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 (Amax) was found. Both tree species showed increased net photosynthetic rates (An) under elevated [CO2] (L. decidua: +39%; P. mugo: +35%). Stomatal conductance (gH2O) was insensitive to changes in [CO2], thus transpiration rates remained unchanged and intrinsic water-use efficiency (iWUE) increased due to higher An. Soil warming affected neither An nor gH2O. Unresponsiveness of gH2O to [CO2] and warming was confirmed by δ18O needle and tree ring values. Consequently, under sufficient water supply, elevated [CO2] induced sustained enhancement in An 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.