invasive plants from edge population ofG. quadriradiata did not significantly differ when growing with new or old competitors (Figs 2-4). The similar performance of the edge population in the face of different competitors and different nitrogen conditions may facilitate the edge population to better adapt to the uncertain environmental factors they may face in the process of range expansion (Alfaro & Marshall, 2019).
Our study explores the competitive relationship between an invasive plant G. quadriradiata and eight competitors with different co-evolutionary histories, which represents a large amount of work. However, the conclusion about the influence of the co-evolution history of invasive plants and competitors on their interaction still needs careful extrapolation, because their interaction is also affected by environmental conditions, plant rarity, functional traits and other factors (Oduor, 2013; Zhang & van Kleunen, 2019). And this study only inferred from the phenotypic traits that the co-evolutionary history of competitors and G. quadriradiata influenced the relationship between them. However, phenotypic trait expression is influenced by heritable genetic and epigenetic factors (Oduor, 2022). Overall, future studies need to be carried out on co-evolution history of other invasive plants and competitors in more habitats, and multi-generation growth of test plants can be used to test whether trait differences persist over time, and the genetic mechanism should be further explored.
Synergisticeffect of nitrogen, population differentiation, and co-evolutionary history
Many abiotic and biotic factors may affect the distribution and spread of invasive plants in the context of disturbance and global climate change (Liu et al., 2017b; Mitchell et al., 2006). These factors often overlap in time and space, and may exert additive, synergistic, or offsetting effects on invasive plants, resulting in complex and unpredictable results (Darling & Cote, 2008; Kersting et al., 2015; NeSmith et al., 2018). For example, in a field experiment in the Californian grassland, the synergistic effect of water and fertilization increased the growth performance of the invaders, which in the presence of surrounding competitors greatly offset the effect (Eskelinen & Harrison, 2014). In our study, we investigated the synergistic effects of nitrogen addition, invasive plant population differentiation, and co-evolutionary history of co-occuring plants on G. quadriradiata . All three factors seem to play an important role in the expansion of G. quadriradiata from central range to edge range in China. Population differentiation contributed to the stronger invasion potential of the center population, which was further enhanced by increases in nitrogen deposition. However, the invasive species dominance was diminished to some extent in the presence of old competitors. Although the invasive potential of the edge population was relatively low, it was enhanced under increases of nitrogen deposition. Our results echo other studies showing that when studying invasive plants, exploring the interaction of multiple influencing factors and their cumulative effects is pressing (Darling & Cote, 2008; Kersting et al., 2015).
CONCLUSION
In this study, we showed that the nitrogen addition significantly promoted the growth and reproductive performance of G. quadriradiata , while central populations had a larger magnitude of response than the edge population when grown in single-culture. However, in the presence of competitors, the response of G. quadriradiatato nitrogen addition was reduced. When grown in mixed settings with old competitors that shared a longer co-evolutionary history, G. quadriradiata both growth and reproductive performance were worse than when growing with new competitors.
In general, the central population of G. quadriradiata had higher population growth, reproductive performance and competitiveness than the edge population. These results suggest that G. quadriradiata show trait differentiation among populations in the process of range expansion, which might benefit populations to adapt to different habitat conditions and resident communities.
When considering both the stage of the range expansion and the co-evolutionary history, old competitors more strongly inhibited the central population performance via competition than the new competitors. This effect, however, was not pronounced for the edge population. The difference between the competition strength of new and old competitors on the invasive G. quadriradiata confirms that the role of co-evolutionary history between competitors and invaders cannot be ignored when studying the range expansion of invasive plants.