The purpose of this study was to explain large differences in growth and decline of the pedunculate oak (Quercus robur L) and the sessile oak (Quercus petraea Liebl) observed in the forest as a result of drought. In addition, northern red oak (Quercus rubra L) was compared with the 2 indigenous oaks. The effects of controlled soil water deficits on growth and water relations of young plants of these 3 species growth in large boxes have been studied. The plants were old enough to have developed normal roots systems. Two species were planted in each box, and submitted to very similar patterns of water stress. Predawn leaf water potential, stomatal conductance, net assimilation rates, shoot elongation and mortality were monitored. The effect of an overall improvement in mineral nutrition on these parameters was also tested. During water deficit (decrease in predawn leaf water potential), the pattern of decrease of gas exchange was similar for the 3 species. Thus, their ability to limit water deficit by reduction of transpiration was similar. On the other hand, shoot growth of Quercus rubra was more reduced than that of Quercus robur for similar predawn leaf water potential; growth of Quercus petraea was the least sensitive However, increase of mineral nutrition improved the growth of both Quercus robur and Quercus rubra, but not that of Quercus petraea. For the 3 species, no mortality was noted as long as predawn leaf water potentials remained >-3.6 MPa. Belows this limit, the mortality rate was highest in Quercus robur, Quercus petraea and lowest in Quercus rubra. These differences in mortality between species are due to differences in tolerance to water stress, not in avoidance.
