Lack of photosynthetic or stomatal regulation after 9 years of elevated [CO2] and 4 years of soil warming in two conifer species at the alpine treeline

Alpine treelines are temperature‐limited vegetation boundaries. Understanding the effects of elevated CO₂] and warming on CO₂ and H₂O gas exchange may help predict responses of treelines to global change. We measured needle gas exchange of Larix decidua Mill. and Pinus mugo ssp. uncinata DC trees after 9 years of free air CO₂ enrichment (575 µmol mol^?1) and 4 years of soil warming (+4 °C) and analysed δ¹³C and δ¹⁸O values of needles and tree rings. Tree needles under elevated CO₂] showed neither nitrogen limitation nor end‐product inhibition, and no down‐regulation of maximal photosynthetic rate (A_max) was found. Both tree species showed increased net photosynthetic rates (A_n) under elevated CO₂] (L. decidua: +39%; P. mugo: +35%). Stomatal conductance (g_H2O) was insensitive to changes in CO₂], thus transpiration rates remained unchanged and intrinsic water‐use efficiency (iWUE) increased due to higher A_n. Soil warming affected neither A_n nor g_H2O. Unresponsiveness of g_H2O to CO₂] and warming was confirmed by δ¹⁸O needle and tree ring values. Consequently, under sufficient water supply, elevated CO₂] induced sustained enhancement in A_n and lead to increased C inputs into this ecosystem, while soil warming hardly affected gas exchange of L. decidua and P. mugo at the alpine treeline.