Keenan et al. reply

Abstract

replying to C. D. Holmes Nature507,http://dx.doi.org/10.1038/nature13113(2014) Forests have become more efficient at using water over the past two decades1. A series of hypotheses exist to explain this trend, but the only credible explanation to date is a response to rising atmospheric CO2. Keenan et al.1 show that the observed trend is physiologically plausible, but is much larger than expected from conventional theory and experimental evidence. This has led to suggestions that processes other than increased atmospheric CO2 may have contributed to the observed trend2. One such process that has yet to be examined is the effect of tropospheric ozone on forest water-use efficiency (WUE). In the accompanying Comment3, Holmes reports that ozone concentrations have declined in the northeastern and midwestern USA by about 50% from 1995 to 2010. Using empirical relationships, he estimates that this decline could explain roughly 15% of the reported increase in WUE over North America, and a significantly lower proportion of the trend in Europe.