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.