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The Triggering Effect of Gaps on Seedling Germination of the Soil Seed Bank in Tropical Rain Forests, Hainan Island, South China

Light, soil temperature, and soil moisture likely change when a gap or clearing is formed in closed-canopy forests, triggering seed germination in the soil seed bank. However, which factors induce such seed germination remains elusive. In this study, we explored the triggering mechanism of gaps on seed germination in the soil seed bank without felling trees. Power sources were supplied in the forest, and three types of lamps were used to simulate the photo-thermal, light, and temperature environments of gaps (i.e., photo-thermal separation in the field), respectively. The photo-thermal separation experiment was carried out in the tropical rain forests of Bawangling in Hainan Island, South China. Three common pioneers and one late-successional species of the Bawangling area were selected for an indoor photo-thermal separation experiment. The field experiment results showed a significant difference in the average seedling number between groups exposed to light and the control group (13.2 ± 4.0 and 1.4 ± 1.7, respectively; p< 0.01), indicating that light in gaps can initiate seed germination of some species in the soil seed bank. Further indoor validation experiments supported this conclusion. No significant difference was observed in the average seedling number between the thermal group (2.1 ± 1.6) and the control group. The indoor validation experiment showed that changes in temperature alone could not trigger the seed germination of the three pioneer species in darkness. However, a higher average seedling number was observed in the photo-thermal group (15.7 ± 5.6) compared to the light group, indicating that the combined effect of light and temperature can initiate seed germination in the soil seed bank, which was also supported by the indoor verification experiment. We further showed that the ratio of species requiring only light for seed germination to those requiring both light and temperature was 2:3. More case studies are necessary to determine if such outcomes are common in forest soil seed banks.