8
Jaume Terradas
high variability can be expected under different climate conditions and management (Chap. 4).
Vegetative growth occurs in May-June in the montane north-facing slopes
where most of our experimental sites are located (Chap. 2), and about one
month earlier in the lowlands. After the growth of dolicoblasts and the
opening of new leaves in May-June shoot growth ceases in July. A second
flush of shoot growth, carrying a smaller generation of leaves, can be produced in late summer or early autumn (De Lillis and Fontanella 1992), but
this is not common for adult trees in montane sites. Maximum leaf fall is in
May-June, coinciding with the growth of new shoots. So, old leaves yielding
low carbon returns are discarded just before the summer drought and at a
time when the growth demands for nutrients and mobile carbon are highest.
Old leaves, twigs and small branches, as well as stem and roots, can act as
nutrient reservoirs useful for regenerating the tree crowns after disturbance.
Leaf lifespan ranges from less than 1 to 4 years, being usually around 2 years
since most leaves are shed at the start of their third year. Leaf turnover rates
change with crown position, habitat characteristics and weather, with a high
interannual variability. So, changes in leaf features within the canopy or between sites are sometimes the result of changes in the age structure of the
leaf population.
Flower buds and flowers appear mostly in May-June on the newlyelongated branches. Flowering can last until late June, when acorn growth begins.
Probably, holm oak has genetic auto incompatibility, and it shows large differences in flowering time between trees and between years; both factors favour gene combination within populations. This is an important characteristic, because it ensures high genetic variability right from the beginning of a
colonization process and onwards (Michaud et al. 1992). Genetic introgression in oaks can explain much of their intraspecific variation (Schwarz 1964).
Genetic varieties of holm oak are probably distinguishable not only by their
leaf morphology, but also by fruit forms.
Acorns mature and fall mostly in November-January. Acorn production
has a great individual and interannual variability (Chap. 6). High acorn production has been said to occur once every (2)4-6 years, but long series of
systematic records are lacking. As in other Quercus species, acorns remain viable for only a short time (1-2 months). The highest germination rates are
obtained within 2 months after collecting the fruits. Predation on acorns can
be a problem in some cases. Germination is stimulated under reduced light
levels and the associated increase in soil moisture that occurs at canopy closure (Bran et al. 1990). Survival is also better under closed canopies. There,
the shade-tolerant holm oak seedlings can survive even decades without significant growth (Espelta et al. 1993; Chap. 7). So, there is usually no significant recruitment into the canopy under the selection thinning management
currently used in most of Catalonia, and only substantial reductions in tree
density could lead to an effective recruitment. When a disturbance occurs,
new genets can develop from acorns but, as noted above, most regeneration
Jaume Terradas
high variability can be expected under different climate conditions and management (Chap. 4).
Vegetative growth occurs in May-June in the montane north-facing slopes
where most of our experimental sites are located (Chap. 2), and about one
month earlier in the lowlands. After the growth of dolicoblasts and the
opening of new leaves in May-June shoot growth ceases in July. A second
flush of shoot growth, carrying a smaller generation of leaves, can be produced in late summer or early autumn (De Lillis and Fontanella 1992), but
this is not common for adult trees in montane sites. Maximum leaf fall is in
May-June, coinciding with the growth of new shoots. So, old leaves yielding
low carbon returns are discarded just before the summer drought and at a
time when the growth demands for nutrients and mobile carbon are highest.
Old leaves, twigs and small branches, as well as stem and roots, can act as
nutrient reservoirs useful for regenerating the tree crowns after disturbance.
Leaf lifespan ranges from less than 1 to 4 years, being usually around 2 years
since most leaves are shed at the start of their third year. Leaf turnover rates
change with crown position, habitat characteristics and weather, with a high
interannual variability. So, changes in leaf features within the canopy or between sites are sometimes the result of changes in the age structure of the
leaf population.
Flower buds and flowers appear mostly in May-June on the newlyelongated branches. Flowering can last until late June, when acorn growth begins.
Probably, holm oak has genetic auto incompatibility, and it shows large differences in flowering time between trees and between years; both factors favour gene combination within populations. This is an important characteristic, because it ensures high genetic variability right from the beginning of a
colonization process and onwards (Michaud et al. 1992). Genetic introgression in oaks can explain much of their intraspecific variation (Schwarz 1964).
Genetic varieties of holm oak are probably distinguishable not only by their
leaf morphology, but also by fruit forms.
Acorns mature and fall mostly in November-January. Acorn production
has a great individual and interannual variability (Chap. 6). High acorn production has been said to occur once every (2)4-6 years, but long series of
systematic records are lacking. As in other Quercus species, acorns remain viable for only a short time (1-2 months). The highest germination rates are
obtained within 2 months after collecting the fruits. Predation on acorns can
be a problem in some cases. Germination is stimulated under reduced light
levels and the associated increase in soil moisture that occurs at canopy closure (Bran et al. 1990). Survival is also better under closed canopies. There,
the shade-tolerant holm oak seedlings can survive even decades without significant growth (Espelta et al. 1993; Chap. 7). So, there is usually no significant recruitment into the canopy under the selection thinning management
currently used in most of Catalonia, and only substantial reductions in tree
density could lead to an effective recruitment. When a disturbance occurs,
new genets can develop from acorns but, as noted above, most regeneration