Holm Oak and Holm Oak Forests: An Introduction
9
comes from resprouts (Chap. 5), which are produced abundantly from
stumps or root crowns. Resprout leaves benefit from an increased root to
shoot ratio, and resprout growth rates are high during the first year (Castell
et al. 1994).
1.5 Responses of Holm Oak to Environmental Conditions
The genus Quercus contains a large variety of leaf types, from summerdeciduous (leaves withering in spring and falling in summer), to evergreen
(with leaves persisting more than a year), to semi-evergreen (with leaves
withering in autumn, persisting on the tree through winter and falling in
spring), and to winter deciduous (withering and falling in autumn). A transition between these types seems to occur along environmental gradients of
vegetative period and drought season lengths. Holm oak is one of a number
of evergreen oaks distributed in a rather large area between tropical and
temperate regions of the world. To the continental European species (Q. ilex,
Q. suber, Q. calliprinos, Q. coccifera) we can add Q. alnifolia from Cyprus,
Q. baloot from the Middle East, and many species from North America, Japan, etc. There is usually an inverse correlation between summer and winter
stresses (Mitrakos 1980) that determines the changes in leaf type dominance.
Winter-deciduous oaks correspond mostly with cold-temperate climates,
while evergreen and semi-evergreen or summer-deciduous oaks extend well
into subtropical regions. In evergreen species, leaf lifespan lengthens from
mesic to drier sites. Resistance to desiccation and to drought injury increases
from deciduous to evergreen oaks. Larcher (1960) measured very large differences in these aspects between the frequently coexisting Q. humilis (winter
deciduous) and holm oak (evergreen). Whereas cuticular transpiration in
Q. humilis is 30% of open-stomata transpiration, in holm oak it is only 3%.
Drought injury delay - measured by the ratio (available water content at time
of complete stomatal closure)f{cuticular transpiration) - for Q. ilex is 15
times higher than for Q. humilis.
Holm oak has been considered a Mediterranean paradigm, partly because
of its evergreen habit. Evergreenness can be advantageous because, under
appropriate conditions, it permits carbon fixation at any season. Evergreen
dominance is also usually explained as a result of low nutrient loss rates in
nutrient-poor environments, with slower decomposition rates. Although in
Mediterranean-type ecosystems leaf chemistry and leaf decomposition may
not differ consistently between evergreen and deciduous species (Aerts
1995), deciduous oaks do have leaves of higher nutrient concentrations than
evergreen oaks. Therefore, in the Mediterranean Basin the evergreen habit
could be more related to the double-stress resistance than to oligotrophy
(Mitrakos 1980). The interrelationships between water and nutrient economies are, however, very complex, and we need to know much more about
9
comes from resprouts (Chap. 5), which are produced abundantly from
stumps or root crowns. Resprout leaves benefit from an increased root to
shoot ratio, and resprout growth rates are high during the first year (Castell
et al. 1994).
1.5 Responses of Holm Oak to Environmental Conditions
The genus Quercus contains a large variety of leaf types, from summerdeciduous (leaves withering in spring and falling in summer), to evergreen
(with leaves persisting more than a year), to semi-evergreen (with leaves
withering in autumn, persisting on the tree through winter and falling in
spring), and to winter deciduous (withering and falling in autumn). A transition between these types seems to occur along environmental gradients of
vegetative period and drought season lengths. Holm oak is one of a number
of evergreen oaks distributed in a rather large area between tropical and
temperate regions of the world. To the continental European species (Q. ilex,
Q. suber, Q. calliprinos, Q. coccifera) we can add Q. alnifolia from Cyprus,
Q. baloot from the Middle East, and many species from North America, Japan, etc. There is usually an inverse correlation between summer and winter
stresses (Mitrakos 1980) that determines the changes in leaf type dominance.
Winter-deciduous oaks correspond mostly with cold-temperate climates,
while evergreen and semi-evergreen or summer-deciduous oaks extend well
into subtropical regions. In evergreen species, leaf lifespan lengthens from
mesic to drier sites. Resistance to desiccation and to drought injury increases
from deciduous to evergreen oaks. Larcher (1960) measured very large differences in these aspects between the frequently coexisting Q. humilis (winter
deciduous) and holm oak (evergreen). Whereas cuticular transpiration in
Q. humilis is 30% of open-stomata transpiration, in holm oak it is only 3%.
Drought injury delay - measured by the ratio (available water content at time
of complete stomatal closure)f{cuticular transpiration) - for Q. ilex is 15
times higher than for Q. humilis.
Holm oak has been considered a Mediterranean paradigm, partly because
of its evergreen habit. Evergreenness can be advantageous because, under
appropriate conditions, it permits carbon fixation at any season. Evergreen
dominance is also usually explained as a result of low nutrient loss rates in
nutrient-poor environments, with slower decomposition rates. Although in
Mediterranean-type ecosystems leaf chemistry and leaf decomposition may
not differ consistently between evergreen and deciduous species (Aerts
1995), deciduous oaks do have leaves of higher nutrient concentrations than
evergreen oaks. Therefore, in the Mediterranean Basin the evergreen habit
could be more related to the double-stress resistance than to oligotrophy
(Mitrakos 1980). The interrelationships between water and nutrient economies are, however, very complex, and we need to know much more about