Vegetation browning patterns under compound soil and atmospheric dryness in northern permafrost ecosystems

Significant changes in vegetation greenness and browning have been observed across the northern permafrost zone, with important implications for ecosystem functioning and carbon uptake. While recent research has improved our understanding of the drivers of greening, the processes behind browning - especially the low-stature shrubs and herbaceous vegetation, which is more directly exposed to soil and atmospheric moisture deficits - remain less clear. To characterize browning patterns, we integrate multiple remote sensing datasets - including normalized difference vegetation index (NDVI), solar-induced chlorophyll fluorescence (SIF), and foliar chlorophyll concentration (FCC) - with gross primary productivity (GPP) simulations from CMIP6 Earth system models (ESM). We identify significant browning trends (-0.033 to - 0.025 decade-1, from MODIS NDVI) from 2001 to 2018, affecting approximately 20 % (~600,000 km²) of the study region. Browning is primarily modulated by compound soil and atmospheric dryness, reflected by declining soil moisture concurrent with increasing vapor pressure deficit. We further show that regional warming and changes in precipitation, together with permafrost-related constraints on infiltration and storage, modulate the spatial heterogeneity of compound dryness. CMIP6 projections suggest that compound dryness is likely to persist or intensify in permafrost ecosystems, implying continued risk of productivity loss, especially when combined with pulse disturbances such as wildfires.