Drought lag and its cumulative effects on vegetation dynamics and response to atmospheric circulation factors in Yinshanbeilu, Inner Mongolia

The Yinshanbeilu region is arid, with little precipitation; its ecological environment is very fragile. This region is an important ecological security barrier in the north of China. To explore the cumulative and delayed effects of drought on vegetation, this study employed wavelet coherence, copula, and partial correlation analysis. Our findings indicated that whereas the annual mean value of the standardized precipitation evapotranspiration index (SPEI) exhibited a declining trend over time, the annual maximum value of the normalized difference vegetation index (NDVI) showed a rising tendency. The two spatial trends were also found to be substantially similar. At various timelines, there were significant variations in the association between the cumulative SPEI and NDVI, with an overall trend that first increased and subsequently decreased. The longest lag times were localized at 5, 8, and 4 months for grassland, forestland, and farmland, respectively. The likelihood of NDVI degradation rises as drought intensity rises. At short and medium scales, sunspots have the greatest influence on the SPEI-NDVI relationship, followed by Pacific Decadal Oscillation and El Niño Southern Oscillation.