Characteristics of Vegetation Evolution and Time-Lag Response to Hydrothermal Effects in Tianjin, China

[Objective] Clarifying dynamic changes of vegetation activities and time lag in response to hydrothermal conditions in Tianjin can provide a scientific basis for regional ecological environment management and sustainable development. Previous studies have investigated urban vegetation dynamic differentiation based on different data sources and time scales, while spatiotemporal patterns of vegetation over long time series, as well as the direction and extent of vegetation response to changes in hydrothermal conditions, still remain uncertain. Most current studies lack specific quantification of the lag duration of climate change. [Methods] This paper took the MODIS13 dataset, air temperature, and precipitation dataset of Tianjin from 2000 to 2022 as research objects. Based on the MatLab platform, methods such as the Hurst index, Theil-Sen trend analysis, Mann-Kendall test, geographic information system (GIS) spatial analysis, and time-lagged partial correlation analysis were adopted to investigate the characteristics of vegetation dynamics and their response to hydrothermal effects in Tianjin over the past 23 years. Additionally, this study analyzed vegetation evolution characteristics and their response to climate change, providing references for understanding ecological environment response under climate change. [Results] The results showed that: (1) annual average NDVI in Tianjin was 0.782, showing an overall trend of first increasing and then decreasing. Areas with high vegetation cover were mainly distributed in northern Jizhou District, northern Baodi District, and Ninghe District. The intra-annual fluctuation trend showed a certain positive correlation with changes in temperature and precipitation. (2) The mean Hurst index for the entire city was 0.493, with 45.225% of the areas having a Hurst index >0.5. Overall, vegetation in Tianjin showed a trend of past degradation but future improvement. Areas with continuous NDVI degradation were mainly distributed in Xiqing District, Ninghe District, and Dongli District. (3) 10.943% and 61.408% of the areas in Tianjin had time-lag effects on temperature and precipitation, respectively. The average lag time of vegetation response to temperature across the city was 2.737 months. The vegetation response to precipitation had a time lag of 1 to 3 months, with an average lag time of 1.016 months. Overall, vegetation was more sensitive to precipitation, and the average lag time of vegetation response to precipitation in the city was shorter than that to air temperature. (4) The responses of different vegetation types to air temperature and precipitation varied. Broad-leaved forests had the shortest response time to precipitation, while herbaceous cover had the longest response time. Sparse vegetation had the fastest response to air temperature, and broad-leaved forests had the slowest response. [Conclusion] From 2000 to 2022, the overall vegetation cover level in Tianjin was good, with NDVI showing a trend of first increasing and then decreasing, indicating past degradation but future improvement. The correlation between precipitation and vegetation is higher, and the lag time of air temperature is longer. Vegetation in Tianjin exhibits lag response characteristics to both air temperature and precipitation, and lag time varies among different vegetation types.