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Warming-Enhanced Carbon Uptake by Earth's Temperate Forests

Paper Reviewed
Keenan, T.F., Gray, J., Friedl, M.A., Toomey, M., Bohrer, G., Hollinger, D.Y., Munger, J.W., O'Keefe, J., Schmid, H.P., Wing, I.S., Yang, B. and Richardson, A.D. 2014. Net carbon uptake has increased through warming-induced changes in temperate forest phenology. Nature Climate Change 4: 598-604.

In a study published in Nature Climate Change, Keenan et al. (2014) note that (1) "long-term ground observations of phenology have shown an increase in growing season length," and that (2) "independent studies based on satellite reflectance corroborate this evidence, showing an earlier spring and later autumn in temperate and boreal forests." However, they also report that (3) "long-term impacts of changes in phenology on temperate forest carbon uptake and storage have yet to be quantified at the regional scale," which they thus proceed to do.

More specifically, the twelve researchers assessed multi-decadal phenological trends in temperate forests of the eastern US, and quantified their subsequent impacts on regional carbon cycling. This they did by combining three different remote sensing greenness indices, two data extraction techniques and the MODIS land cover dynamics product (Ganguly et al., 2010) with two decades of ground observations of individual tree phenology and measurements of CO2 exchange between forests and the atmosphere at seven long-term research sites.

Across all scales studied (organism, ecosystem and landscape), Keenan et al. detected a consistent trend of earlier springs and later autumns over the past two decades. And using the observed ecosystem-atmosphere carbon exchange, they quantified the impacts that inter-annual variability and long-term changes in phenology are having on forest photosynthesis, respiration and, consequently, CO2 uptake. And what did they thereby learn?

In summing up their findings and the significance of those findings, Keenan et al. write that "our analysis of the temperature-phenology-carbon coupling suggests a current and possible future enhancement of forest carbon uptake due to changes in phenology," which phenomenon, as they state in the concluding sentence of their paper, "constitutes a negative feedback to climate change, serving to reduce the growth rate of atmospheric CO2 and slow future warming."

Reference
Ganguly, S., Friedl, M.A., Tan, B., Zhang, X. and Verma, M. 2010. Land surface phenology from MODIS: Characterization of the Collection 5 global land cover dynamics product. Remote Sensing of Environment 114: 1805-1816.

Posted 11 November 2014