The negative effects of enhanced ultraviolet-B (UV-B) on plant growth and development have been reported with many species. Considering the ability of jasmonic acid (JA) to improve plant stress tolerance, the hypothesis that JA pretreatment could alleviate the adverse effects of UV-B on S. baicalensis was tested in this study with photosynthesis and growth characteristics. The results showed that UV-B or JA alone both induced photosynthesis inhibition and decreased biomass in stems and leaves. However, the photosynthetic reduction caused by increased UV-B was mainly related to the effect of nonstomatal-limitation, while that of JA was a stomatal-limitation effect. JA pretreatment prior to UV-B could remit the photosynthetic inhibition via the recovery of chlorophyll content, stomatal conductance; and intercellular CO2 concentration (especially the maximum electron transport rate increase). Furthermore, the coaction of JA and enhanced UV-B alleviated some disadvantageous effects on the leaf and did not aggravate the growth damage induced by their separate actions.

Question In the Taibai Mountains, birch forests dominated by Betula albo‐sinensis and Betula utilis are considered unsustainable because there are few seedlings or saplings in the forests. Tree‐fall gaps play a central role in the natural regeneration of trees. In this study, seed germination and seedling emergence in gap and non‐gap plots were investigated to explore the role of tree‐fall gaps in the natural regeneration of birch forests. Location Taibai Mountains, Shaanxi Province, China. Methods A field survey was conducted to reveal the relationship between tree‐fall gaps and birch seedlings. Field and laboratory experiments were conducted to investigate seed germination and seedling establishment in gap and non‐gap plots. Results In the field survey, occurrence of birch seedlings was correlated with gap size, which confirms that tree‐fall gaps play a crucial role in the regeneration of birch forests. However, in the field experiment, seed germination was extremely low in both gap and non‐gap plots, and all seedlings died within 60 d in a seedling establishment test. The only difference between gap and non‐gap plots in the field experiment was that more seeds survived winter in the gap plots than in the non‐gap plots. The laboratory experiment helped us understand results of the field experiment. In the seed germination tests, Betula seeds germinated well in light treatments, but very few germinated in darkness, indicating that Betula seeds are photoblastic. No seeds germinated at 15/5 °C (day/night temperature regime), demonstrating the reason for lack of germination in the field experiment. The cover of forest litter reduced the seed germination rate. Seeds stored in dry conditions maintained high survival rates after long‐term storage at low temperature. However, the survival of imbibed seeds decreased with decreasing storage temperature. Conclusions Seed germination and seedling establishment of Betula require a dry and light environment. The main regeneration barriers to seed germination and seedling growth in the non‐gap plots are chilling injury and lack of light. Tree‐fall gaps are better habitats for Betula seed survival and seedling establishment. This indicates that regeneration of birch forest relies on disturbance.