Plant functional traits are adaptive and effect traits that affect plant survival, growth and reproduction, reflecting plant responses to the environment and measurable traits that can influence ecosystem function (Liu and Ma, 2015). It has proved useful for predicting plant community assembly (Lavorel and Garnier, 2010), which can respond strongly to environmental and climate change and can serve as an excellent ecological indicator role (Joswig et al., 2022). Functional traits are morphological, physiological, phenological and behavioral characteristics that respond to the environment and directly or indirectly affect plant adaptation and related ecosystem properties (Violle et al., 2007, Moore et al., 2018). Variation in plant functional traits is the result of natural selection and feedback from plants to their environment and reflects variation in the relative of plant’s adaptive mechanisms (Liu et al., 2010, Meyerson et al., 2016). Quantifying the variation pattern of critical traits related to morphological traits, nutrient contents and stoichiometric ratios at each of multiple spatial scales and driving factors could provide an opportunity to evaluate the relative importance of these drivers for species evolutionary adaptation (Ackerly and Cornwell, 2007, Joswig et al., 2022).
The cosmopolitan species Phragmites australis is a tall wetland grass with high intraspecific variation, making it a suitable model species for studying the underlying mechanisms of intraspecific trait variation (Kueffer et al., 2013, Eller et al., 2017). The species occurs over an extensive latitudinal range and in a broad range of ecological niches and habitats(Packer et al., 2017). Species with general distributions tend to be more traits variable and have high intraspecific variation and show a continuous geographic gradient, especially across different latitudes as a consequence of acclimation across a broad range of environmental conditions (Ren et al., 2020, Siefert et al., 2015). Although plant traits have been extensively studied under different treatments (e.g. grazing, nutrient addition, etc.), intraspecific variation at the regional scale under natural conditions have rarely been considered (Pigliucci, 2010). Many studies have shown that large-scale variation in individual plant traits is associated with environmental gradients. Early plant biogeographers believed that climate and soil together determined the form and function of plants (Bruelheide et al., 2018, Simpson et al., 2016). However, the study of plant functional trait variability characteristics and their adaptation to external conditions is significant for understanding plant community construction in different regions and their response mechanisms to habitats (Suding et al., 2008, Kong et al., 2021).
The concept of economic spectrum of plant leaves has attracted wide attention from ecologists. By applying the "investment-benefit trade-off" theory of economics to the study of plant resource allocation, the interrelationships among plant functional traits and ecological adaptation strategies can be quantitatively analyzed (Osnas et al., 2013, Niinemets, 2015). Wright et al. (2004) defined a continuously varying spectrum of leaf functional trait combinations on a global scale, the "leaf economic spectrum (LES)", to illustrate trade-off strategies between vascular plant resource acquisition and storage, which can be expressed through the range of variation in trait indicators and their quantitative relationships (Wright et al., 2004). Since then, leaf economic spectrum studies have been extended to stems (SES), roots (RES) and whole plants (WPES), community structures and ecosystem types at different levels by related ecologists (Li et al., 2019, Pérez-Ramos et al., 2012, Kong et al., 2019). One end of the economic spectrum represents the "fast investment-gain" strategy of plants, where species with this feature have cheap tissue investment and fast return on investment (acquisition strategy); while the other end represents the "slow investment-gain" strategy of plants, where species with this feature have expensive tissue investment and slow return on investment (conservative strategy) (Wright et al., 2004, Reich and Cornelissen, 2014). As a kind of organ-level trade-off strategy spectrum, plant economic spectrum studies can better describe and generalize plant traits and functional types, providing new thinking perspectives and explore avenues for researching plant functional traits and ecological adaptation strategies of species.
The Inner Mongolia Plateau is located in the semi-arid and arid climate region of northern China, with a large longitude span, and the climate from east to the west shows the transition characteristics from semi-arid to the arid region. Most of the lakes in the region are inland-type lakes, and the species and community structure of the lakeshore wetlands are significantly influenced by the geographical location and climate environment. As a dominant species in the lakeshore wetlands of the arid and semi-arid region of Inner Mongolia, P. australis provides suitable conditions for studying possible latitudinal variation in morphology, growth and phenological traits in population distribution and climate response. In this study, the variation in morphological, nutrient and stoichiometry traits of P. australis under geographic location and the drivers of trait variation were analyzed, and the ecological adaptation strategies under different regional characteristics were elucidated by economic spectrum analysis. We surveyed reed community sample sites in 13 lakeshore wetlands and monitored 26 plant functional traits including phenotypic traits, nutrient content and stoichiometric ratios. These traits were from plant organs (leaves, stems and roots) that are particularly sensitive to environmental change, and essential for plant growth and reproduction capacity. We specifically assessed the following hypotheses: (1) The functional traits of P. australis are variable at different scales. At the local scale, at a broadly similar climate regime, we expect edaphically driven heterogeneity in moisture availability to primarily influence variation in morphological and nutrient economy traits of species in lakeshore wetlands, while under the latitudinal gradient dominated by climatic factors. (2) The economic spectrum theory also applies to leaves, stems, roots and whole plant individuals at the population level of reeds, and the ecological adaptation strategies for plant traits are different in semi-arid than in arid regions.