Species diversity plays an important role in urban ecosystem stability and sustainability. Research on woody plant diversity among different administrative districts is vital to effectively improving urban biodiversity. In this study, a stratified random sampling method was used to set the sample plot, a spatial mapping of the diversity indices was conducted, and methods of principal component analysis and similarity analysis were used to analyze the species composition and dissimilarity among different administration districts. Finally, the model based on the relationship between the diversity indices and forest landscape pattern indices was built. The results showed that woody plant species diversity differed significantly among different administrative districts in Changchun. The species diversity levels in the Qikai (QK), Lvyuan (LY), and Kuancheng (KC) districts were higher, and those in the Nanguan (NG) and Erdao (ED) districts were lower. The species composition of every district basically complied with the 10/20/30 “rule of thumb”. The most common species, genera, and families in urban forests should not exceed the proportion of the total abundance of all species. There was a significant correlation between the diversity indices and the forest landscape indices of each district. The Margalef index (d_Ma) and the Shannon–Wiener index (H′) decreased significantly while CONTIG-MN increased, and increased significantly while LSI increased. Species evenness index J decreased significantly while LPI increased, and increased significantly while DIVISION increased. In future tree species selection, native tree species should be given priority as much as possible, the planting of Salicaceae should be reduced, and the species diversity of each district should be increased. Chaoyang (CY), ED, and LY districts should strengthen the links between forest patches.

Alien plants invasion have become a hot issue in the field of ecology. The invasion of alien plants is usually accompanied by changes in the physical and chemical properties of the soil, the ensuing negative feedback creates a favorable environment for its own growth and expansion. Invasive plans have a strong ability to sequester carbon, which can greatly affect the original local ecological environment. In this study, we selected Rhus typhina, an invasive plant widely used for greening, as the experimental subject and natural growing grassland as the control. The aims were to investigate the effects of different degrees of invasion of R. typhina on soil physicochemical properties and carbon emissions, and to explore the influential factors on carbon emission. The results showed that R. typhina invasion significantly increased soil pH, total nitrogen content, easy extraction of glomalin-related soil protein (EEG) and cumulative CO₂ emissions (CEM). It is worth noting that the CEM increased significantly during the severe invasion by R. typhina. The significant increase in soil NH₄⁺-N content and the decrease in soil NO₃⁻-N content indicate that the soil after the invasion of R. typhina has better uptake of NH₄⁺-N. Temperature and soil moisture content had significant direct effects on CEM, while NH₄⁺-N, NO₃⁻-N, EEG and temperature sensitivity of soil organic carbon mineralization Q₁₀ (30 °C/20 °C) had a direct but non-significant effect on CEM. The above findings suggest that R. typhina can generate positive feedback by influencing the physicochemical properties and CEM of the soil, opening the way for its own expansion, which can be targeted to prevent the destruction of local ecosystems during the introduction of cultivation and subsequent management.

Middle-aged trees refer to trees aged between 50 and 99 years, which are the reserve resources of old trees (trees ≥ 100 years of age). They are vital parts of the urban ecosystem, with important ecological, landscape, cultural, and historical value. Conservation of middle-aged trees in urban areas is important for the development of large old trees in the future. In this study, we investigated the middle-aged trees in Changchun city and analyzed the species composition and diversity characteristics of different urban green space types and tree age classes. The results showed that there were 72 species and 22,376 plants of middle-aged trees in Changchun city, and the coniferous species prevailed. The top five species with a high importance value (IV) were Pinus tabuliformis var. mukdensis, Lavix olgensis, Salix matsudana, Ulmus pumila, and Abies holophylla. Green space type and tree age were important factors influencing the richness and diversity of middle-aged trees. Tree growth spaces were relatively sufficient, and land use was stable for park green spaces (PGS) and attached green spaces (AGS), which resulted in the abundant, richer, and diverse species richness (SR) of middle-aged trees. Road green spaces (RGS) and square green spaces (SGS) had fewer trees and lower species richness, Margalef richness index (dMa), Shannon–Wiener index (H_e) and evenness index (J_e) which could be attributed to the high intensity of human interference and poor environmental quality. The SR of middle-aged trees decreased with an increase in age class, and the values of SR in Age Class 80–89 years and Age Class 90–99 years were lower than in Age Class 50–59 years. Age Class 70–79 years had the lowest values of dMa, H_e, and J_e, which need to be protected urgently. The results of this study can provide a basis for the conservation and management of middle-aged trees in urban areas and the choice of species for urban greening.