Rainfall drives variation in rates of change in intrinsic water use efficiency of tropical forests

Rates of change in intrinsic water use efficiency ( W ) of trees relative to those in atmospheric [CO 2 ] ( c a ) have been mostly assessed via short-term studies (e.g., leaf analysis, flux analysis) and/or step increases in c a (e.g., FACE studies). Here we use compiled data for abundances of carbon isotopes in tree stems to show that on decadal scales, rates of change ( dW/dc a ) vary with location and rainfall within the global tropics. For the period 1915–1995, and including corrections for mesophyll conductance and photorespiration, dW/dc a for drier tropical forests (receiving ~ 1000 mm rainfall) were at least twice that of the wettest (receiving ~ 4000 mm). The data also empirically confirm theorized roles of tropical forests in changes in atmospheric 13 C/ 12 C ratios (the 13 C Suess Effect). Further formal analysis of geographic variation in decade-to-century scale dW/dc a will be needed to refine current models that predict increases in carbon uptake by forests without hydrological cost. How the water use efficiency of trees changes with atmospheric CO 2 variations has mostly been studied on short time scales. Here, a newly compiled data set covering 1915 to 1995 shows how rates of change in water use efficiency vary with location and rainfall over the global tropics on a decadal scale.