Allelopathy is an important process in plant communities. The effects of allelopathy on seed germination and seedling development have been extensively investigated. However, the influences of extract soaking time and concentration on the foregoing parameters are poorly understood. Here, we conducted a seed germination assay to determine the allelopathic effects of the donor herbs Achnatherum splendens (Trin.) Nevski, Artemisia frigida Willd., and Stellera chamaejasme L., from a degraded grassland ecosystem in northern China, on lettuce (Lactuca sativa L.) seed germination and early seedling growth. Extract soaking times (12 h or 24 h) did not exhibit significantly different effects on lettuce seed germination or seedling development. However, all aqueous herb extracts inhibited lettuce seed germination and root length (RI < 0) and promoted lettuce shoot length, stem length, leaf length, and leaf width (RI > 0) at both low (0.005 g mL−1) and high (0.05 g mL−1) concentrations. Moreover, A. splendens extracts increased seedling biomass (RI > 0) and synthetical allelopathic effect (SE > 0) at both concentrations. In contrast, both A. frigida and S. chamaejasme extracts had hormesis effects, which stimulate at low concentrations (RI > 0) but inhibit at high concentrations (RI < 0) on seedling biomass and synthetical allelopathic effect (SE). The results suggest that allelopathic potential may be an important mechanism driving the dominance of A. frigida and S. chamaejasme in degraded grasslands. Reseeding allelopathy-promoting species such as A. splendens may be beneficial to grassland restoration. The present study also demonstrated that seedling biomass, root and shoot length, and seed germination rate are the optimal bioindicators in allelopathy assays and could be more representative when they are combined with the results of multivariate analyses.

Changes in species composition and diversity-productivity relationships during vegetation succession are of great significance for the restoration of degraded grasslands. However, the human intervention of grassland restoration mostly considers the single ecological (diversity) or production (biomass) function of grasslands, and does not fully consider the relationships between the two to improve the restoration effects. In this study, three vegetation types (desert grassland, sandy grassland and temperate savanna) were considered to simulate the three stages of restoration of degraded temperate savanna (early, middle and late restoration). Changes in species composition, diversity, and diversity-productivity relationships during the temperate savanna restoration were explored through spatiotemporal substitution. The results showed that with the progress of succession, xerophytes decreased significantly, mesophytes and hygrophytes increased, C4 plants decreased gradually, and C3 plants increased. The change of species richness was the main reason for community differences in the process of community succession, and species replacement mainly occurred between communities in the early and middle stages of succession. Species richness increased significantly from early to late stage of restoration. Latitude was significantly negatively correlated with species diversity. In the process of vegetation restoration, the impact of species richness, Pielou’s evenness, and β-diversity on biomass gradually developed positively and continued to increase, while the Simpson index showed the opposite effect. This study provides references for the restoration of temperate savanna and emphasizes the ecological and productive functions of the grassland should be taken into account in the restoration strategy.

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