Intrinsic water-use efficiency estimated with 13C discrimination is unrelated to stand-level water use efficiency for Eucalyptus globulus in Western Australia

Background/Question/Methods In our largely water limited world, plants that could photosynthesize and produce more biomass with less water would be a boon for agriculture and forestry.  Carbon isotopic discrimination during photosynthesis is linearly related to the intrinsic water use efficiency of photosynthesis at the leaf level, and ¹³C discrimination is simple and inexpensive to measure.  Because of these facts, much attention has focused on using this metric to improve water use efficiency at the plant level.  We examine the link between intrinsic water use efficiency and wood growth per unit of water use in a study of Eucalyptus globulus across five sites in Western Australia.  Precipitation and potential evapotranspiration differed across sites, and leaf area and water use differed within each site with the number of trees, fertility and forest age.  These experimental conditions created a broad gradient in wood growth, water use, and water use efficiency.  Wood growth was measured using conventional forestry techniques, water use as the balance between precipitation and soil water to 18 m, and ¹³C measured in phloem sap sampled from five trees per plot (three plots per treatment) five times over two years during peak plantation growth.

Results/Conclusions Intrinsic water use efficiency was either uncorrelated or negatively correlated with actual forest level wood growth per water use (forest-level water use efficiency).  Annual wood production across the gradient in water availability and water use was highly correlated with annual water use.  Forest-level water use efficiency decreased slightly as water use increased (and wood growth increased).  The lack of correlation of intrinsic (leaf-level) and forest-level water use efficiency results from several processes important at larger scales of space and time.   Total leaf area, partitioning to biomass or a specific component of biomass, and the pattern of water use, photosynthesis and partitioning through time are all important components of growth and water use at these larger scales.   Intrinsic water use efficiency cannot indicate forest water use or the water efficiency of wood production without information on leaf area, partitioning, and seasonal water use.