How did the diversity of woody plants in urban forests vary with the urban-rural gradient in Qingdao, China?

The distribution and diversity of woody vegetation are crucial for understanding the structure and ecology of urban forests. As urbanization accelerates, the construction and composition of urban forests vary significantly along the urban-rural gradient. Qingdao's urban forests offer an opportunity to test the relationship between the diversity of woody plants and the urban-rural gradient. We classified the urban-rural gradient using imperviousness and construction time, then investigated the diversity of woody plants in Qingdao's urban forests under different urban-rural gradients and tested the reasonableness of their allocation. Correlation analysis found that the diversity index of woody plants in urban forests was highly connected to the urban-rural gradient (by imperviousness: r_{Margalef Index} = -0.589, r_{Shannon-Wiener Index} = -0.373, r_Pielou _Index = -0.170, r_{Simpson Index} = 0.272/by construction time: r_{Margalef Index} = -0.530, r_{Shannon-Wiener Index} = -0.360, r_Pielou _Index = -0.148, r_{Simpson Index} = 0.272/0.174). With a decrease in urbanization density, the Margalef (<i>H</i>), Shannon-Wiener (<i>H'</i>), and Simpson (<i>D</i>) indices all decreased while the Pielou (<i>E</i>) index increased. The four diversity indices showed a substantial correlation with one another, but not with the Margalef and Pielou indices. The analysis utilizing the 10/20/30 rule of empirical demonstrates a clear irrationality in allocating shrub species in Qingdao's urban forests, and the distribution of tree species is reasonable. Based on the study results, strategies for optimizing and enhancing urban forests in Qingdao are proposed for different urban-rural gradients, respectively. This study can provide a scientific framework for urban biodiversity conservation and management in Qingdao and serve as a guide for urban forests and greening with comparable climates.