Effects of grazing exclusion years on module structure and growth pattern of Stipa grandis clones in a typical steppe

In this study, we selected different grasslands subjected to varying periods of grazing exclusion (10 and 20 years) and grazed grassland (control) as sites on the Loess Plateau. We harvested the whole clones to study the effects of number of grazing exclusion years on module structures and growth patterns of Stipa grandis clones, which provides a theoretical basis for studying module structures and resource allocation. We found that: 1) compared with grazing, grazing exclusion for 10 years significantly increased the size, reproductive height, and inflorescence height of S. grandis (P < 0.05), and grazing exclusion for 20 years markedly improved the size, total tillers, reproductive tillers, vegetative tillers, total buds, reproductive height, inflorescence height, total aboveground biomass, biomass of vegetative tillers, and biomass of inflorescence (P < 0.05). In addition, grazing exclusion reduced the coefficient of variation (CV) of reproductive tillers, reproductive height, inflorescence height, total aboveground biomass, biomass of reproductive tillers, and biomass of inflorescence. 2) For the biomass allocation ratio, vegetative tiller significantly increased, reproductive tiller decreased, and inflorescence showed no significant change (P > 0.05) with increase in grazing exclusion years (P < 0.05). 3) The biomass of vegetative tillers was significantly higher in grazing exclusion grasslands than in grazed grasslands (P < 0.05); however, there was no significant difference in the biomass of reproductive tillers and total tillers (P > 0.05). 4) The S. grandis size showed a significant positive correlation with the total aboveground biomass, reproductive tiller number, biomass of reproductive tiller, and biomass of inflorescence (P < 0.001). It also showed a significant positive correlation with biomass of vegetative tillers (P < 0.01) and a positive correlation with total tiller numbers (P < 0.05). Total aboveground biomass had a significant positive correlation with reproductive tiller numbers, vegetative tiller numbers, total tillers, and biomass of vegetative tillers (P < 0.001); it also had a significant positive correlation with total buds, biomass of reproductive tillers, and biomass of inflorescence (P < 0.01). Finally, total bud numbers showed a significant positive correlation with vegetative tiller numbers, total tiller numbers, and biomass of vegetative tillers (P < 0.001) and a significant positive correlation with total aboveground biomass (P < 0.01). In conclusion, short-term grazing exclusion improves the reproductive allocation of S. grandis and results in S. grandis rapidly becoming the dominant species, while long-term exclusion results in generation of S. grandis mainly through asexual reproduction.