Sub-continental patterns of carbon-isotope discrimination across Australia in relation to precipitation and soil nutrients

Carbon isotope ratios of C₃ plants can been used to infer intrinsic water-use efficiency. Several transects have been established across Australia to study the sensitivity of intrinsic-water use efficiency to mean annual precipitation. These investigations showed a surprising divergence in the sensitivity of carbon-isotope discrimination to mean annual precipitation among sub-continental regions. Here, we combine previous observations with measurements along a new transect in northeastern Australia to show that such sub-continental scale sensitivity in the response of intrinsic water-use efficiency to precipitation depends on regional-scale soil phosphorus concentrations. The influence of soil phosphorus appears to operate through modulation of stomatal conductance, rather than, or in addition to, photosynthetic capacity. We hypothesize that Australian woody plant species have evolved to use high transpiration rates to facilitate phosphorus foraging in phosphorus-impoverished, ancient soils. Our analyses suggest that this strategy interacts with the well know strategy of increasing intrinsic water-use efficiency in response to decreasing mean annual precipitation.