10
Jaume Terradas
them. Water relationships are discussed in Chapters 10 and 11. Holm oak is
much more resistant to drought than winter-deciduous trees, but not enough
to survive in the drier areas of the Mediterranean region. Extreme droughts,
such as that of 1994 in Spain, can produce high rates of crown withering and
tree mortality. High losses of hydraulic conductivity can occur during summer drought and losses can be much larger than in other Mediterranean
trees (Ceratonia siliqua, Olea oleaster, Quercus suber). Sala and Tenhunen
(1994) suggest that when xylem water potential is below -3 MPa cavitation
may occur and strong leaf shedding can be observed. Salleo and Lo Gullo
(1990) consider that sclerophylly does not have the ecological meaning of
water saving. Holm oak does not behave as a drought avoider but as a
drought tolerant, because its stomata remain open at least twice as long as
those of Q. humilis or Q. suber, and because it has a greater capacity to recover from water losses and to maintain a higher minimum diurnal relative
water content.
Such relatively stable water status is possible because the deep roots of
holm oak can tap gro\1.ndwater, enabling it to resist drought. However, the
Californian Q. agrifolia and Q. chrysolepis are drought avoiders, unable to tap
groundwater to a large extent, with lower recovery values than the droughtdeciduous Q. douglasii and the deciduous Q. keUoggii (Knops and Koenig
1994). This shows that conclusions about an evergreen oak cannot be extrapolated to other species with an apparently similar strategy nor can we
explain the evergreenness of Mediterranean trees just by reasoning on water
budget aspects.
Dense holm oak forests seem to use around 500-600 mm of water a year
(Chap. 19). When annual rainfall is on average below 400-450 mm, holm oak
forests cannot maintain closed canopies, and the species becomes scarce and
restricted to the best sites. Mature vegetation is then formed by Mediterranean pines and sclerophyllous or summer-deciduous shrubs with phryganatype malacophyllous chamaephytes or steppe-type perennial grasses.
Holm oak resists quite low temperatures, down to -15°C approximately,
and it can, in fact, survive temperatures as low as -20 to -25°C in winter if
cold periods do not last long enough to freeze the thick trunks (Larcher and
Mair 1969). Apparently, holm oak suffers from water stress due to frostdrought more than from frost injury (Save et al. 1988). Under very cold temperatures, all leaves may die, but even then trees can still survive. Holm oak is
more cold-resistant than other sclerophyllous species which frequently coexist with it, like Q. suber, Rhamnus alaternus, Q. coccifera or Ceratonia siliqua (Larcher and Mair 1969). Nevertheless, soil surface temperatures of only
-4°C can kill the seedlings. The regular occurrence of -8 to -10°C temperatures over a number of years makes the survival of young trees impossible (Larcher 1995). Seasonal hardening for cold resistance is described by
Larcher (1970).
Nutrient availability could also be a factor limiting holm oak distribution,
but simultaneous variation of water- and nutrient-holding capacity of soils
Jaume Terradas
them. Water relationships are discussed in Chapters 10 and 11. Holm oak is
much more resistant to drought than winter-deciduous trees, but not enough
to survive in the drier areas of the Mediterranean region. Extreme droughts,
such as that of 1994 in Spain, can produce high rates of crown withering and
tree mortality. High losses of hydraulic conductivity can occur during summer drought and losses can be much larger than in other Mediterranean
trees (Ceratonia siliqua, Olea oleaster, Quercus suber). Sala and Tenhunen
(1994) suggest that when xylem water potential is below -3 MPa cavitation
may occur and strong leaf shedding can be observed. Salleo and Lo Gullo
(1990) consider that sclerophylly does not have the ecological meaning of
water saving. Holm oak does not behave as a drought avoider but as a
drought tolerant, because its stomata remain open at least twice as long as
those of Q. humilis or Q. suber, and because it has a greater capacity to recover from water losses and to maintain a higher minimum diurnal relative
water content.
Such relatively stable water status is possible because the deep roots of
holm oak can tap gro\1.ndwater, enabling it to resist drought. However, the
Californian Q. agrifolia and Q. chrysolepis are drought avoiders, unable to tap
groundwater to a large extent, with lower recovery values than the droughtdeciduous Q. douglasii and the deciduous Q. keUoggii (Knops and Koenig
1994). This shows that conclusions about an evergreen oak cannot be extrapolated to other species with an apparently similar strategy nor can we
explain the evergreenness of Mediterranean trees just by reasoning on water
budget aspects.
Dense holm oak forests seem to use around 500-600 mm of water a year
(Chap. 19). When annual rainfall is on average below 400-450 mm, holm oak
forests cannot maintain closed canopies, and the species becomes scarce and
restricted to the best sites. Mature vegetation is then formed by Mediterranean pines and sclerophyllous or summer-deciduous shrubs with phryganatype malacophyllous chamaephytes or steppe-type perennial grasses.
Holm oak resists quite low temperatures, down to -15°C approximately,
and it can, in fact, survive temperatures as low as -20 to -25°C in winter if
cold periods do not last long enough to freeze the thick trunks (Larcher and
Mair 1969). Apparently, holm oak suffers from water stress due to frostdrought more than from frost injury (Save et al. 1988). Under very cold temperatures, all leaves may die, but even then trees can still survive. Holm oak is
more cold-resistant than other sclerophyllous species which frequently coexist with it, like Q. suber, Rhamnus alaternus, Q. coccifera or Ceratonia siliqua (Larcher and Mair 1969). Nevertheless, soil surface temperatures of only
-4°C can kill the seedlings. The regular occurrence of -8 to -10°C temperatures over a number of years makes the survival of young trees impossible (Larcher 1995). Seasonal hardening for cold resistance is described by
Larcher (1970).
Nutrient availability could also be a factor limiting holm oak distribution,
but simultaneous variation of water- and nutrient-holding capacity of soils