Lack of photosynthetic or stomatal regulation after 9 years of elevated [ CO 2 ] 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 [ CO 2 ] and warming on CO 2 and H 2 O gas exchange may help predict responses of treelines to global change. We measured needle gas exchange of L arix decidua   M ill. and P inus mugo ssp. uncinata   DC trees after 9 years of free air CO 2 enrichment (575  µ mol mol −1 ) and 4 years of soil warming (+4 °C) and analysed δ 13 C and δ 18 O values of needles and tree rings. Tree needles under elevated [ CO 2 ] 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 [ CO 2 ] ( L . decidua : +39%; P . mugo : +35%). Stomatal conductance ( g H2O ) was insensitive to changes in [ CO 2 ], 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 [ CO 2 ] and warming was confirmed by δ 18 O needle and tree ring values. Consequently, under sufficient water supply, elevated [ CO 2 ] 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.