MORPHOLOGICAL ADJUSTMENTS OF MATURE QUERCUS-ILEX TREES TO ELEVATED CO2

It is still not known whether mature forest trees. respond to increasi ng atmospheric CO2 concentrations in similar ways as seedlings do. Mat ure Mediterranean oaks (Quercus ilex) growing in a CO2 enriched atmosp here around natural CO2 vents since the seedling stage showed a modera te, age dependent increase in stem biomass production, but had signifi cantly lower biomass of 6-year-old branches, decreased branching, and lower leaf area per unit branch biomass, compared to control trees at a nearby unenriched site. Our data indicate that trees in natural fore st stands morphologically adjust to increasing CO2 and reduce CO2 indu ced initial growth stimulations. Allometric adjustments such as reduct ion in leaf area may be regarded as a ''down-regulation'' of canopy ph otosynthesis and may be an effective mechanism for saving water.