| Tropical forests are terrestrial ecosystems harboring the most abundant biodiversity on Earth and play an important role in maintaining the Earth’s ecological health and biodiversity. Gaps play an important role in maintaining the dynamic cycle and diversity of forest. After a gap formed, a large number of seedlings which keep dormant or resting before emerged from soil seed banks. Three environmental factors are of particular importance:temperature, light, and moisture. All three of these factors are affected when a gap or clearing is formed in closed-canopy forest. Therefore, gap formation should influence seed germination. But how the gap affect the seedling emerged from soil seed banks and which factor played a decisive role. So far, these problems still have not been satisfactorily answered. In order to study the triggering effect of gap on seedling emergence of the soil seed bank in tropical rain forest, we investigated the species diversity of aboveground vegetation and soil seed banks of tropical montane rain forest in Bawangling, Hainan island, south China, carried out the germination experiment of soil seed banks in this tropical rain forest by photo-thermal separation and selected four species from different ecological forms to conduct seed germination in artificial climate chamber. The main conclusions are as follows,1There are a large number of viable and resting seeds of pioneer species in soil seed banks. Seed density of soil seed banks is1694/m2. Pioneer species were the dominant emerging seedlings and up to86%of these were from five species:Thysanolaena maxima, Melastoma affine, Melastoma sanguineum, Trema orientalis and Scleria biflora. On the vertical distribution characteristics of the soil seed banks, the species amount is the most in topsoil, the second in middle soil layer and the last in subsoil, but the seed amount is the last in topsoil, the second in middle soil layer and the most in subsoil. The similarity of species composition between aboveground vegetation s and its soil seed bank was low, similarity index was only0.05, indicating that the soil seed bank and aboveground vegetation was not closely linked. The similarity index was lower between soil seed banks and the seedling bank, indicates that the soil seed bank and aboveground vegetation similarities will become smaller as the community succession.2In germination experiment of soil seed banks, light intensity of searchlight group was up to about954umolm-2s-1, daily maximum temperature of topsoil was between26.7℃-34.6℃, the amplitude of diurnal variation in temperature was between7.4℃-19.4℃. light intensity of heat lamps group was around1.3umolm-2s-1, daily maximum temperature was between29.3℃-41.7℃, the amplitude of diurnal variation in temperature was between7.8℃-24.6℃. In the group of illumination energy-saving lamps, light intensity was about75umolm-2s-1, the daily average temperature of topsoil was between13.9℃-22.2℃, the amplitude of diurnal variation in temperature was between2℃-3.9℃. Compared with100m2measured gaps, the light intensity and temperature of searchlight group was similar with gaps, successfully simulated the light and temperature characteristics of gaps. The light intensity of saving lamp treatment was similar with gaps, but the temperature of surface was similar with understory topsoil, successfully simulated the light characteristics of gaps. The temperature of heat lamp treatment was similar with gaps, but light intensity was similar with understory, successfully simulated the temperature characteristics of gaps.3The results of the soil seed bank germination experiments in forest under photo-thermal separation showed that:(1) Light is the key factor to trigger the most seedling emergence from the soil seed banks in tropical rain forest. In energy-saving lamp treatment (light group, LG), a large number of seedlings emerged from soil seed bank when light intensity increased, a total of119seedlings belonged to six species emerged from all samples;(2) The combined effects of light and temperature can trigger more larger number of seedlings belonged to more species emerged from soil seed banks. In the searchlight treatment (photo-thermal group, PTG), light intensity and soil temperature increased at the same time, more seedlings emerged from the soil seed banks, a total of141seedlings belonged to12species from all samples;(3) It was difficult to judge the effect of temperature on seedling emergence from ceramic heating lamp treatment (temperature group, TG), because soil volumetric water content was only0.097in TG while soil moisture were more higher in other three groups. So it is necessary to do indoor experiment to explore the effect of temperature on seedling emergence from seed banks.(4) Soil moisture can not start the seedling emergence, but higher soil moisture was favor for seedling emergence, in understory germination experiment the number of emerged seedlings was more in the two groups with higher soil moisture(soil volumetric water content was0.150and0.140) than the group with low soil moisture (soil volumetric water content was0.097), the indoor humidity control experiments also verified this result, there is a certain number of seedlings emerged (24.8seedlings) when moisture content was0.195, more seedlings emerged(34.6-36.8seedlings) when the water content was between0.232to0.275.4The results of the seed germination experiments of different ecological species in artificial climate chamber showed that:(1) Light in gaps triggered the seed germination of herbal pioneer Thysanolaena maxima and shrub pioneer Melastoma sanguineum, temperature and soil moisture had no significant effect on the germination of their seeds.(2) Temperature in gaps triggered the seed germination of pioneer tree Trema tomentosa. Seeds of Trema tomentosa only germinated in high amplitude of temperature variation, high light intensity can reduce the demand for temperature. There was no seed germination in40%soil moisture, indicating that high soil moisture inhibit seed germination of Trema tomentosa.(3) Seed germination of non-pioneer tree Cryptocarya chinensis was not subject to the regulation of environmental factors in gaps in the forest. FG was not significantly different in three light levels and temperature. There was no seed germination in40%soil moisture, indicating that high soil moisture inhibit seed germination of Cryptocarya chinensis.5The results of field experiments and indoor experiments all showed no seedling emerged in dark or forest understory, light intensity of gaps triggered a large number of seedlings emerged, combination of light intensity and soil temperature in gaps triggered more seedling belonged to more species emerged, temperature can trigger seed germination of Trema tomentosa, soil moisture changes in gaps had no significant effect on seedling emergence in soil seed banks. Therefore, in the environmental factors of gaps, the light, the temperature and their combined effects triggered seedlings emerging from soil seed banks in tropical montane rain forest in Bawangling, Hainan Island, south China. |
