Abstract: Aims: We examine how root system demography and morphology are affected by air warming and multiple, simultaneous climate change drivers. Methods: Using minirhizotrons, we studied root growth, morphology, median longevity, risk of mortality and standing root pool in the upper soil horizon of a temperate grassland ecosystem for 3 years. Grassland monoliths were subjected to four climate treatments in a replicated additive design: control (C); elevated temperature (T); combined T and summer precipitation reduction (TD); combined TD and elevated atmospheric CO2 (TDCO2). Results: Air warming (C vs T) and the combined climate change treatment (C vs TDCO2) had a positive effect on root growth rate and standing root pool. However, root responses to climate treatment varied depending on diameter size class. For fine roots (≤ 0.1 mm), new root length and mortality increased under warming but decreased in response to elevated CO2 (TD vs TDCO2); for coarse roots (> 0.2 mm), length and mortality increased under both elevated CO2 and combined climate change drivers. Conclusions: Our data suggest that the standing roots pool in our grassland system may increase under future climatic conditions. Contrasted behaviour of fine and coarse roots may correspond to differential root activity of these extreme diameter classes in future climate.