98
Javier Retana, Josep Maria EspeJta, Marc Gracia and MiqueJ Riba
during the initial years following thinning: seedling recruitment is highest in
old-cut stands (16-20 and >20 years-since-Iast-thinning stands) and lowest
in medium-cut stands (6-11 years-since-Iast-thinning stands). As shown in
Fig. 7.5, this pattern is mainly due to differences in density of 1- and 2-yearold seedlings. Exposure to full sunlight in recently cut stands not only has a
negative effect on survival of already established holm oak seedlings, but also
can prevent seed germination. The fact that holm oak acorns are large seeds
has long been thought to provide an adaptive advantage for seedling establishing under shade (Westoby et al. 1992; Bazzaz and Miao 1993; Leishman
and Westoby 1994). Acorns have a short life span when exposed to full sunlight because they dry out very quickly (Vuillemin 1982), and they germinate
better under tree cover, with reduced light levels and the associated increase
in soil moisture content (Bran et al. 1990). Broncano (1995) has confirmed
experimentally these results: the percentage of holm oak acorn germination
at low light levels (8% PAR) is considerably greater than that at full sunlight
(23 vs. 1 % acorn germination, respectively). The higher overall seedling density in older stands resUlts from a remarkable increase in the recruitment of
1- and 2-year-old seedlings rather than the variation of older ones (Fig. 7.5).
Holm oak has always been considered a shade-tolerant species (Ceballos and
Ruiz de la Torre 1979; Bran et al. 1990), which can survive under low light by
means of efficient light interception and reduced respiration. However,
probably the most important reason for such abundant regeneration under
covered conditions is the avoidance of desiccation due to high moisture levels rather than higher light-use efficiency.
7.5 The Sapling Stage: Is There a Lack of Recruitment
in Holm Oak Forests Under Present Management?
Various authors (Lorimer 1984; Crow 1988; Ward 1992) suggest that oaks can
be established under a sylvicultural system that greatly reduces overstory
densities and creates gaps in the canopy, favouring successful release of
seedlings. Nevertheless, a high number of seedlings before thinning in recently cut holm oak forests is no guarantee of good regeneration. Young
seedlings are not able to outgrow the new stump sprouts, which grow much
more rapidly than seedlings during the initial years following the disturbance (Fig. 7.6), chiefly by virtue of the root systems of the parent trees. Only
vigorous holm oak saplings present prior to thinning can successfully compete with sprouts and reach the forest canopy. Nevertheless, in welldeveloped holm oak forests, present conditions of genet density and thinning
management do not allow seedlings older than 15 years to survive (Espelta
et al. 1995).
Holm oak lies at one end of the spectrum of those species identified by
Canham (1988) that can survive under canopy cover: those for which shade
Javier Retana, Josep Maria EspeJta, Marc Gracia and MiqueJ Riba
during the initial years following thinning: seedling recruitment is highest in
old-cut stands (16-20 and >20 years-since-Iast-thinning stands) and lowest
in medium-cut stands (6-11 years-since-Iast-thinning stands). As shown in
Fig. 7.5, this pattern is mainly due to differences in density of 1- and 2-yearold seedlings. Exposure to full sunlight in recently cut stands not only has a
negative effect on survival of already established holm oak seedlings, but also
can prevent seed germination. The fact that holm oak acorns are large seeds
has long been thought to provide an adaptive advantage for seedling establishing under shade (Westoby et al. 1992; Bazzaz and Miao 1993; Leishman
and Westoby 1994). Acorns have a short life span when exposed to full sunlight because they dry out very quickly (Vuillemin 1982), and they germinate
better under tree cover, with reduced light levels and the associated increase
in soil moisture content (Bran et al. 1990). Broncano (1995) has confirmed
experimentally these results: the percentage of holm oak acorn germination
at low light levels (8% PAR) is considerably greater than that at full sunlight
(23 vs. 1 % acorn germination, respectively). The higher overall seedling density in older stands resUlts from a remarkable increase in the recruitment of
1- and 2-year-old seedlings rather than the variation of older ones (Fig. 7.5).
Holm oak has always been considered a shade-tolerant species (Ceballos and
Ruiz de la Torre 1979; Bran et al. 1990), which can survive under low light by
means of efficient light interception and reduced respiration. However,
probably the most important reason for such abundant regeneration under
covered conditions is the avoidance of desiccation due to high moisture levels rather than higher light-use efficiency.
7.5 The Sapling Stage: Is There a Lack of Recruitment
in Holm Oak Forests Under Present Management?
Various authors (Lorimer 1984; Crow 1988; Ward 1992) suggest that oaks can
be established under a sylvicultural system that greatly reduces overstory
densities and creates gaps in the canopy, favouring successful release of
seedlings. Nevertheless, a high number of seedlings before thinning in recently cut holm oak forests is no guarantee of good regeneration. Young
seedlings are not able to outgrow the new stump sprouts, which grow much
more rapidly than seedlings during the initial years following the disturbance (Fig. 7.6), chiefly by virtue of the root systems of the parent trees. Only
vigorous holm oak saplings present prior to thinning can successfully compete with sprouts and reach the forest canopy. Nevertheless, in welldeveloped holm oak forests, present conditions of genet density and thinning
management do not allow seedlings older than 15 years to survive (Espelta
et al. 1995).
Holm oak lies at one end of the spectrum of those species identified by
Canham (1988) that can survive under canopy cover: those for which shade