Atmospheric CO enrichment of alpine treeline conifers.

•   Experimental CO enrichment of mature Larix decidua and Pinus uncinata trees and their understory vegetation was used to test the carbon limitation hypothesis of treeline formation at the alpine treeline in Switzerland. •   Forty plots (each 1.1 m ) were established; half of them were exposed to elevated (566 ppm) atmospheric CO using a free air CO enrichment (FACE) system releasing pure CO , and the other half were treated as controls at current ambient [CO ]. •   Reliable and adequate CO control was achieved, with 63% and 90% of 1-min averages having a [CO ] within ±10% and ±20% of the target value, respectively, which is comparable to previous FACE systems. Both tree species showed higher net photosynthesis, lower stomatal conductance, and increased accumulation of nonstructural carbohydrates in response to CO in the first year of treatment. Quite unexpectedly, shoot length increment increased significantly at elevated CO (up to 23%) compared with controls in both species. •   The pure CO release technology proved suitable for CO enrichment of native trees on this remote mountain slope. Our results suggest an improved C balance and growth of treeline trees in response to elevated CO . However, it is unclear whether this initial growth stimulation will persist in the longer term.