VULNERABILITY OF XYLEM TO EMBOLISM IN RELATION TO PLANT HYDRAULIC RESISTANCE IN QUERCUS-PUBESCENS AND QUERCUS-ILEX COOCCURRING IN A MEDITERRANEAN COPPICE STAND IN CENTRAL ITALY

The seasonal patterns of xylem embolism and xylem transport properties in Quercus pubescens Willd. and Quercus ilex L. trees growing in a na tural mixed coppice stand in conditions of severe water stress were in vestigated, Xylem embolism was evaluated in both dehydrating branches and in apical twigs during a whole year. Measurements of xylem water p otential were conducted from predawn to sunset on selected sunny days. On the same days, diurnal courses of leaf conductance were monitored. Measurements of half-hourly sap flow were made by the heat-pulse tech nique throughout the summer. At the onset of summer, a sharp decrease in water potential was observed in both species. Full recovery of wate r potentials was observed for both species after the first major rainf all event in September. Both experienced serious embolism throughout t he year, ranging between minima of c. 60% (expressed as percentage los s of hydraulic conductivity) after the rains in autumn and after bud b urst in spring, and maxima of c. 80 % during summer and after freezing -thawing events during the winter season. A significant negative linea r relationship was found between water potential and xylem embolism in branches dehydrating in air for Q. pubescens and Q. ilex. Q. pubescen s had greater efficiency in hydraulic transport (higher specific condu ctivity and leaf specific conductivity) by the xylem than Q. ilex. In June, leaf conductance was high early in the morning and decreased gra dually during the day. Midday depression of leaf conductance, as a res ult of high evaporative demand combined with water deficit, was observ ed in both species. In August, leaf conductance of both species was gr eatly reduced, as water potential dropped to extremely low values, and the stomata were almost completely closed during the afternoon. No hy steresis resulting from plant capacitance was observed in the relation ship between shoot water potential and sap flow. Q. pubescens exhibite d very high values of whole-tree hydraulic resistance between July and September, whereas Q. ilex generally showed lower values. The effect of soil moisture depletion on the relationship between sap flow and sh oot water potential appears as a lowering of water potential at zero f low. A significant decrease of whole-tree hydraulic resistance in both species was observed with the onset of the autumn, preceding the part ial recovery of twig hydraulic conductivity. The results demonstrate t hat both Q. pubescens and Q. ilex, although highly tolerant of severe water stress and tissue dehydration, operate at the limits of safety w hich are surpassed under severe droughts, and prolonged climatic stres s might predispose these Quercus species to decline.