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

Rising atmospheric CO2 stimulates photosynthesis and productivity of forests, o setting CO2 emissions. Elevated CO2 experiments in temperate planted forests yielded ∼23% increases in productivity over the initial years. Whether similar CO2 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 CO2 concentration in amature 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 CO2 in three years, despite a sustained 19% increase in leaf photosynthesis. Moreover, tree growth in ambient CO2 was strongly P-limited and increased by ∼35% with added phosphorus. The findings suggest that P availability may potentially constrain CO2-enhanced productivity in P-limited forests; hence, future atmospheric CO2 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. Limited understanding of the size of the CO2-induced fertilization effect on forest carbon sinks remains among the largest quantitative uncertainties in terms of terrestrial feedbacks to the carbon (C) cycle–climate system. Coupled climate–C cycle models project a 24–80% increase of net primary productivity (NPP) for forests in the next 50 years with rising atmospheric CO2 concentration, with substantial atmospheric CO2 responses expected for forests in the tropics. These model projections are partly based on elevated CO2 (eCO2) experiments in young temperate planted forests, which have yielded on average ∼23% increases in production over several years with 200 μmolmol increases in atmospheric CO2 concentrations. Due to the lack of experimental evidence, at present we do not know how large the eCO2 fertilization response is for mature forests that grow on soils where phosphorus (P) is limiting productivity, as is the case for many evergreen broadleaved forests. This knowledge gap creates major uncertainties in future climate projections because evergreen broadleaved forests comprise over a third of global forest area, and dominate the atmospheric CO2 sink at lower latitudes. Many eCO2 experiments have taken place in young tree plantations on relatively P-rich soils, but unlike aggrading forests, mature forests are more likely near nutritional equilibrium with their underlying soils. Hence, mature forests may be more appropriate for understanding in situ nutrient limitations to productivity and C storage with rising atmospheric CO2. Without clear understanding of this nutrient feedback to the C cycle in evergreen broadleaved forests, we cannot accurately estimate the trajectory of future atmospheric CO2, thus limiting our ability to estimate climate change mitigation by such forests and constrain internationally allowable CO2 emissions. Soil nutrient limitation may restrict eCO2-induced biomass enhancement and related C storage processes, but it is unclear if the type of nutrient limitation is important. Studies in a temperate grassland and a forest ecosystem under contrasting CO2 and N supply suggest a large initial stimulation in productivity, often followedby reducedCO2 stimulationwhenN is limiting. Limited P supply might affect tree growth and ecosystem C sequestration processes differently than the N-supply limitation that has thus far been demonstrated in eCO2 experiments on N-poor soils. In heavily weathered soils common in tropical and subtropical regions, P is typically bound to Fe and Al oxides, hydroxides and secondary minerals and not available to plants. One possibility is that increased plant carbohydrate availability from eCO2 leads to increased plant investment in the secretion of organic acids from roots or the investment in P acquisition bymycorrhizal symbionts. This would thereby reduce P limitation to broadleaved evergreen forest productivity by increasing plant access to scarce soil P. Consistent with this idea, there is evidence that recent rising CO2 may have driven a substantial portion of the observed historical increase in tropical forest carbon stocks, although future increases remain in question. Although there is considerable variation in soil fertility across the world, tree growth in highly weathered tropical and subtropical soils may be limited by P availability in addition to, or rather than, N availability. Hence, nutrient availability and the type of nutrient limitation may both be important in regulating forest CO2 fertilization responses in those regions. There is still little agreement on how to appropriately represent P limitations to productivity in Earth systems models, and there has been no direct experimental test of the CO2 fertilization effect in P-limited forests (Supplementary Fig. 1). To help fill this gap, we established a free-air CO2 enrichment experiment on six circular 25-m-diameter plots in mature Eucalyptus forest (EucFACE) on a low-P soil near Sydney, Australia (23m elevation; 33 37 4" S, 150 44 25" E) (Supplementary Fig. 2).