Urbanized and climatic influences on vegetation changes of urban green space under the increase of atmospheric NOX: A case study in Tianjin

• NO X inhibits the growth of forests, causing vegetation browning of urban green space. • Urbanization and climate change are positive with NO X , co-limiting forest production. • The responses of forest understory layers to environmental changes are different. • Urbanization increases leaf herbivory and lead to severe pests of urban green space. Nowadays, atmospheric concentrations of nitrogen oxides (NO X ) have increased rapidly and exceeded the threshold of ecosystems, inevitably affecting the plant growth and community stability, especially in urban green space. The increase of atmospheric NO X is also accompanied by natural and anthropogenic environmental change, which also bring severe environmental stresses on green vegetation. To better understand drivers that contribute to the vegetation changes, this study explored the interaction among atmospheric NO X , climate, terrain, and urbanization, and its impact on the tree leaf and forest community traits, using a dataset with 118 plots of tree communities composed of 5781 trees across distinct types of urban green space in Tianjin, a highly urbanized metropolis in China. The results indicated that atmospheric NO X concentration showed a positive association with urbanization and temperature, which co-limited the tree growth and resulted in the vegetation browning of urban green space. The responses of coniferous and broadleaved forests were different, whose understory herbaceous diversity and height had contrary tendencies to environmental change. Meanwhile, urbanization intensified atmospheric N deposition and heat island effect, increasing the leaf chlorophyll contents of tree species in urban green spaces. While leaf herbivory showed positive correlation with urbanization, temperature, and evaporation only in the high NO X regime. Overall, this study found that atmospheric NO X could be used as a good indicator of urbanization. These findings also advanced our recognition of the environmental driving factors of green vegetation dynamics, and provided references for the research of elsewhere.