Carbon Gain, Carbon Use, and Persistence: How Elevated CO₂ and Warming Reshape High-Elevation Plants.

High-elevation plants operate within a tightly constrained carbon economy shaped by low temperature, short growing seasons, shallow soils, and reduced atmospheric CO2 partial pressure. Elevated CO2 and climate warming are widely expected to enhance carbon uptake in these systems, yet persistent biomass gains and long-term carbon sequestration remain inconsistent. Here, we synthesise experimental, observational, and modelling evidence to propose a three-gate framework: carbon gain (Gate 1), carbon use (Gate 2), and persistence (Gate 3) to explain alpine plant responses to global change. Elevated CO2 frequently increases leaf-level photosynthesis and intrinsic water-use efficiency, while moderate warming can transiently stimulate early-season productivity. However, sink limitation, nutrient imbalance, elevated respiration, and rhizosphere priming constrain structural carbon deployment. Excess assimilates are often diverted to non-structural carbohydrate storage, antioxidant systems, and secondary metabolism rather than durable biomass accumulation. Critically, demographic filters, phenological mismatch, recruitment failure, and stage-specific mortality govern long-term persistence. We argue that persistence, not productivity, ultimately determines alpine carbon stability. Incorporating allocation constraints and demographic processes into ecosystem models is essential for improving projections of mountain carbon-climate feedbacks.