Changes in vegetation phenology are not reflected in atmospheric CO2
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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.
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