Forest productivity response to elevated CO2 in free-air CO2 enrichment experiments: the 23 percent solution, revisited.

A synthesis of free-air CO2 enrichment (FACE) experiments describing the response of forest net primary productivity (NPP) to elevated atmospheric CO2 published in 2005 has provided a valuable benchmark for ecosystem models used to address the impact of future atmospheric CO2 levels on forest productivity and the feedback to the atmosphere and climate change. However, that analysis was limited to young, temperate zone tree plantations, and its applicability to other biomes can be questioned. Now, after 20 years, this new analysis includes two sites in much older, mature forests and expanded and updated analyses from the original sites. The original conclusion from 2005 remains valid with only a minor modification. After normalizing to a common CO2 enrichment of 41%, NPP increased 21.8% in elevated CO2 across a wide range of forest productivity. The response declined with increasing mean annual temperature, but did not decline with forest age as expected. The response of wood production (18.2%) was somewhat less than that of NPP, but there was no evidence of a CO2 effect on carbon allocation between long- and short-term carbon pools. This analysis should inform and generate testable hypotheses for new FACE experiments such as the AmazonFACE experiment in a tropical forest.