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?