Sand burial is a common phenomenon in sand dune ecosystems all over the world. Although many studies concerning the effects of sand burial on seed germination and seedling emergence have been conducted in many parts of the world, such studies are rare in China. In the summer of 2003, we studied the effects of sand burial depth (0, 1, 2, 4, 6, 8, 10 and 12 cm) and seed size on seed germination and seedling emergence of Psammochloa villosa, an important psammophyte that establishes on mobile sand dunes in deserts and sandy lands of North China. Seeds were collected in their natural habitat in Mu-Us Sandland, Ordos Plateau, in 2002 and stored at -18 ℃. The average seed mass of P. villosa was (5.51±0.05) mg. Seeds were sorted into 3 groups based on their mass: small (4 - 4.9 mg, mean = (4.489 ± 0.012) mg), medium (5 - 5.9 mg, mean = (5.457 ± 0.012) mg), and large (6 - 6.9 mg, mean = (6.415 ± 0.011) mg). We conducted two experiments, one in the field and the other in a non-heated greenhouse. In the second experiment, burial depth was 1, 2, 4, 6 and 8 cm. Before burial experiments, all seeds were treated with cold stratification in 5 ℃ for 4 weeks in order to break dormancy.
We found that sand burial depth had significant effects on seed germination and seedling emergence ofP. villosa. Experiments revealed that the highest percentage of seeds that germinated and seedlings that emerged occurred at 2 cm burial depth. Few seeds lying on the surface of the sand germinated, and, those that did germinate, could not anchor their roots into the soil and seedlings did not establish successfully. The next greatest germination and emergence percentage occurred at the 1 cm burial depth. The timing of emergence was delayed with deeper burial depth. As burial depth increased from 2 to 12 cm, the percentage of seeds that germinated and emerged declined, a higher number of seeds remained dormant, and the number of days before first emergence increased. As sand burial depth increased, some seeds were able to germinate but the seedlings were not able to emerge possibly because the seeds did not have enough energy. Seedlings that could not emerge, etiolated, died and decomposed in the sand. At all depths, the seeds that could not germinate remained dormant. Possible reasons for sustained dormancy include poor aeration, high sand moisture, low soil temperature, poor O₂ content, higher CO₂ levels in soil and low light intensity. The dormant seeds, which form soil seed banks, are very important for the long-term survival of this species. The dormant seeds can germinate when sand erosion decreases the burial depth and the seeds are exposed to optimal environmental conditions, such as sand depth. However, our experiment revealed that seeds buried at deeper depths had higher mortality rates (15 - 35%). The high mortality might be due to fungal infection, water-soluble germination inhibitors in the seed coat, and an unfavorable microenvironment and low O₂ concentrations.
Seed size did not affect the percentage of seeds that germinated or remained dormant, but had a remarkable effect on the percentage of seedlings that emerged at different sand burial depths. At greater depths (>6 cm), large seeds had higher rates of seedling emergence than medium and small seeds. From 1 to 6 cm depth, the seedling shoots that originated from large seeds were longer than those from small seeds; also, at 2 to 6 cm depth, seedling shoots that originated from medium sized seeds were longer than those from small seeds. Therefore, we suggest larger seeds have ecological advantages in sand dune ecosystems. Larger seeds have a larger endosperm, contain more energy, and thus can emerge from deeper burial depths. This will benefit seedling growth in early stages and help them to successfully establish under extreme desert conditions. Sand burial might be a selective pressure on seed size; therefore, seed polymorphism as an evolutionary strategy can benefit the renewal ofP. villosa populations. In sand dune habitats, P. villosa produced different sizes of seeds that could emerge from different depths of sand burial. The diversity of seed size could enhance opportunities for seedling establishment in this heterogeneous environment allowing plants to respond to different sand burial depths, enhance their ability to adapt to the changing environment, and increase survival.