Drought thresholds shape divergent responses in ecosystem water use efficiency

Abstract. Ecosystem water use efficiency (WUE), the ratio of gross primary productivity (GPP) to evapotranspiration (ET), couples the land carbon and water cycles and is a key indicator of water–carbon coupling and ecosystem resilience. Yet how WUE responds to intensifying drought has remained unresolved, with studies reporting both increases and decreases. Here, combining multi-source global satellite, reanalysis, and model-fusion datasets, we show that across 57.5 % of vegetated land WUE first rises and then falls as drought deepens, reversing only once drought reaches extreme severity. This reconciles the apparent contradiction in earlier work, whose reported increases and decreases are the rising and falling limbs of one threshold response rather than competing outcomes. The reversal arises from an ecohydrological asymmetry, in which the early rise is driven by ET declining faster than GPP while continued GPP loss under severe drought overturns it. The depth of the threshold is not uniform but set by aridity, radiation, leaf area, and atmospheric dryness overlaid on vegetation type, so that denser, more productive ecosystems tolerate more severe drought before reversing. Because these thresholds cluster in the semi-arid and transitional biomes that dominate the variability of the land carbon sink, a rising WUE signals only transient coping rather than durable resilience, and the threshold marks an early indicator of where intensifying drought begins to erode ecosystem carbon uptake.