Evapotranspiration of a declining Quercus robur (L.) stand from 1999 to 2001. II. Daily actual evapotranspiration and soil water reserve

The components of actual evapotranspiration (ET) - interception (In%), tree transpiration (T) and forest floor ET - were measured from 1999 to 2001 in a mixed stand dominated by declining pedunculate oaks. Sap flux density measurements (oaks and maple) and forest floor ET [38] were extrapolated to season and stand scales by regressions with potential evapotranspiration (PET) and leaf area index (ET/ PET: 0.75 - 1.23). Oak transpiration (21 - 38% of stand transpiration) is lower than forest floor ET: forest floor can therefore not be neglected in this stand water balance. The soil water reserve dynamics deduced from those measurements reflects the inter- annual changes of water use. No water stress have been calculated on the 1999 - 2001 period, but it is suspected to have occurred previously; together with soil constraints and caterpillar defoliation, it could in part explain the severe oak decline symptoms.