4.1 Effects of environmental factors on functional traits ofP. australis
Phragmites australis is a type of salt-tolerant plant (Mauchamp et al., 2001; Santos et al., 2016); higher salinity inhibits their growth. Phragmites australis is more likely to be found in sites of surface ground water discharge and appears to access deeper, less saline water (Burdick et al., 2001; Guo et al., 2018; Veldhuis et al., 2019). Salinity stress results in a clear stunting of plants (Wang et al., 2016) and reduced leaf area expansion (Parida et al., 2005; Gong et al., 2020). As previously reported, the height reduction of P. australis growth was a marked feature under salinity stress (Lissner et al., 1997; Guo et al., 2018; Sdouga et al., 2019). Leaf traits are closely related to the growth, ability to utilize resources, and survival strategies of plants under environmental changes (Mason et al., 2013). Specific leaf area associated with allocation strategy tends to scale positively with leaf nitrogen concentration, and negatively with leaf longevity (Pérez-harguindeguy et al., 2013; Spasojevic et al., 2014). Leaf thickness plays a key role in determining the physical strength of leaves, being higher in sunnier, drier, and less fertile habitat, as well as in longer-lived leaves (Wu et al., 2008). Our results clearly verified these conclusions, as indicated by the average height of P. australis being negatively correlated with electrical conductivity (Figure 3). Based on our results, salinity affects plant leaves through reduced specific leaf area and leaf area, but increased leaf water content and leaf thickness enhance the ability to resist environmental stress, altering the living strategy from growth to survival.
Stem traits play a vital role in the entire plant life cycle, supporting ground organization, holding water and nutrients, and conducting water and elements (Liu et al., 2015). Based on our results, the stem traits had positive correlations among themselves (Table 1), which were generally linear and logarithmic relationships with electrical conductivity (Figure 3). Under low electrical conductivity, the plant had a relatively high growth rate, with stem elongation, an increased number of sections, and thickened base stem diameter to support more leaves and better compete for resources. Under high-salinity environments, plants changed their growth strategy to thicken and strengthen their structures to preserve internal resources to survive. The results of the present study verified that the electrical conductivity of environments was the dominating factor controlling the functional properties of P. australis in the Yellow River Delta.
Many studies have reported that community structure (Pérez-Ramos et al., 2012), species composition (Li et al., 2008), and vegetation growth (Gong et al., 2014a) were affected by soil water content. Generally, a non-linear relationship with an obvious soil moisture threshold value appeared for most plants (Yu et al., 2012), and plant growth could be limited by both a water deficit and excess soil water. The relationship between soil water content and growth traits was approximately linear in dry habits (Gong et al., 2014b). However, based on our laboratory experiments, P. australis has no obvious specific correlation between soil water content and functional traits, except for the leaf water content (Table 1, Figure 3). P. australis had a wide range of niche adaptations to water and had high tolerance to both drought and waterlogged conditions; the soil water conditions used in our research were not extreme enough to cause stress. The results indicated a high adaptability to soil water content for the growth of P. australis , however, soil water content was not the limiting factor based on the 96 samples in our research.