Transpiration and tree water relations in a Pinus pinaster chronosequence

Although the age-related decline in aboveground net primary productivity has long been known to forest ecologists, its functional determinants are still being investigated. Growth decline has been suggested to stem from a reduction in leaf gas exchange in older trees, itself determined by the greater hydraulic resistance of taller trees. The present research is an attempt to better understand the age-related patterns of gas exchange and growth in maritime pine (Pinus pinaster Ait.), a commun natural and plantation species in the Mediterranean region, as part of the EU-CarboAge European project. Water relations are being studied in a P. pinaster chronosequence, comprising 5 stands ranging in age between 8 and 70 years. Sap flow is countinuously recorded in the stem of 8 plants per stand by the Granier method. The dataset will be used to analyze the response of transpiration to key environmental parameters in stands of different age. The diurnal pattern of needle water potential is also being measured in intensive field campaigns by the pressure chamber technique. Plant hydraulic resistance will be estimated from the relationship between sap flow and needle water potential for plantsdifferents plant age and dimension. A summary of preliminary results will be presented.