Carbon isotope discrimination and wood anatomy variations in mixed stands of Quercus robur and Quercus petraea

The two most common oak species in western Europe, Quercus robur and Quercu s petraea, display different ecological behaviours, particularly with respe ct to their responses to drought. The ecophysiological basis of this niche difference is not understood well. Here we test the hypothesis that these t wo species present distinct water use efficiencies (WUEs), using the carbon isotope discrimination approach. Leaves and 13 dated ring sequences were s ampled in 10 pairs of adult trees growing side by side. Carbon isotope comp osition was measured on cellulose extracts. In addition, relationships betw een carbon isotope discrimination and wood anatomy were assessed at the tre e level. Quercus robur displayed a 1.0 parts per thousand larger isotopic d iscrimination than Q. petraea and therefore a lower intrinsic WUE (-13%). T his interspecific difference of isotopic discrimination was quite stable wi th time and independent of tree radial growth and climate fluctuations. A s trong positive correlation was observed between average tree values of earl ywood vessel surface area and C-13 isotopic discrimination. This correlatio n was even higher with C-13 of the 1976 dry year (r = 0.86). These observat ions led to the hypothesis that hydraulic properties of xylem could exert a constraint on leaf gas exchange, resulting in a larger WUE for individuals with smaller vessel cross-section area.