title: Changes in vegetation phenology are not reflected in atmospheric CO2and13C/12C seasonality.
abstract: Northern terrestrial ecosystems have shown global warming-induced advances in start, delays in end, and thus increased lengths of growing season and gross photosynthesis in recent decades. The tradeoffs between seasonal dynamics of two opposing fluxes, CO2uptake through photosynthesis and release through respiration, determine the influence of the terrestrial ecosystem on the atmospheric CO2and13C/12C seasonality. Here, we use four CO2observation stations in the Northern Hemisphere, namely Alert, La Jolla, Point Barrow, and Mauna Loa Observatory, to determine how changes in vegetation productivity and phenology, respiration, and air temperature affect both the atmospheric CO2and13C/12C seasonality. Since the 1960s, the only significant long-term trend of CO2and13C/12C seasonality was observed at the northern most station, Alert, where the spring CO2drawdown dates advanced by 0.65 ± 0.55 days yr-1, contributing to a nonsignificant increase in length of the CO2uptake period (0.74 ± 0.67 days yr-1). For Point Barrow station, vegetation phenology changes in well-watered ecosystems such as the Canadian and western Siberian wetlands contributed the most to13C/12C seasonality while the CO2seasonality was primarily linked to nontree vegetation. Our results indicate significant increase in the Northern Hemisphere soil respiration. This means, increased respiration of13C depleted plant materials cancels out the12C gain from enhanced vegetation activities during the start and end of growing season. These findings suggest therefore that parallel warming-induced increases both in photosynthesis and respiration contribute to the long-term stability of CO2and13C/12C seasonality under changing climate and vegetation activity. The summer photosynthesis and the soil respiration in the dormant seasons have become more vigorous which lead to increased peak-to-through CO2amplitude. As the relative magnitude of the increased photosynthesis in summer months is more than the increased respiration in dormant months, we have the increased overall carbon uptake rates in the northern ecosystems.
authors.affiliation: State Key Laboratory of Remote Sensing Science, Institute of Remote Sensing and Digital Earth, Chinese Academy of Sciences, Beijing, 100101, China.|Department of Geography and Planning, University of Toronto, Toronto, ON, M5S 3G3, Canada.
authors.forename: Alemu
authors.identifier: 0000-0002-2461-618X
authors.initials: A
authors.lastname: Gonsamo
authors.affiliation: Institute of Agricultural Sciences, Swiss Federal Institute of Technology Zurich, 8092, Zurich, Switzerland.
authors.forename: Petra
authors.initials: P
authors.lastname: D'Odorico
authors.affiliation: Department of Geography and Planning, University of Toronto, Toronto, ON, M5S 3G3, Canada.
authors.forename: Jing M
authors.initials: JM
authors.lastname: Chen
authors.affiliation: State Key Laboratory of Remote Sensing Science, Institute of Remote Sensing and Digital Earth, Chinese Academy of Sciences, Beijing, 100101, China.
authors.forename: Chaoyang
authors.initials: C
authors.lastname: Wu
authors.affiliation: Institute of Agricultural Sciences, Swiss Federal Institute of Technology Zurich, 8092, Zurich, Switzerland.
authors.forename: Nina
authors.initials: N
authors.lastname: Buchmann
mesh_terms: D002170:Canada; D057486:Carbon Cycle; D002245:Carbon Dioxide; D017753:Ecosystem; D010788:Photosynthesis; D010944:Plants; D012621:Seasons
chemical_list: D002245:Carbon Dioxide
keywords: Alert station; CO2 seasonality; La Jolla station; Mauna Loa Observatory; Point Barrow station; carbon isotope; climate change; phenology; soil respiration; δ13C