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

Nowadays, atmospheric concentrations of nitrogen oxides (NOX) 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 NOX 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 NOX, 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 NOX 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 NOX regime. Overall, this study found that atmospheric NOX 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.