Warming offsets productivity losses from high evaporative demand in a widespread C3 pasture grass

• Atmospheric warming and high vapour pressure deficit (VPD) often co-occur and threaten forage production by constraining photosynthetic capacity and stomatal regulation. Yet their relative effects on plant productivity remain poorly resolved, necessitating a mechanistic understanding of how plants respond to heat and atmospheric dryness.
• We experimentally isolated the effects of warming and VPD on growth and physiology of the perennial C₃ pasture grass Dactylis glomerata by growing plants in controlled-environment chambers at 26 °C and 30 °C under low (1 kPa) and high (2.4 kPa) VPD.
• High VPD reduced productivity more strongly at ambient than at elevated temperature, driven by a higher respiration-to-photosynthesis ratio, revealing an antagonistic interaction between warming and VPD. At ambient temperature, high VPD induced conservative water-use strategies that restricted stomatal conductance and suppressed photosynthesis. Under warming, however, thermal acclimation enhanced carbon assimilation and partially offset the negative effects of high VPD.
• Our results demonstrate that rising VPD poses a major threat to forage productivity primarily through stomatal limitation. Although reduced stomatal sensitivity under high VPD curbed water loss, sustained stomatal closure constrained carbon assimilation and growth. Warming partially mitigated these effects, indicating that atmospheric dryness—not temperature alone—may dominate future constraints on plant production.