title: 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.
abstract: Alpine treelines are temperature-limited vegetation boundaries. Understanding the effects of elevated [CO2 ] and warming on CO2 and H2 O 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 δ(13) C and δ(18) O 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 δ(18) O 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.
authors.affiliation: Laboratory of Atmospheric Chemistry, Paul Scherrer Institute (PSI), 5232, Villigen, Switzerland.
authors.forename: Kathrin
authors.initials: K
authors.lastname: Streit
authors.forename: Rolf T W
authors.initials: RT
authors.lastname: Siegwolf
authors.forename: Frank
authors.initials: F
authors.lastname: Hagedorn
authors.forename: Marcus
authors.initials: M
authors.lastname: Schaub
authors.forename: Nina
authors.initials: N
authors.lastname: Buchmann
mesh_terms: D057486:Carbon Cycle; D002245:Carbon Dioxide / Q000378:metabolism*; D019069:Cell Respiration; D057231:Climate Change; D028221:Larix / Q000378:metabolism / Q000502:physiology*; D017711:Nonlinear Dynamics; D010100:Oxygen / Q000378:metabolism; D010788:Photosynthesis; D028223:Pinus / Q000378:metabolism / Q000502:physiology*; D054046:Plant Stomata / Q000502:physiology*; D012044:Regression Analysis; D012987:Soil
chemical_list: D012987:Soil; D002245:Carbon Dioxide; D010100:Oxygen
keywords: carbon and oxygen isotopes; carbon cycle; down-regulation; isotopic fractionation; photosynthesis; stomatal conductance; stomatal sensitivity; vapour pressure deficit; water-use efficiency