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.