Abstract
The timing of phenological events exerts a strong control over ecosystem
function and leads to multiple feedbacks to the climate system1. Phenology is
inherently sensitive to temperature (though the exact sensitivity is disputed2)
and recent warming is reported to have led to earlier spring, later autumn3,4
and increased vegetation activity5,6. Such greening could be expected to
enhance ecosystem carbon uptake7,8, though reports also suggest decreased
uptake for boreal forests4,9. Here we assess changes in phenology of temperate
forests over the eastern US during the past two decades, and quantify the
resulting changes in forest carbon storage. We combine long-term ground
observations of phenology, satellite indices, and ecosystem-scale carbon
dioxide flux measurements, along with 18 terrestrial biosphere models. We
observe a strong trend of earlier spring and later autumn. In contrast to
previous suggestions4,9 we show that carbon uptake through photosynthesis
increased considerably more than carbon release through respiration for both
an earlier spring and later autumn. The terrestrial biosphere models tested
misrepresent the temperature sensitivity of phenology, and thus the effect on
carbon uptake. Our analysis of the temperature-phenology-carbon coupling
suggests a current and possible future enhancement of forest carbon uptake
due to changes in phenology. This constitutes a negative feedback to climate
change, and is serving to slow the rate of warming.