4.3 The model of P. australis life strategies
The trade-off between growth and survival is the heart of the economic spectrum theory of leaves (Reich, 2014; Díaz et al., 2016). Three core leaf traits, SLA, LDMC, and LA, were used as criteria to determine the ecological strategies of individuals (Pierce et al., 2017). The application of the CSR strategy in community construction has been controversial. Many studies have verified the explanation of CSR strategy classification for community species composition in habitats where species strategy distribution was concentrated (Negreiros et al., 2014, Paula et al., 2015). However, CSR strategies are usually used to research the different species in communities, and there are few studies on the strategies of the same species under different environmental stresses.
According to the top-level succession theory, the plant community in the Yellow River Delta should be in the early stage of succession and salt is a limiting factor for most herbaceous species (Tan et al., 2012). Therefore, the plants with strong salinity stress tolerance under the S/CS strategy have more advantages, and the plants near the S strategy have greater survival opportunities. S is the dominant strategy forP. australis in the Yellow River Delta. Plants near the S strategy end have lower LDMC, while plants near the C strategy end have higher SLA traits, representing revenue capacity, than those near the S strategy end. With the decrease of soil salinity and the increase of soil water content, the strategy of P. australis in the Yellow River Delta gradually shifted to C. For the C strategy, plant selection and growth are mainly aimed at rapidly occupying resources, and a relatively large SLA is needed to grow rapidly (Xu et al., 2019). Plants under the R strategy are adapted to environments with strong disturbance, tend to invest many resources in reproduction to offset the effects of disturbances on the population, achieve the purpose of survival and continuation of the population (Pierce et al., 2017). In the Yellow River Delta, P. australis have a minor ruderal (R) strategy.
As a perennial herb, P. australis tends to grow rapidly and spread by underground stems, forming a single community, which is common in the Yellow River Delta. Due to the habits of P. australis , the R strategy rarely appears in the Yellow River Delta. There are similar modes of strategies of P. australis related to the soil salinity and the soil water content. We believe that environmental factors partly determined the C or S strategy of P. australis , while with the decrease of environmental stress, the mainly strategy gradually shifts from S to C. This indicated that P. australis has stronger tolerance and competitive ability in the communities from the coastal wetland to inland areas, therefore becoming the dominant species in the Yellow River Delta.