Elevated CO2 does not increase eucalypt forest productivity on a low-phosphorus soil

Experimental evidence from a mature, phosphorous-limited, eucalypt forest finds that aboveground productivity was not significantly stimulated by elevated CO 2 . Findings suggest that this effect may be limited across much of the tropics. Rising atmospheric CO 2 stimulates photosynthesis and productivity of forests, offsetting CO 2 emissions 1 , 2 . Elevated CO 2 experiments in temperate planted forests yielded ∼23% increases in productivity 3 over the initial years. Whether similar CO 2 stimulation occurs in mature evergreen broadleaved forests on low-phosphorus (P) soils is unknown, largely due to lack of experimental evidence 4 . This knowledge gap creates major uncertainties in future climate projections 5 , 6 as a large part of the tropics is P-limited. Here, we increased atmospheric CO 2 concentration in a mature broadleaved evergreen eucalypt forest for three years, in the first large-scale experiment on a P-limited site. We show that tree growth and other aboveground productivity components did not significantly increase in response to elevated CO 2 in three years, despite a sustained 19% increase in leaf photosynthesis. Moreover, tree growth in ambient CO 2 was strongly P-limited and increased by ∼35% with added phosphorus. The findings suggest that P availability may potentially constrain CO 2 -enhanced productivity in P-limited forests; hence, future atmospheric CO 2 trajectories may be higher than predicted by some models. As a result, coupled climate–carbon models should incorporate both nitrogen and phosphorus limitations to vegetation productivity 7 in estimating future carbon sinks.