CO2 enhancement of forest productivity constrained by limited nitrogen availability

Stimulation of terrestrial plant production by rising CO2 concentration is projected to reduce the airborne fraction of anthropogenic CO2 emissions. Coupled climate-carbon cycle models are sensitive to this negative feedback on atmospheric CO2, but model projections are uncertain because of the expectation that feedbacks through the nitrogen (N) cycle will reduce this so-called CO2 fertilization effect. We assessed whether N limitation caused a reduced stimulation of net primary productivity (NPP) by elevated atmospheric CO2 concentration over 11 y in a free-air CO2 enrichment (FACE) experiment in a deciduous Liquidambar styraciflua (sweetgum) forest stand in Tennessee. During the first 6 y of the experiment, NPP was significantly enhanced in forest plots exposed to 550 ppm CO2 compared with NPP in plots in current ambient CO2, and this was a consistent and sustained response. However, the enhancement of NPP under elevated CO2 declined from 24% in 2001-2003 to 9% in 2008. Global analyses that assume a sustained CO2 fertilization effect are no longer supported by this FACE experiment. N budget analysis supports the premise that N availability was limiting to tree growth and declining over time -an expected consequence of stand development, which was exacerbated by elevated CO2. Leaf-and stand-level observations provide mechanistic evidence that declining N availability constrained the tree response to elevated CO2; these observations are consistent with stand-level model projections. This FACE experiment provides strong rationale and process understanding for incorporating N limitation and N feedback effects in ecosystem and global models used in climate change assessments.