Invasive plants often show high adaptability and phenotypic plasticity so that they can survive under altered environmental conditions (Davidson et al. 2011; Liu and van Kleunen 2017), but different invasive plants vary in their responses to environmental change, such as climate change (Dukes and Mooney 1999). Numerous case studies have demonstrated that invasive plants benefit from climate change (Dukes and Mooney 1999; Sorte et al. 2013; Liu et al. 2016; Mozdzer et al. 2016), especially N deposition and elevated CO2. Elevated CO2 can facilitate plant invasion by increasing plant photosynthesis, water use, growth rates, resource use efficiency, productivity, seed production and seed banks (Stewart and Potvin 1996; Smith et al. 2000; Weltzin et al. 2003; Ziska 2003; Dukes et al. 2011; Smith et al. 2013). N deposition can facilitate plant invasion by increasing N availability, plant-soil feedbacks, plant growth and competitive ability (Limpens et al. 2003; Zhang et al. 2010; He et al. 2011; Wan et al. 2019). In most research, these two factors are studied independently. Understanding how invasive plants respond to the interactive effect of elevated CO2 and N deposition is important but still underappreciated. Plant growth and biomass accumulation can be enhanced by elevated CO2, and the rates of these processes may be constrained by soil N availability (Luo et al. 2004; Feng et al. 2015). The invasion of Eupatorium adenophorum may be exacerbated by CO2 enrichment and N deposition (Lei et al. 2012).
Successful invasion by one plant not only is related to its adaptability to the environment but also depends on its ability to compete with other plants, including other invaders. As biological invasions increase in frequency, most habitats are invaded by multiple invasive plants, so interactions between different invasive species become more important (Shea and Chesson 2002). Invasive species can interact with each other (Russell et al. 2014; Kuebbing and Nunez 2014). One alien invasive species can be facilitated by another species, which is described as invasional meltdown (Simberloff and Von Holle 1999). Most known cases of invasional meltdown involve plants and organisms at other trophic levels, and there are few examples involving plant-plant interactions (Simberloff 2006). Invasion by one species can be negatively impacted by the presence of another invader, known as invasional interference (Yang et al. 2011; Rauschert and Shea 2012), especially when they require similar niches (Rauschert and Shea 2016). In some cases, these negative relationships may result in an invasive species replacing another invasive species, known as “overinvasion”, which is used to describe this phenomenon in an animal context (Russell et al. 2014). Many authors have noted that a decline in one nonnative species results in a rapid increase in another, which indicates that competition among invasive plants may be common (Kuebbing and Nunez 2014). Invasional interference and overinvasion are understudied in plants. For example, the replacement of Spartina anglica by Spartina alterniflora was deduced (Zhi et al. 2007) in China. It is also possible that the interactions are neutral, as supported by a meta-analysis on the interactions of invasive animals (Jackson 2015).
The types of interactions among plants depend on environmental, species and individual features (Callaway and Walker, 1997). A series of studies have compared responses to climate change between invasive plants and native plants (Dukes and Mooney 1999; Liu et al. 2016) or naturalized alien plants and native plants (Sorte et al., 2013). However, different invasive plants are seldom compared in altered environments. Competitive ability varies with changes in environmental conditions; for example, an increased competitive advantage of invasive species has been attributed to higher N availability (Quinn et al., 2007). Variations in the environment can influence the intensity of competitive interactions (James and Richards 2007) and competitive relationships of plants because competing species respond differently to environmental change (Miller et al. 2007; Mamolos 2006). Moreover, competition relationships and elevated CO2 and N deposition often occur at the same time, and how these factors impact invasive species still needs to be determined.
To study the relationship of two notorious invasive plants, Ambrosia artemisiifolia (common ragweed) and Amaranthus retroflexus (redroot pigweed), and the impacts of elevated CO2 and N deposition on their relationship, we planted these two plants in monoculture and mixed culture under different environmental treatments, namely, elevated CO2, N deposition, elevated CO2 + N deposition and a control. The growth characters, biomass and relative yield of the two invasive species were all measured, and we also examined the seed production of common ragweed under competition and elevated CO2 and N deposition. We aimed to answer the following questions: (1) Do these two invasive alien plants respond to elevated CO2, N deposition and competition in the same way? (2) Does climate change impact the relationship between two invasive alien plants? (3) How do elevated CO2, N deposition and competition affect the reproduction of common ragweed?