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

Rising atmospheric CO ₂ stimulates photosynthesis and productivity of forests, offsetting CO ₂ emissions. Elevated CO ₂ experiments in temperate planted forests yielded ∼23% increases in productivity 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. This knowledge gap creates major uncertainties in future climate projections 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 in estimating future carbon sinks.