INTRAANNUAL AND INTERANNUAL VARIATIONS OF TRANSPIRATION, LEAF-AREA INDEX AND RADIAL GROWTH OF A SESSILE OAK STAND (QUERCUS-PETRAEA)

Bud-burst, leaf area index (LAI), transpiration, soil water content an d radial growth of a 35-year-old Quercus petraea stand were measured d uring 5 successive years (1989-1993). At the beginning of 1992, half o f the stand was thinned. The increase of stand transpiration during sp ring was linearly correlated to the development of LAI. During the sec ond part of the season, although LAI continued to increase because of rhythmic shoot development, transpiration was strongly reduced as soil water content decreased. The transpiration/potential evapotranspirati on (T/PET) ratio decreased sharply as soon as relative extractable wat er (REW) dropped below 0.4. Likewise, cumulated stand transpiration va ried among years because of variability in soil water availability, LA I and canopy structure. A linear relationship, similar to the one obse rved for weekly variations, was noted between T/PET and LAI; maximum L AI ranged from 3.3 to 6 in this ring-porous species. Seasonal circumfe rence measurements showed that 43% of the annual increment was achieve d before leaf development, hence before canopy transpiration and CO2 a ssimilation were started. Tree ring area was significantly correlated to the cumulated transpiration; a water-use efficiency variable was de fined at both tree and stand level.