199
sex ratio. The size of the individuals ranged from 5 to 100 cm DBH with an average
of 38.7 cm. The thicker (older) individuals were usually found at about 500 m out
of the current riverbed, in the floodplain. In these sites, the soil is more or less consolidated, and the vegetation is shrubby and typical of the Patagonian steppe. The
average annual diameter growth measured in wood cores at breast height of 15
prospected adult trees from different sites and rivers was 9 mm, reaching in some
sites and years up to 22 mm. The average height was 6 m and the tallest individuals
reached 13.6 m. The harsh environmental conditions, with constant strong winds,
produce serious damages on the willow shoots. The bark of the adult trees, especially the oldest ones, had clear signs of the grinding effect of the sand moved by the
wind, making irregular smooth plates. Most trees lost a part of their canopy, and had
numerous dead branches, probably due to the drought stress experienced during the
summer. The average proportion of living canopy in these old trees was around
45%. Most trees had a single main stem, and in some cases, large old trees were
fully or partially prostrated with some lateral branches taking a dominant role as
main stems (Orellana et al. 2016). Natural regeneration was found only in 3 out of
the 23 monitored sites, with less than 100 juveniles, severely affected by livestock
herbivory and always restricted to the edge of the current riverbed. Putative hybrids
between S. humboldtiana and S. x fragilis f. fragilis were observed also in three sites.
7.2.3 Landscape Genetic Structure
The huge invasion of all river shores by exotic willow clones is important for the
hybridization process given the high demographic imbalance with hundred thousands of exotic Salix trees and just few surviving native willows. The probability
that an insect and/or the wind carrying native willow pollen fecundates a native
female tree is very small. On the contrary, the probability of fecundating the female
exotic willow clone is very high. Therefore, in most of the rivers, when phenological windows overlap, hybrid seeds are produced.
Salix humboldtiana hybridizes specially with the introduced Salix species of the
section Salix (Wu et al. 2015) with a remarkable introgression and consequently
genetic dilution of its genetic information. In order to identify hybridization processes among the invasive willow taxa (S. alba, S. fragilis, S. babylonica, S. matsudana, S. viminalis) and S. humboldtiana, a molecular toolbox for taxa discrimination
was created. It combines three maternally inherited chloroplast microsatellite with
ten biparentally inherited nuclear microsatellite markers. This toolbox is suitable to
unambiguously distinguish first-generation S. humboldtiana hybrids. The taxa can
be discerned by chloroplast microsatellite markers, and in addition, S. humboldtiana presented a private allele at one of the nuclear microsatellite loci (Bozzi et al.
2012). The analysis of 1300 individuals from five Patagonian rivers allowed detecting 86 Salix humboldtiana hybrids: 76% were juvenile and only 24% were adults.
In most cases, S. humboldtiana was the pollen donor (Bozzi et al. 2012), which is
7 Araucaria araucana and Salix humboldtiana…
sex ratio. The size of the individuals ranged from 5 to 100 cm DBH with an average
of 38.7 cm. The thicker (older) individuals were usually found at about 500 m out
of the current riverbed, in the floodplain. In these sites, the soil is more or less consolidated, and the vegetation is shrubby and typical of the Patagonian steppe. The
average annual diameter growth measured in wood cores at breast height of 15
prospected adult trees from different sites and rivers was 9 mm, reaching in some
sites and years up to 22 mm. The average height was 6 m and the tallest individuals
reached 13.6 m. The harsh environmental conditions, with constant strong winds,
produce serious damages on the willow shoots. The bark of the adult trees, especially the oldest ones, had clear signs of the grinding effect of the sand moved by the
wind, making irregular smooth plates. Most trees lost a part of their canopy, and had
numerous dead branches, probably due to the drought stress experienced during the
summer. The average proportion of living canopy in these old trees was around
45%. Most trees had a single main stem, and in some cases, large old trees were
fully or partially prostrated with some lateral branches taking a dominant role as
main stems (Orellana et al. 2016). Natural regeneration was found only in 3 out of
the 23 monitored sites, with less than 100 juveniles, severely affected by livestock
herbivory and always restricted to the edge of the current riverbed. Putative hybrids
between S. humboldtiana and S. x fragilis f. fragilis were observed also in three sites.
7.2.3 Landscape Genetic Structure
The huge invasion of all river shores by exotic willow clones is important for the
hybridization process given the high demographic imbalance with hundred thousands of exotic Salix trees and just few surviving native willows. The probability
that an insect and/or the wind carrying native willow pollen fecundates a native
female tree is very small. On the contrary, the probability of fecundating the female
exotic willow clone is very high. Therefore, in most of the rivers, when phenological windows overlap, hybrid seeds are produced.
Salix humboldtiana hybridizes specially with the introduced Salix species of the
section Salix (Wu et al. 2015) with a remarkable introgression and consequently
genetic dilution of its genetic information. In order to identify hybridization processes among the invasive willow taxa (S. alba, S. fragilis, S. babylonica, S. matsudana, S. viminalis) and S. humboldtiana, a molecular toolbox for taxa discrimination
was created. It combines three maternally inherited chloroplast microsatellite with
ten biparentally inherited nuclear microsatellite markers. This toolbox is suitable to
unambiguously distinguish first-generation S. humboldtiana hybrids. The taxa can
be discerned by chloroplast microsatellite markers, and in addition, S. humboldtiana presented a private allele at one of the nuclear microsatellite loci (Bozzi et al.
2012). The analysis of 1300 individuals from five Patagonian rivers allowed detecting 86 Salix humboldtiana hybrids: 76% were juvenile and only 24% were adults.
In most cases, S. humboldtiana was the pollen donor (Bozzi et al. 2012), which is
7 Araucaria araucana and Salix humboldtiana…