The seasonal patterns of xylem embolism and xylem transport properties
in Quercus pubescens Willd. and Quercus
ilex L. trees growing in a natural mixed coppice stand in
conditions of severe water stress were investigated. Xylem
embolism was evaluated in both dehydrating branches and in
apical twigs during a whole year. Measurements of
xylem water potential 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
technique throughout the summer. At the onset of summer, a
sharp decrease in water potential was observed in
both species. Full recovery of water potentials was observed
for both species after the first major rainfall event in
September. Both experienced serious embolism throughout the year,
ranging between minima of c. 60%
(expressed as percentage loss of hydraulic conductivity) after
the rains in autumn and after bud burst in spring,
and maxima of c. 80% during summer and after freezing-thawing
events during the winter season. A significant
negative linear relationship was found between water potential
and xylem embolism in branches dehydrating in
air for Q. pubescens and Q. ilex. Q. pubescens
had greater
efficiency in hydraulic transport (higher specific
conductivity and leaf specific conductivity) by the xylem than
Q. ilex. In June, leaf conductance was high early in
the morning and decreased gradually during the day. Midday
depression of leaf conductance, as a result of high
evaporative demand combined with water deficit, was observed in
both species. In August, leaf conductance of
both species was greatly reduced, as water potential dropped to
extremely low values, and the stomata were almost
completely closed during the afternoon. No hysteresis resulting
from plant capacitance was observed in the
relationship between shoot water potential and sap flow. Q.
pubescens exhibited 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
shoot water potential appears as a lowering of water
potential at zero flow. A significant decrease of whole-tree
hydraulic resistance in both species was observed with
the onset of the autumn, preceding the partial recovery of twig
hydraulic conductivity. The results demonstrate
that both Q. pubescens and Q. ilex, although highly tolerant
of
severe water stress and tissue dehydration, operate
at the limits of safety which are surpassed under severe droughts,
and prolonged climatic stress might predispose
these Quercus species to decline.