Introduction
Invasive species are a major threat to the ecosystems and are one of the major environmental challenges of global change (Funk et al., 2016). Global environmental changes could create novel environments that directly increase the availability resources for invasive plants. High resources availability facilitate the invasive plant species to grow faster due to their higher phenotypic plasticity (Si et al., 2014) and higher competitive ability (Dostál et al., 2013). Numerous experimental studies have described that global environmental changes enhanced plant invasion. For instance, a recent meta-analysis comparing the growth performance of 74 invasive and 117 native plant species in response to global environmental changes i.e flooding, increase in atmospheric CO2-concentration, nitrogen enrichment and temperature variations, noted that these changes help invasive species to grow well (Liu et al., 2017). Functional traits play an important role in the success of invasive plant species over native plant species under these global environmental changes (Wan et al., 2018). However, it is very interesting to understand the role of functional traits in enhancing invasive plant species growth under these global environmental changes. Functional traits such as growth characteristics are key indicators of root and shoot development, and leaf functional traits are key indicators of transpiration, evaporation, and photosynthesis (Wang et al., 2016a).
Strong functional traits, especially physiological and growth traits, show the success of invasive species in different environmental conditions. Therefore, to determine the success of invasive species, functional traits and resources are also considered as major factors in the native ecosystems (Jia et al., 2016). The resource ratio hypothesis is one of the explanations for the mechanisms for success of invasive species (Harpole, 2006). According to the resource ratio hypothesis, plants vary in their nutrient requirements and with the increased availability of a certain nutrient some plants may be benefited and these benefiting plants will inhibit other plants through competitive interactions (Wan et al., 2019). Invasive plant species show better performances under nutrient enrichment and water fluctuations because of higher phenotypic plasticity than native species, and out compete natives species due to interspecific competition (Liu et al., 2018b). Phenotypic plasticity and relative competition intensity are two main features that make invasive plant species to cope with different environmental changes (Liu and van Kleunen, 2017, Van Kleunen et al., 2015), so because of higher phenotypic plasticity and better competition intensity under nutrient enrichment and water fluctuations may create a more suitable environment for invasive species, making them dominant over native species. Relative competition intensity is considered an important factor in determining plant community structure and in promoting invasive potential (Luo et al., 2014).
Mostly competitive ability of invasive and native species depends on the resources of the habitat. Invasive species like to grow in resource-rich habitats. Thus, the competition ability of invasive species may change with the availability of resources. There are several examples where a change in nutrient availability affects the performance of invasive plants. For example, nitrogen (N) enrichment enhanced invasion ofBerberis thunbergii, Robinia pseudoacacia , and competitive ability of Centaurea stoebe (He et al., 2012) and addition of potassium made Taraxacum officinale[9]successful in grassland, because of higher phenotypic plasticity. Several nutrients have a role in the success of invasive species but N is a major element of global change that disturbs plant community structure, and especially it enhances the abundance of exotic species and decreases the species richness (Duprè et al., 2010). Nitrogen enrichment plays a vital role in the spread of invasive plants. Continued increase in nitrogen may change soil properties that help invasive plants to grow faster in different habitats (Lu et al., 2014). Previous studies have reported that invasive species were more successful than native species in nitrogen-enriched environments because of better competitive ability and these were less successful compared to the native species in the nitrogen-poor habitats (James et al., 2011, Kolb et al., 2002).
Water availability is considered as a key factor responsible for shaping plant communities (Kimball et al., 2014, Rahlao et al., 2010). Water is a vital part of ecosystem facilities, and changes in its availability will modify nutrient cycles resulting in decreased or increased nutrient uptake due to low or high moisture contents in the soil (Ledger et al., 2013, Waraich et al., 2011). The water regime of a habitat can be characterized by the depth, duration, and frequency of flood (Casanova and Brock, 2000). Flooding is the most important factor that plays vital role for the success of invasive species because frequent flooding can produce a more stable environment and shifts in species dominance and species composition (Van Geest et al., 2005). Flooding created more favorable environments, which mostly inhibited the growth of emergent macrophytes (Casanova and Brock, 2000) and preferred submersed aquatic macrophytes, such as evergreen perennial species. Flooding can be observed as a disturbance to the plant community and these disturbances are mostly the main mechanism that facilitates invasion through replacing the native species by the invasive one (Capers et al., 2007). Therefore, understanding the role of nitrogen enrichment and flooding is important to understand the success of invasive species.
Functional traits of invasive plant species play a vital part in its successful invasion in the different environments (Dai et al., 2016b). The response of functional traits of invasive plant species under flooding along with nitrogen enrichment is not well understood. Therefore, we conducted an experiment to examine the functional traits of the invasive speciesWedelia trilobata and its congener native Wedelia chinensis under nitrogen enrichment, flooding, and their interaction, in different plant culture. We hypothesized that flooding along with increased nitrogen concentration, promotes the success of invasive species on native plants due to better physiological and growth responses, which confer them a higher competitive effect, because of higher phenotypic plasticity. In this study, we addressed the following questions: (1) How functional traits of an invasive plant and its native competitor respond to nitrogen enrichment along with flooding? (2) Do flooding and nitrogen enrichment make invasive species more competitive than the native species?