## 5 Conclusions  
In this study, we investigated the trends of drought extent (SPEI), vegetation cover (LAI), evapotranspiration (ET), and photosynthesis (GPP) in the Songnen Plain of northeastern China from 2000 to 2020 at the pixel scale. The bidirectional causality, time-lag, and cumulative response among the factors were analyzed to reveal the interactive relationship between drought and ecosystem function. In the last 21 years, the drought situation in the Songnen Plain had improved in different degrees. Vegetation cover changed insignificantly, whereas both ET and GPP showed increasing trends. Some degradation were found in the forest land. There was a certain degree of bidirectional causality between SPEI and LAI, ET, and GPP, among them the number of pixels were highest in the responding relationship of LAI-SPEI and SPEI-GPP. The response time and intensity of the variables to the shocks varied significantly among species. The greatest response intensity were basically concentrated between 6 th and 8 th months, roughly having a positive mutual contribution. The cumulative impulse response coefficients showed significant spatial differences and positive value of pixels took predominantly, indicating that LAI, ET, and GPP in most areas increased in 12 months after the impact as the Songnen Plain became wetting. On the other hand, the increase in values of ecosystem function indicators would promote the study area toward a wetter direction. The negative response areas were mainly distributed in the northeast (edge of the Xiaoxing'an Mountains) and southeast border areas, dominated by woody plants. The above results indicate that there existed some time-lag and cumulative effect between water conditions and plant interactions in temperate semi-arid and semi-humid transition areas, and they were varied among regions and vegetation species.  
The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/rs14225887/sl](https://www.mdpi.com/article/10.3390/rs14225887/sl)]([https://www.mdpi.com/article/10.3390/rs14225887/sl](https://www.mdpi.com/article/10.3390/rs14225887/sl)). Figure S1. Area changes of major land use/cover during 2001-2020. Figure S2. Spatial distribution and the area percentage of major land use type change types over three different periods in the Songnen Plain. Figure S3. Change trends of: (a)Temperature, (b) Precipitation from 2000 to 2020. Figure S4. Spatial distribution of: (a) Temperature, (b) Precipitation from 2000 to 2020 (\(p<0.05\)).  
H.L.: Conceptualization, Methodology, Investigation, Writing-original draft, Visualization. F.H.: Conceptualization, Resources, Writing--review & editing, Supervision, Funding acquisition. X.H.: Methodology, Writing--review & editing. P.W.: Data processing, Writing--review & editing. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the Natural Science Foundation of Jilin Province, China (Grant No. 20220101155 JC), the National Natural Science Foundation of China (Grant No. 41571405), and the Education Department of Jilin Province, China (Grant No. JJKH20211289 KJ).  
Not applicable.  
The authors would like to thank the reviewers and editors for their valuable comments and suggestions.  
The authors declare no conflict of interest.