Individual versus combined effects of drought, warming and eCO2 on grassland water uptake and fine roots

In a changing climate, grasslands are expected to experience major shifts in water supply and demand. To date, little is known about how projected future conditions of severe drought, climate warming, and rising CO2 affect grassland water uptake, and whether adaptations of fine roots affect the capacity to extract water from soil. Using a multifactor global-change experiment in a managed montane C3 grassland, we studied the individual and combined effects of drought, warming (+3 ), and elevated CO2 (eCO2; +300 ppm) on root water uptake (RWU) over three growing seasons. RWU was assessed across different layers of the main rooting horizon using diel soil moisture dynamics during non-rain periods. We also investigated treatment effects on fine roots (production, traits), fine-root-to-shoot ratios, and consequences for RWU capacity. By increasing vapour pressure deficit (VPD) and its effect on RWU rates normalized to soil water content (RWUSWC), warming reduced RWU during hot periods. Under sustained warming, grassland decreased specific root length, and increased root diameters and fine-root-to-shoot ratios. Conversely, eCO2 slowed RWUSWC at high VPD, though fine-root adaptations were negligible. Compared to warming alone, future conditions (warming, eCO2) increased RWUSWC to a lesser extent and induced no fine-root adaptations, but reduced RWU to a similar degree. Drought reduced RWU (-6675%) and increased water sourcing from deeper soil layers; however, a hot season amplified any RWU reductions under future conditions by 20%. Altogether, our study demonstrates that (i) RWU in C3 grasslands declines in a warmer, drier future, though (ii) eCO2 will mitigate the need for fine-root adaptations, maintaining RWU capacity. However, (iii) rising temperatures will exacerbate RWU reductions under drought. Therefore, hot droughts should have significant repercussions for water dynamics in C3 grasslands.