Vertical root distribution underpins belowground resistance to extreme drought

Abstract

Extreme droughts are becoming more frequent and strongly affect ecosystem productivity, yet their impact on belowground productivity—key to the carbon cycle—remain underexplored. Using a coordinated drought experiment across six Eurasian grassland sites spanning a precipitation gradient, we found that extreme drought had little effect on belowground net primary productivity (BNPP) or root biomass across the 0-20 cm soil profile because drought-induced shifts in root allocation compensated across soil depths. Specifically, drought induced site- and depth-specific shifts: in xeric grasslands, BNPP and root biomass increased in the 0-10 cm layer but declined in the 10-20 cm layer, whereas mesic grasslands exhibited the opposite pattern. These contrasting responses were consistent with shifts in depth-specific soil moisture. Consequently, the apparent resistance of belowground productivity to drought masks a redistribution of root growth along the vertical soil profile. Accounting for such depth-dependent root dynamics is essential for understanding ecosystem functioning, including carbon cycling, plant persistence, and ecosystem responses to intensifying climate extremes.