IMPACT OF LONG-TERM DROUGHT ON XYLEM EMBOLISM AND GROWTH IN PINUS-HALEPENSIS MILL

The present study was carried out to elucidate the response mechanisms of 50-year-old Pinus halepensis Mill. trees to a long-term and severe drought. The amount of water available to trees was artificially rest ricted for 12 months by covering the soil with a plastic roof. Over th e short term a direct and rapid impact of drought was evident on the w ater relations and gas exchanges of trees: as the soil dried out in th e Spring, there was a concurrent decrease of predawn water potential; transpiration was strongly reduced by stomatal closure. Seasonal chang es in the water volume fractions of twig and stem xylem were observed and interpreted as the result of cavitation and refilling in the xylem . When droughted trees recovered to a more favourable water status, re filling of embolized xylem was observed; twig predawn water potentials were still negative in the period when the embolism was reversed in t he twig xylem. A few months after the removal of the covering, no diff erences in whole plant hydraulic resistance were observed between drou ghted and control trees. Needle and shoot elongation and stem radial g rowth were considerably reduced in droughted trees; no strategy of tre es to allocate carbon preferentially to the stem conducting tissues wa s apparent throughout the experiment. An after-effect of the drought o n growth was observed.