Intra- 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 and radial growth of a 35-year-old Quercus petraea stand were measured during 5 successive years (1989-1993). At the beginning of 1992, half of the stand was thinned. The increase of stand transpiration during spring was linearly correlated to the development of LAI. During the second 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 evapotranspiration (T/PET) ratio decreased sharply as soon as relative extractable water (REW) dropped below 0.4. Likewise, cumulated stand transpiration varied among years because of variability in soil water availability, LAI and canopy structure. A linear relationship, similar to the one observed for weekly variations, was noted between T/PET and LAI; maximum LAI ranged from 3.3 to 6 in this ring-porous species. Seasonal circumference measurements showed that 43% of the annual increment was achieved before leaf development, hence before canopy transpiration and CO2 assimilation were started. Tree ring area was significantly correlated to the cumulated transpiration; a water-use efficiency variable was defined at both tree and stand level.