No growth stimulation of tropical trees by 150 years of CO2 fertilization but water-use efficiency increased

The biomass of undisturbed tropical forests has likely
increased in the past few decades1,2, probably as a result
of accelerated tree growth. Higher CO2 levels are expected
to raise plant photosynthetic rates3 and enhance water-use
eciency4, that is, the ratio of carbon assimilation through
photosynthesis to water loss through transpiration. However,
there is no evidence that these physiological responses do
indeed stimulate tree growth in tropical forests. Here we
present measurements of stable carbon isotopes and growth
rings in the wood of 1,100 trees from Bolivia, Cameroon and
Thailand. Measurements of carbon isotope fractions in the
wood indicate that intrinsic water-use eciency in both
understorey and canopy trees increased by 30–35% over the
past 150 years as atmospheric CO2 concentrations increased.
However, we found no evidence for the suggested concurrent
acceleration of individual tree growth when analysing the
width of growth rings. We conclude that the widespread
assumption of a CO2-induced stimulation of tropical tree
growth may not be valid.