Radiation modifies the effect of water availability on the carbon isotope composition of beach (Fagus sylvatica)

Water availability and flux and carbon isotope and other chemical compositions of wood, foliage and phloem sap were quantified in a replicated field test examining the effects of exposure (north-east, south-west) and stem density on beech ecophysiology ( Fagus sylvatica). Standard techniques for quantifying water potential, water flux in stems and carbon isotope composition of wood and foliage were complemented with chemical analysis of phloem sap collected using the 'phloem bleeding' technique. Phloem sap was similar in composition to other hardwoods. The delta C-13 signatures of sap reflected short-term fluctuations in water availability and intercepted radiation, whereas foliar delta C-13 was a poor reflection of current environmental conditions. delta C-13 of tree rings suggested that the south-west-facing site experienced a greater shortage of water during summer than the paired north-east-facing site. However, phloem delta C-13 analysis showed that interpreting the effects of water availability using delta C-13 might be confounded by radiation. Better integration of hydrological models with reliable indices of water stress in beech will be required for reliable predictions of growth, especially if (when) climates change.