The lagged and cumulative effects of permafrost and seasonally frozen soil on water use efficiency in the Qilian Mountains, Northwest China

Permafrost in the Qilian Mountains is highly sensitive to climate change, affecting ecosystem energy, water, and carbon cycling. Using MODIS data and the self-calibrated Palmer Drought Severity Index (scPDSI), we analyzed WUE dynamics and drought responses in permafrost and seasonally frozen soil regions during 2001–2020. WUE showed an overall gradual decline and an intra-annual increase-decrease–increase pattern, with growth rates peaking in November and the lowest values occurring in June. A threshold of 0.6 gC m −2 mm −1 demarcated the two frozen-soil types. Delayed drought responses occurred in 50.77% of vegetation, with a mean lag of 6.82 months; seasonally frozen soil mainly showed non-lagged and long-term responses, whereas permafrost mainly showed non-lagged and short-term responses. Cumulative effects affected 40.49% of vegetation, with a mean duration of 5.06 months. These findings provide a mechanistic basis for understanding drought impacts on vegetation WUE across contrasting frozen-soil environments.