What can we learn from the historical dynamics of Abies alba? Is there some
type of human management that had previously benefited or harmed the firs? In any
case, human impact, more than climate, appears to be the primary factor in local
extinction and the loss of genetic diversity and, consequently, the loss of adaptability (Carcaillet and Muller 2005).
5.2.2 Are Baseline and Range of Natural Variability
Appropriate Concepts for Secondary Communities?
Although Costa et al. (1998) reported that Betula is widely distributed across the
northern and medial zones of the European subcontinent, in the south its presence
becomes progressively weaker. In the Iberian peninsula, these trees often dot other
forest formations, such as Eurosiberian or sub-Mediterranean, although it is also
possible to find them in more or less pure stands. Therefore, from the viewpoint of
current forest management, knowing the origin of the small birch woods that are
scattered across the panorama of other forests or whether there are stable birch
forests is of great interest to conservation efforts.
In the Pyrenees, we find two species: Betula pendula and Betula pubescens.
Being heliophilous plants, they have serial behaviors, meaning that in the process of
succession that allows the original forest to recover, they act as pioneers in the
successional sequence after disturbances in other forest masses or other sites that
are difficult to colonize. Birch is a eurioic genus, capable of rapid natural expansion
under favorable conditions, i.e., heavy production of light, winged seeds, vegetative
regeneration, and easy germination (Costa et al. 1998). Therefore, the discussion
concerns whether birch should be considered a secondary formation or the optimum
forest vegetation, which would not be unusual in the presence of rexistasia and
geomorphological activity (landslides, avalanches, etc.).
From a paleoecological perspective (Fig. 5.3a), birch propagated rapidly at the
beginning of the Holocene, when the last glaciation had relegated to far distant
areas the various forest patterns that today occupy the territory, allowing a broader
distribution than in present times. Later, its distribution was limited to small populations in more favorable local habitats, which contributed to its persistence during
the rapid climate changes that followed. At present, when thinking about ecological
succession after a system has been strongly modified by human action, we tend to
consider changes in forest structure at human scale. That is why birch stands have
not traditionally been considered a stable ‘final’ status but rather an intermediate
phase of a succession. What happens if these secondary dynamics extend more than
100 years? During the beginning of the Holocene, birch woods have been stable
formations in the Pallars Sobirà region, and it was not until recent millennia that
they have adapted to specific, greatly disturbed areas. Therefore, the same species
of this genus may have had two totally differentiated preferences over the course of
time, and a species considered to be mainly secondary in the Pyrenees today might
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A. Pèlachs et al.
type of human management that had previously benefited or harmed the firs? In any
case, human impact, more than climate, appears to be the primary factor in local
extinction and the loss of genetic diversity and, consequently, the loss of adaptability (Carcaillet and Muller 2005).
5.2.2 Are Baseline and Range of Natural Variability
Appropriate Concepts for Secondary Communities?
Although Costa et al. (1998) reported that Betula is widely distributed across the
northern and medial zones of the European subcontinent, in the south its presence
becomes progressively weaker. In the Iberian peninsula, these trees often dot other
forest formations, such as Eurosiberian or sub-Mediterranean, although it is also
possible to find them in more or less pure stands. Therefore, from the viewpoint of
current forest management, knowing the origin of the small birch woods that are
scattered across the panorama of other forests or whether there are stable birch
forests is of great interest to conservation efforts.
In the Pyrenees, we find two species: Betula pendula and Betula pubescens.
Being heliophilous plants, they have serial behaviors, meaning that in the process of
succession that allows the original forest to recover, they act as pioneers in the
successional sequence after disturbances in other forest masses or other sites that
are difficult to colonize. Birch is a eurioic genus, capable of rapid natural expansion
under favorable conditions, i.e., heavy production of light, winged seeds, vegetative
regeneration, and easy germination (Costa et al. 1998). Therefore, the discussion
concerns whether birch should be considered a secondary formation or the optimum
forest vegetation, which would not be unusual in the presence of rexistasia and
geomorphological activity (landslides, avalanches, etc.).
From a paleoecological perspective (Fig. 5.3a), birch propagated rapidly at the
beginning of the Holocene, when the last glaciation had relegated to far distant
areas the various forest patterns that today occupy the territory, allowing a broader
distribution than in present times. Later, its distribution was limited to small populations in more favorable local habitats, which contributed to its persistence during
the rapid climate changes that followed. At present, when thinking about ecological
succession after a system has been strongly modified by human action, we tend to
consider changes in forest structure at human scale. That is why birch stands have
not traditionally been considered a stable ‘final’ status but rather an intermediate
phase of a succession. What happens if these secondary dynamics extend more than
100 years? During the beginning of the Holocene, birch woods have been stable
formations in the Pallars Sobirà region, and it was not until recent millennia that
they have adapted to specific, greatly disturbed areas. Therefore, the same species
of this genus may have had two totally differentiated preferences over the course of
time, and a species considered to be mainly secondary in the Pyrenees today might
118
A. Pèlachs et al.