Niche Characteristics of Dominant Plant Populations in Desert Steppe of Ningxia with Different Enclosure Times Understory vegetation community structure and diversity in low-value lichee (Litchi chinensis Sonn.) plantation in Dongguan of South China Determining niches of the major tree species based on a flow accumulation technique in a natural secondary forest in Northeastern China Study on the Niche Characteristics of Main Tree Populations in Tropical Mountain Rain Forest at Jianfengling, Hainan lsland