Variation in plant functional traits explains the substitution distribution and allocation strategy of <i>Stipa</i> species across natural grasslands of Ningxia, Northern China

Functional traits reflect plants' adaptability to their environment, and environmental gradients influence their distribution. But few studies have investigated the link between these traits and species substitution patterns or the relevant ecological factors. We measured the aboveground (leaf) and belowground (root) functional traits of <i>Stipa</i> species in 17 plots across natural grasslands in Ningxia in Northern China. Redundancy analysis was used to explore the relationships between <i>Stipa</i>'s functional traits and its species substitution distribution. Then, on the species substitution gradient, principal component analysis (PCA) was used to verify and quantify the leaf economic spectrum (LES), root economic spectrum (RES), and whole-plant economic spectrum (WPES), with the relation between these spectra investigated by fitting standardized major axis regressions. The effects of aboveground, belowground, and whole-plant ecological factors were quantified and ranked by variance decomposition and hierarchical partitioning. Our results showed that functional traits drive the substitution distribution of <i>Stipa</i> species, in being variously coupled with its desert, typical, and meadow steppe habitat types. The leaf, root, and whole-plant economic spectra of <i>Stipa</i> species in desert steppe exhibit a "quick investment-acquisition" strategy. In typical steppe, the leaf and whole-plant economic spectra of <i>Stipa</i> species correspond to a "fast investment-acquisition" strategy, whereas the root economic spectrum adopts a "slow investment-acquisition" strategy. On meadow steppe, the leaf, root, and whole-plant economic spectra of <i>Stipa</i> species similarly adopt a "slow investment-acquisition" strategy. Finally, when considering the environmental factors involved, we find that the substitution distribution of <i>Stipa</i> spp. is chiefly a response to shifting soil patterns, these mainly driven by soil total nitrogen and nitrogen/phosphorus ratio. Collectively, these findings provide an important reference for the ecological restoration and reconstruction of grassland ecosystems, to better understand the relationship between plant functional traits and ecological niche attributes, and thus guide the reasonable restoration of grassland vegetation.