Changes in vegetation phenology are not reflected in atmospheric CO₂ and ¹³ C/¹² C seasonality

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, CO₂ uptake through photosynthesis and release through respiration, determine the influence of the terrestrial ecosystem on the atmospheric CO₂ and ¹³ C/¹² C seasonality. Here, we use four CO₂ observation 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 CO₂ and ¹³ C/¹² C seasonality. Since the 1960s, the only significant long-term trend of CO₂ and ¹³ C/¹² C seasonality was observed at the northern most station, Alert, where the spring CO₂ drawdown dates advanced by 0.65 ± 0.55 days yr^-1 , contributing to a nonsignificant increase in length of the CO₂ uptake 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 to ¹³ C/¹² C seasonality while the CO₂ seasonality was primarily linked to nontree vegetation. Our results indicate significant increase in the Northern Hemisphere soil respiration. This means, increased respiration of ¹³ C depleted plant materials cancels out the ¹² C 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 CO₂ and ¹³ C/¹² C 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 CO₂ amplitude. 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.