Role of relative competitive intensity and plasticity index
Relative competition intensity (RCI) of WT and WC indicated improved competition intensity under interspecific competition due to decrease in plant height, leaf nitrogen and increase in SLA and chlorophyll content (Fig.4). Flooding and nitrogen enrichment have great impact on the interspecific competition in the water fluctuation habitat (Zhou et al., 2017). Relative competition intensity is the trade-off among plants between competition and facilitation, which means that under higher resource availability plant shows competition and under stressful condition plant exhibits facilitation (Gratani, 2014). RCI of most functional traits under additional nitrogen and combination of flooding (2N and F.2N) were negative of WT that indicated the WT was more competitive than WC, especially when both plants grew in mixed culture, because of its survival and better competition ability under flooding and nitrogen enrichment conditions. WC might be repressed by two factors; one is competition with WT and the other sensitivity to flooding with higher nutrient availability. Mainly this type of outcome happens under these conditions because every plant species have different tolerance ability under adverse environments (Sun et al., 2019). Here WT appears to be more dominant due to greater tolerance of flooding and nitrogen enrichment. Furthermore, RCI under combination of flooding and additional nitrogen (F.N and F.2N), LN and CHI of WT had negative values, but WC had positive values for these parameters, which indicated that WT became competitor but WC behaved like facilitator. Thus, the competition intensity of WTdecreased under combination of flooding and additional nitrogen (F.N and F.2N), but WC increased (Fig. 4). This can be explained by the flaring of the functional divergence between WTand WC under combination of nitrogen and flooding treatments.
Functional traits observed in this study revealed phenotypic plasticity to some extent (Fig. 3). Phenotypic plasticity is the traits mechanism that make plant able to cope with biotic and abiotic environments (Gratani, 2014) and main factor for the success of different plant under different habitats (Legay et al., 2014). According to the results, it was obvious that phenotypic plasticity may play a vital part to adapt to the adverse changes in the environments. However, it was noted that the phenotypic plasticity in functional traits of WT was higher than WC. Phenotypic plasticity and relative competitive intensity are closely related to each other because both make invasive plant species able to alter above and below ground functional traits to cope with a wide range of environmental changes (Lamarque et al., 2013). Conflicting to prediction, phenotypic plasticity of LN and CHI in WT were lower than in WC. These lower ranges may indicate a fitness cost for plastic physiological traits under complex environments. Leaf construction costs and plant growth rate may be quiet due to lower phenotypic plasticity of LN and CHI (Drenovsky et al., 2012). WT compensated the negative effects of these adverse environments due to the limited plasticity of functional traits (Quan et al., 2015). Thus, this facilitates invasion and the development of populations in new habitats (Wang et al., 2017). However, plasticity of other indices of WT was significantly different from WC (Fig. 3). Although invasive species mostly did not show higher range of plasticity compared with the natives, here WT showed higher plasticity than WCbecause of availability of nitrogen and water. WT and WC showed also higher plasticity in plant growth under combination of W × N, which that enhanced their competitiveness (Čuda et al., 2017, Wan et al., 2019). Previous studies also confirmed that invasive and native species could positive respond under competition (Liu et al., 2018a).