Advances in spring leaf phenology are mainly triggered by elevated temperature along the rural-urban gradient in Beijing, China.

Urbanization-induced phenological changes have received considerable attention owing to their implications for determining urban ecosystem productivity and predicting the response of plants and ecosystem carbon cycles to future climate change. However, inconsistent rural-urban gradients in plant phenology remain, and phenological drivers other than temperature are poorly understood. In this study, we simultaneously observed the micro-climate and spring leaf phenology of seven woody plant species at 13 parks along a rural-urban gradient in Beijing, China. The minimum (T) and mean (T) air temperature and the minimum (VPD) and mean (VPD) vapor pressure deficit increased significantly along the rural-urban gradient, but the maximum air temperature (T) and maximum vapor pressure deficit (VPD) did not. All observed leaf phenological phases for the seven species were significantly advanced along the rural-urban gradient by 0.20 to 1.02 days/km. Advances in the occurrence of leaf phenological events were significantly correlated with increases in T (accounting for 57-59% variation), T (21-26%), VPD (12-16%), and VPD (3-5%), but not with changes in T or VPD. Advances in spring leaf phenology along the rural-urban gradient differed between non-native species and native species and between shrubs and trees. The reason may be mainly that the sensitivities of spring leaf phenology to micro-climate differ with species origin and growth form. This study highlights that urbanization-induced increases in T and T are the major contributors to advances in spring leaf phenology along the rural-urban gradient, exerting less influence on native species than on non-native species.