INTERACTIVE EFFECTS OF ELEVATED CO2 AND SOIL DROUGHT ON GROWTH AND TRANSPIRATION EFFICIENCY AND ITS DETERMINANTS IN 2 EUROPEAN FOREST TREE SPECIES

The responses of growth and transpiration efficiency (W = biomass accu mulation/water consumption) to ambient and elevated atmospheric CO2 co ncentrations (350 and 700 mumol mol-1, respectively) were investigated under optimal nutrient supply in well-watered and in drought conditio ns in two temperate-forest tree species: Quercus petraea Liebl. and Pi nus pinaster Ait. Under well-watered conditions, doubling the CO2 conc entration for one growing season increased biomass growth by 138% in Q . petraea and by 63% in P. pinaster. In contrast, under drought condit ions, elevated CO2 increased biomass growth by only 47% in Q. petraea and had no significant effect on biomass growth in P. pinaster. Transp iration efficiency was higher in Q. petraea than in P. pinaster in all treatments. This difference was linked (i) to lower carbon isotope di scrimination (DELTA), and thus lower values of the intercellular/ambie nt CO2 concentration (c(i)/c(a)) ratio, in Q. petraea, (ii) to lower v alues of leaf mass ratio (LMR, leaf mass/whole plant mass), which we s uggest was positively related to the proportion of daytime carbon fixa tion lost by respiration (PHI), in Q. petraea, and (iii) to slightly l ower C concentrations in Q. petraea than in P. pinaster. The CO2-promo ted increase in W was higher in Q. petraea (+80%) than in P. pinaster (+50%), and the difference was associated with a more pronounced decre ase in PHI in response to elevated CO2 in Q. petraea than in P. pinast er, which could be linked with the N dilution effect observed in Q. pe traea. Because PHI also directly affects growth, the CO2-induced enhan cement of PHI in Q. petraea is a crucial determinant of the growth sti mulation observed in this species. Leaf gas exchange regulation was no t the only factor involved in the responses of growth and W to elevate d CO2 and drought, other physiological processes that have crucial rol es include carbon and N allocation and respiration.