Seedling Recruitment
95
ported for shade-tolerant species (Walters et al. 1993; Kitajima 1994; Osunkjoya et al. 1994).
Seedling characteristics which affect carbon assimilation, namely leaf area
ratio and photosynthetic rates, may be highly responsible for the poor performance of holm oak seedlings at high radiation levels. Nevertheless, we
must take into account other plant features that might also affect plasticity. In
this sense, the pattern of biomass partitioning in the roots, stem and leaves of
the holm oak appears soon after acorn germination, and the plasticity of
such traits may influence the response of seedlings to light and water availability (Leishman and Westoby 1994). Holm oak, as other Quercus species,
produces heavy acorns without photosynthetic cotyledons. Both these features are commonly interpreted as being adaptations to poor light environments (Kitajima 1994; Leishmann and Westoby 1994; Osunkoya et al. 1994).
The main characteristic of biomass partitioning in this species is the large
amount of resources devoted to roots in all environments. Thus, it has been
reported that root biomass can account for nearly 55% of the whole seedling
weight in shady environments, and up to 70% in high light levels (Broncano
1995). Due to the increasing respiration rates under high light conditions
(Givnish 1988), a great investment in non-photosynthetic structures (mainly
roots) may lead to important construction and maintenance costs (Lambers
and Poorter 1992), resulting in a small net carbon gain. Thus, as previously
reported for other shade-tolerant species (Kitajima 1994), the optimum performance of holm oak seedlings under moderate light regimes seems to be
the result of multiple features, both anatomical and physiological, which result in a high survival rate but which constrain growth rates.
7.4 Effects of Disturbance on Seedling Density and Dynamics
Man-induced disturbances play an important role in holm oak seedling
population dynamics. The two major disturbances affecting holm oak forests
in NE Spain are thinning and fire. Grazing has declined in importance in recent decades as domestic herbivores, which have a much greater impact on
holm oak than wild ones (Cuartas and Garda-Gonzalez 1992), are no longer
common in these forests. Both thinning and fire have direct effects on seedlings. Thinning causes high seedling mortality through dragging of tree
trunks and, more importantly, through the presence of slash piles, which
cover and finally kill many seedlings. Yet, the direct effects of fire are even
more highly destructive, as most of the aerial parts of seedlings are burned
or die in the extreme heat. These disturbances also have indirect effects on
the seedlings via changes in the post-disturbance environment. When disturbances such as fire or thinning cause the opening of the canopy, most of
the factors that affect seedling recruitment change drastically. The availability of light is increased, associated with exposure to environmental extremes
95
ported for shade-tolerant species (Walters et al. 1993; Kitajima 1994; Osunkjoya et al. 1994).
Seedling characteristics which affect carbon assimilation, namely leaf area
ratio and photosynthetic rates, may be highly responsible for the poor performance of holm oak seedlings at high radiation levels. Nevertheless, we
must take into account other plant features that might also affect plasticity. In
this sense, the pattern of biomass partitioning in the roots, stem and leaves of
the holm oak appears soon after acorn germination, and the plasticity of
such traits may influence the response of seedlings to light and water availability (Leishman and Westoby 1994). Holm oak, as other Quercus species,
produces heavy acorns without photosynthetic cotyledons. Both these features are commonly interpreted as being adaptations to poor light environments (Kitajima 1994; Leishmann and Westoby 1994; Osunkoya et al. 1994).
The main characteristic of biomass partitioning in this species is the large
amount of resources devoted to roots in all environments. Thus, it has been
reported that root biomass can account for nearly 55% of the whole seedling
weight in shady environments, and up to 70% in high light levels (Broncano
1995). Due to the increasing respiration rates under high light conditions
(Givnish 1988), a great investment in non-photosynthetic structures (mainly
roots) may lead to important construction and maintenance costs (Lambers
and Poorter 1992), resulting in a small net carbon gain. Thus, as previously
reported for other shade-tolerant species (Kitajima 1994), the optimum performance of holm oak seedlings under moderate light regimes seems to be
the result of multiple features, both anatomical and physiological, which result in a high survival rate but which constrain growth rates.
7.4 Effects of Disturbance on Seedling Density and Dynamics
Man-induced disturbances play an important role in holm oak seedling
population dynamics. The two major disturbances affecting holm oak forests
in NE Spain are thinning and fire. Grazing has declined in importance in recent decades as domestic herbivores, which have a much greater impact on
holm oak than wild ones (Cuartas and Garda-Gonzalez 1992), are no longer
common in these forests. Both thinning and fire have direct effects on seedlings. Thinning causes high seedling mortality through dragging of tree
trunks and, more importantly, through the presence of slash piles, which
cover and finally kill many seedlings. Yet, the direct effects of fire are even
more highly destructive, as most of the aerial parts of seedlings are burned
or die in the extreme heat. These disturbances also have indirect effects on
the seedlings via changes in the post-disturbance environment. When disturbances such as fire or thinning cause the opening of the canopy, most of
the factors that affect seedling recruitment change drastically. The availability of light is increased, associated with exposure to environmental extremes