First record of Ulva californica in the mainland of China:a single alien parthenogenetic population in discontinuous distribution

Molecular investigations have raised concerns about the ecological risks of green tides caused by alien Ulva species in new habitats.The green tide-forming species U.californica Wille was generally considered to be native to North America,but new records have been widely reported in Europe,Asia,and Oceania in recent decades,indicating a strong dispersal capacity of the species.In this study,the first record of U.californica on the coastline of China's Mainland was reported,following a combined identification with multi-molecular markers and morphological characterization.It was shown that this species has a discontinuous distribution pattern along the coast of China's Mainland,with northern populations in the Yellow Sea and southern populations in the East China Sea and South China Sea.According to results of examination for life cycles and identification with mating type(MT)genetic markers,it was indicated that all U.californica samples were male gametophytes,and reproduced themselves through parthenogenesis solely.Combined with the fact that southern and northern populations are highly genetically identical,here we believed that U.californica was a recent alien species to China's Mainland with a rapid local spread.This finding provided evidences that the ability to reproduce in a variety of ways may play an important role in the spread of Ulva species,as well as essential basic data for marine risk management of green tides in China.In addition,according to the phylogeographic analysis,the possible geographical origin and global dispersal routes of U.californica were also proposed.

Effects of Land Cover on Soil Organic Carbon Stock in a Karst Landscape with Discontinuous Soil Distribution

Land cover type is critical for soil organic carbon （SOC） stocks in territorial ecosystems. However, impacts of land cover on SOC stocks in a karst landscape are not fully understood due to discontinuous soil distribution. In this study, considering soil distribution, SOC content and density were investigated along positive successional stages （cropland, plantation, grassland, scrubland, secondary forest, and primary forest） to determine the effects of land cover type on SOC stocks in a subtropical karst area. The proportion of continuous soil on the ground surface under different land cover types ranged between 0.0% and 79.8%. As land cover types changed across the positive successional stages, SOC content in both the 0-20 cm and 20-50 cm soil layers increased significantly. SOC density （SOCD） within O-lOO cm soil depth ranged from 1.45 to 8.72 kg m^-2, and increased from secondary forest to primary forest, plantation, grassland, scrubland, and cropland, due to discontinuous soil distribution. Discontinuous soil distribution had a negative effect on 8OC stocks, highlighting the necessity for accurate determination of soil distribution in karst areas. Generally, ecological restoration had positive impacts on SOC accumulation in karst areas, but this is a slow process. In the short term, the conversion of croplandto grassland was found to be the most efficient way for SOC sequestration.

Sequential city growth： Theory and evidence from the US

City growth patterns are attracting more attention in urban geography studies. This paper examines how cities develop and grow in the upper tail of size distribution in a large-scale economy based on a theoretical model under new economic geography framework and the empirical evidence from the US. The results show that cities grow in a sequential pattern. Cities with the best economic conditions are the first to grow fastest until they reach a critical size, then their growth rates slow down and the smaller cities farther down in the urban hierarchy become the fastest-growing ones in sequence. This paper also reveals three related features of urban system. First, the city size distribution evolves from low-level balanced to primate and finally high-level balanced pattern in an inverted U-shaped path. Second, there exist persistent discontinuities, or gaps, between city size classes. Third, city size in the upper tail exhibits conditional convergence characteristics. This paper could not only contribute to enhancing the understanding of urbanization process and city size distribution dynamics, but also be widely used in making effective policies and scientific urban planning.