DIFFERENCES IN DROUGHT RESISTANCE AMONG 3 DECIDUOUS OAK SPECIES GROWNIN LARGE BOXES

The purpose of this study was to explain large differences in growth a nd decline of the pedunculate oak (Quercus robur L) and the sessile oa k (Q petraea Liebl) observed in the forest as a result of drought. In addition, northern red oak (Q rubra L) was compared with the 2 indigen ous oaks. The effects of controlled soil water deficits on growth and water relations of young plants of these 3 species grown in large boxe s have been studied. The plants were old enough to have developed norm al root systems. Two species were planted in each box, and submitted t o very similar patterns of water stress. Predawn leaf water potential, stomatal conductance, net assimilation rates, shoot elongation and mo rtality were monitored. The effect of an overall improvement in minera l 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 li mit water deficit by reduction of transpiration was similar. On the ot her hand, shoot growth of Q rubra was more reduced than that of Q robu r for similar predawn leaf water potential; growth of Q petraea was th e least sensitive. However, increase of mineral nutrition improved the growth of both Q robur and Q rubra, but not that of Q petraea. For th e 3 species, no mortality was noted as long as predawn leaf water pote ntials remained > -3.6 MPa. Below this limit, the mortality rate was h ighest in Q robur, Q petraea and lowest in Q rubra. These differences in mortality between species are due to differences in tolerance to wa ter stress, not in avoidance.