Abstract:

Aims Zonation patterns of plant communities are determined by abiotic factors, such as soil salinity across the elevation gradient at the coastal salt marsh. Currently, little study has tested the spatial variation of zonation patterns at the costal salt marsh in the Yellow River Delta. In this study, we fill this knowledge gap by conducting a simulated experiment in a greenhouse to investigate the effects of soil type and groundwater depth on the spatial differentiation of salt marsh plant communities.

Methods We constructed plant communities consisting of seven common species in the Yellow River Delta in two soil types (i.e., coastal soil and shell sand soil), and subjected them to eight levels of soil depth. After two months, we counted the number of living species and measured aboveground biomass of each species.

Results (1) Richness and Shannon-Wiener diversity index of plant communities in the shell sand soil were greater than that in the coastal soil, and plant richness exhibited a unimodal pattern as groundwater depth increased in the sand soil. Plant richness did not exhibit the same unimodal pattern for coastal soil. The same unimodal pattern was observed for Shannon-Wiener diversity in response to increasing the groundwater depth in both shell sand soil and coastal soil, peaking at groundwater depths of 60 cm and 70 cm, respectively. (2) With increasing of groundwater depth, plant communities in the shell sand soil changed from halophytic community type dominated by Suaeda salsa and Cynanchum chinense to mesophytic community type dominated by Artemisia mongolica and Ziziphus jujujuba. However, aboveground biomass of plant communities was unaffected by soil type, and only showed a unimodal pattern with increasing groundwater depth. (3) Water content in the shell sand soil was significantly lower than that in the coastal soil, decreasing as groundwater depth increased and stabilizing at a depth of 60 cm. Similarly, water content of the coastal soil showed a negative relationship as groundwater depth increased, keeping stable from a depth of 50 cm. (4) Salt content of the coastal soil was significantly greater than that of the shell sand soil. Salt content of the shell sand soil showed a bimodal pattern as groundwater depth increased. However, salt content of the coastal soil was unaffected by groundwater depth. (5) Bulk density and capillary porosity of the shell sand soil were significantly lower than those of the coastal soil. Shell sand soil had the highest content of coarse sand particles, while coastal soil had the highest content of fine sand particles. These results indicate that the shell sand soil is composed of coarser particles, which determines its low bulk density and capillary porosity.

Conclusion The low bulk density and capillary porosity of the shell sand soil can block capillary action of highly mineralized groundwater to reach the surface, thereby reducing the salt and water content of the shell sand soil and creating continuously changing habitats including saline and dry habitat and thus maintaining high biodiversity. The coastal soil is composed of finer particles, which determines its high capillary porosity. Groundwater can rise to the soil surface and deposit large quantities of salt, forming saline soil and thereby reducing plant diversity. Thus we conclude that both soil type and groundwater depth in typical wetlands of the Yellow River Delta drive changes in soil water and salt transportation patterns, resulting in different ecological effects, which determines spatial differentiation and biodiversity maintenance of plant communities in the typical salt marshes of the Yellow River Delta.

Key words: coastal wetlands, shell sand soil, coastal soil, soil water and salinity transportation, plant diversity