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

• Published in: Plant, cell and environment Vol. 37; no. 2; pp. 315 - 326
• Main Authors: STREIT, KATHRIN, SIEGWOLF, ROLF T. W., HAGEDORN, FRANK, SCHAUB, MARCUS, BUCHMANN, NINA
• Format: Journal Article
• Language: English
• Published: Oxford Blackwell 01.02.2014; Wiley Subscription Services, Inc
• Subjects: Biological and medical sciences; carbon and oxygen isotopes; Carbon Cycle; Carbon Dioxide - metabolism; Cell Respiration; Climate Change; down‐regulation; Fundamental and applied biological sciences. Psychology; isotopic fractionation; Larix - metabolism; Larix - physiology; Nonlinear Dynamics; Oxygen - metabolism; Photosynthesis; Pinus - metabolism; Pinus - physiology; Plant Stomata - physiology; Regression Analysis; Soil; stomatal conductance; stomatal sensitivity; vapour pressure deficit; water‐use efficiency; Relative humidity; Vapor pressure; Carbon dioxide; water-use efficiency; stomatal sensitivity; Oxygen isotopes; carbon and oxygen isotopes; Gymnospermae; down-regulation; Species; Warming; Stomatal conductance; Plant ecology; Stomata; Increase; Carbon; Carbon cycle; Soils; Sensitivity; Top down control; Isotopic fractionation; Spermatophyta; Coniferales; Photosynthesis; vapour pressure deficit; stomatal conductance; photosynthesis; isotopic fractionation; carbon cycle
• ISSN: 0140-7791; 1365-3040
• DOI: 10.1111/pce.12197

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. In the context of global change (elevated [CO2] and warming) we investigated C and H2O gas exchange and C and O isotope ratios of organic matter of Larix decidua and Pinus mugo trees at the alpine treeline (at 2180 m a.s.l.). Trees were exposed nine years to elevated [CO2] concentration (580 ppm) and four years to soil warming. In contrast to our expectations we found neither a down‐regulation of photosynthesis nor of stomatal conductance (gH2O) under elevated [CO2] or any response to soil warming. Our findings suggest a weaker coupling between stomatal conductance and [CO2] as generally presumed and a lack of end‐product inhibition, at least for Larix and Pinus trees in an alpine treeline ecosystem with abundant water supply but harsh climatic conditions. Commentary: CO2 enrichment at treeline: help or hindrance for trees on the edge?