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

  • Ellsworth, David S.
  • Anderson, Ian C.
  • Crous, Kristine Y.
  • Cooke, Julia
  • Drake, John E.
  • Gherlenda, Andrew N.
  • Gimeno, Teresa E.
  • Macdonald, Catriona A.
  • Medlyn, Belinda E.
  • Powell, Jeff R.
  • Tjoelker, Mark G.
  • Reich, Peter B.
Publication Date
Jan 01, 2017
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Rising atmospheric CO2 stimulates photosynthesis and productivity of forests, offsetting CO2 emissions1, 2. Elevated CO2 experiments in temperate planted forests yielded ~23% increases in productivity3 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 evidence4. This knowledge gap creates major uncertainties in future climate projections5, 6 as a large part of the tropics is P-limited. Here, we increased atmospheric CO2 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 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 productivity7 in estimating future carbon sinks.

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