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Historical Changes in the Water-Use Efficiencies of Tropical Trees
Reference
Hietz, P., Wanek, W. and Dunisch, O.  2005.  Long-term trends in cellulose δ13C and water-use efficiency of tropical Cedrela and Swietenia from Brazil.  Tree Physiology 25: 745-752.

What was done
From a primary rain forest in Aripuana, Brazil, the authors collected samples of wood from 37 tropical cedar (Cedrela odorata L.) trees that were between 11 and 151 years old in 2001 and from 16 big-leaf mahogany (Swietenia macrophylla King) trees that were between 48 and 126 years old at that time, after which they measured the wood samples' cellulose δ13C in 10-year growth increments.

What was learned
Hietz et al. report that cellulose δ13C decreased by 1.3 per mil in Cedrela and by 1.1 per mill in Swietenia over the past century, with the largest changes occurring during the past 50 years.  Based on these data and known trends in atmospheric CO2 and δ13CO2, they calculate that the intrinsic water-use efficiency of the trees increased by 34% in Cedrela and by 52% in Swietenia over this period, which they say is about the same as what has been deduced from similar measurements of the wood of temperate trees (Freyer, 1979; Bert et al., 1997; Feng, 1999).

What it means
Because, in the words of the authors, "water is probably not a strong limiting factor in tropical rain forest trees," they conclude that "the gain in water use efficiency translates mostly to increased carbon assimilation, which may explain the observed increase in tree growth and turnover (Phillips, 1996; Laurance et al., 2004)," which has also been observed in a number of other studies over the past several decades (see our Editorial of 20 Apr 2005).  As these reports indicate, evidence continues to accumulate for a worldwide stimulation of tree growth over the course of the Industrial Revolution, which has been driven primarily by the historical increase in the air's CO2 concentration.  This being the case, we can logically expect the upward trend in tree growth to continue, as the burning of fossil fuels that is required to sustain the growth of the world's developing economies continues to release ever more CO2 to the atmosphere.

References
Bert, D., Leavitt, S.W. and Dupouey, J.L.  1997.  Variations of wood δ13C and water-use efficiency of Abies alba during the last century.  Ecology 78: 1588-1596.

Feng, X.  1999.  Trends in intrinsic water-use efficiency of natural trees for the past 100-200 years: a response to atmospheric CO2 concentration.  Geochimica et Cosmochimica Acta 63: 1891-1903.

Freyer, H.D.  1979.  On the 13C record in tree rings.  Part I. 13C variations in northern hemispheric trees during the last 150 years.  Tellus 31: 124-137.

Laurance, W.F., Oliveira, A.A., Laurance, S.G., Condit, R., Nascimento, H.E.M., Sanchez-Thorin, A.C., Lovejoy, T.E., Andrade, A., D'Angelo, S., Ribeiro, J.E. and Dick, C.W.  2004.  Pervasive alteration of tree communities in undisturbed Amazonian forests.  Nature 428: 171-175.

Phillips, O.L.  1996.  Long-term environmental change in tropical forests: increasing tree turnover.  Environmental Conservation 23: 235-248.

Reviewed 21 September 2005