484
ing an economic failure. In an enrichment plan, trees may express poor competitive
aptitude and be suppressed under pre-existing vegetation, whether they be trees,
shrubs, or grasses.
Maladaptation can occur very early, even when the seedlings face the first environmental stress, but it can also be verified belatedly, when the plantation faces
extreme climatic events of low recurrence. Thus, an atypical drought or an exceptional frost can kill all the planted seedlings, although their initial survival and
growth has been adequate so far. Possible biological attacks can also reveal maladaptation processes. An eventual outbreak of a pest or the proliferation of a disease
due to predisposing environmental conditions may put at risk the survival or alter
the tree shape of the planted individuals, even though they have been vigorous and
healthy up to that point.
On the other hand, the wrong choice of the genetic pool to propagate can have a
detrimental effect not only on the implanted mass but also on pre-existing forests. If
afforestation is carried out in regions where the same species grows naturally, the
use of a genetic pool from another region entails a “genetic contamination” risk
(Bischoff et al. 2010; Vander Mijnsbrugge et al. 2010), that is, the introgression of
“exotic” (non-local) genes or genotypes in natural forest stands. This process can
occur through both free pollination and natural seed dispersal, and its effect becomes
irreversible, leading to possible irrecoverable loss of the local genetic pool.
The mixing of exotic and local genes can produce the effect of “outbreeding
depression” (Fenster and Galloway 2000), by which advanced hybrid generations
express a lower fitness than that of the progenitors, even when hybrid vigor has been
verified in the first generation of hybridization. This effect appears to be due to the
loss of heterozygosity after F1 and the disruption of co-adapted gene complexes
(Fenster and Dudash 1994; Frankham et al. 2011). The long life cycle of trees may
delay the verification of this effect, being perhaps too belatedly. This risk is particularly important in restoration programs that carry out enrichment afforestation in
degraded forests in order to recompose the forest continuity between the surviving
remnants after fires or other catastrophic events.
In the particular case of enrichment afforestation, extensive planting in degraded
forests, in which the remaining individuals of the species to be planted are relatively
few, can cause what is called “genetic swamping.” In this process, the local gene
pool is diluted in the gene pool brought by the plantation, and if these are different,
loss of allelic variants or multigene complexes may occur, even those with adaptive
value (Lesica and Allendorf 1999). There may also be a swamping process when
plants of different provenances are mixed in the plantation, and some of them have
a significantly better performance than the others, including the local one, leading
in the long term to the imposition of this genetic pool (Falk et al. 2001).
In this sense, the most conservative decision (the one that involves the least risks)
when choosing the genetic material to propagate, both for a productive project and
for a restoration program, is the use of local genetic pools (Vander Mijnsbrugge
et al. 2010). Collecting seeds from the forests that will be restored (or in their immediate vicinity) is a basic rule of ecosystem restoration.
M. J. Pastorino et al.
ing an economic failure. In an enrichment plan, trees may express poor competitive
aptitude and be suppressed under pre-existing vegetation, whether they be trees,
shrubs, or grasses.
Maladaptation can occur very early, even when the seedlings face the first environmental stress, but it can also be verified belatedly, when the plantation faces
extreme climatic events of low recurrence. Thus, an atypical drought or an exceptional frost can kill all the planted seedlings, although their initial survival and
growth has been adequate so far. Possible biological attacks can also reveal maladaptation processes. An eventual outbreak of a pest or the proliferation of a disease
due to predisposing environmental conditions may put at risk the survival or alter
the tree shape of the planted individuals, even though they have been vigorous and
healthy up to that point.
On the other hand, the wrong choice of the genetic pool to propagate can have a
detrimental effect not only on the implanted mass but also on pre-existing forests. If
afforestation is carried out in regions where the same species grows naturally, the
use of a genetic pool from another region entails a “genetic contamination” risk
(Bischoff et al. 2010; Vander Mijnsbrugge et al. 2010), that is, the introgression of
“exotic” (non-local) genes or genotypes in natural forest stands. This process can
occur through both free pollination and natural seed dispersal, and its effect becomes
irreversible, leading to possible irrecoverable loss of the local genetic pool.
The mixing of exotic and local genes can produce the effect of “outbreeding
depression” (Fenster and Galloway 2000), by which advanced hybrid generations
express a lower fitness than that of the progenitors, even when hybrid vigor has been
verified in the first generation of hybridization. This effect appears to be due to the
loss of heterozygosity after F1 and the disruption of co-adapted gene complexes
(Fenster and Dudash 1994; Frankham et al. 2011). The long life cycle of trees may
delay the verification of this effect, being perhaps too belatedly. This risk is particularly important in restoration programs that carry out enrichment afforestation in
degraded forests in order to recompose the forest continuity between the surviving
remnants after fires or other catastrophic events.
In the particular case of enrichment afforestation, extensive planting in degraded
forests, in which the remaining individuals of the species to be planted are relatively
few, can cause what is called “genetic swamping.” In this process, the local gene
pool is diluted in the gene pool brought by the plantation, and if these are different,
loss of allelic variants or multigene complexes may occur, even those with adaptive
value (Lesica and Allendorf 1999). There may also be a swamping process when
plants of different provenances are mixed in the plantation, and some of them have
a significantly better performance than the others, including the local one, leading
in the long term to the imposition of this genetic pool (Falk et al. 2001).
In this sense, the most conservative decision (the one that involves the least risks)
when choosing the genetic material to propagate, both for a productive project and
for a restoration program, is the use of local genetic pools (Vander Mijnsbrugge
et al. 2010). Collecting seeds from the forests that will be restored (or in their immediate vicinity) is a basic rule of ecosystem restoration.
M. J. Pastorino et al.