180
7.1.3 Landscape Genetic Structure: Fragmentation Matters
Holocene glaciations affected South American temperate forests promoting vegetation shifts and forest retractions (e.g. Compagnucci 2011; Folguera et al. 2011),
events still imprinted in the genetic diversity of long-lived species (e.g. Marchelli
et al. 1998; Premoli et al. 2000; Azpilicueta et al. 2009). The glaciation in the
Southern Hemisphere, which at the north of 41°S was mostly restricted to valleys
(Flint and Fidalgo 1964; Rabassa and Clapperton 1990; Markgraf et al. 1996), led
to speculate on forests’ persistence in several small refugia. After the Last Glacial
Maximum (18,000–20,000 BP), recolonization began about 14,000 years BP
(Heusser et al. 1996; Moreno 1997), but the current vegetation structure was established only about 3000 years ago (Villagran 1991; Heusser et al. 1999).
The current structure and fragmentation of Araucaria forests is also due to the
presence of humans, which in the region began some 11,000 years ago with the
settlement of the continental migrations (Montané 1968). However, the real impact
was more significant during the twentieth century, when growing agricultural and
livestock activities led to a higher frequency and intensity of intentional forest fires.
On the eastern border of the Araucaria native range, in Argentina, the situation is
aggravated by a combination of extreme environmental conditions that increase
drought stress in association with a highly intense human impact. This situation
reduces natural regeneration of forests, which is the most obvious sign of Pewen
forest degradation. Moreover, marginal xeric populations of A. araucana are characterized by a higher proportion of clonal growth, as found in other forest trees with
usually less extent of clonality (e.g. Aparicio et al. 2009; Wilmking et al. 2017).
In this scenario of high habitat fragmentation, the species autoecology and
related ecological processes are altered, and different genetic processes can be
affected (Hartl and Clark 1989; Hanski 1998). The isolation between populations in
a fragmented landscape might increase genetic differentiation, inbreeding and levels of genetic drift (Templeton et al. 2001), while gene flow can be favoured due to
opening of the landscape (Robledo-Arnuncio et al. 2004). With the help of molecular markers, it is possible to evaluate the impact of habitat fragmentation on the
genetic diversity and its distribution among populations.
Organelle DNA markers proved to perform best in studies on historic biogeography or phylogeography (Petit and Vendramin 2006). In Araucariaceae, chloroplast
and mitochondrial DNA are paternally inherited according to cytological evidences
(Kaur and Bhatnagar 1984), as well as mother-progeny comparisons with molecular
markers (Marchelli et al. 2010). Therefore, both plastid genomes move with the pollen grain. With the aim of studying if the species persisted throughout glacial times
in scattered and fragmented populations located towards the east of the glacial margins (“rear-edge” populations), a set of 16 populations covering the fragmented area
were studied with organelle markers (Marchelli et al. 2010). Rear-edge populations
are thought to be stable relicts usually isolated and much older than any other population from the rest of the range (Hampe and Petit 2005). Considering the long life
P. Marchelli et al.
7.1.3 Landscape Genetic Structure: Fragmentation Matters
Holocene glaciations affected South American temperate forests promoting vegetation shifts and forest retractions (e.g. Compagnucci 2011; Folguera et al. 2011),
events still imprinted in the genetic diversity of long-lived species (e.g. Marchelli
et al. 1998; Premoli et al. 2000; Azpilicueta et al. 2009). The glaciation in the
Southern Hemisphere, which at the north of 41°S was mostly restricted to valleys
(Flint and Fidalgo 1964; Rabassa and Clapperton 1990; Markgraf et al. 1996), led
to speculate on forests’ persistence in several small refugia. After the Last Glacial
Maximum (18,000–20,000 BP), recolonization began about 14,000 years BP
(Heusser et al. 1996; Moreno 1997), but the current vegetation structure was established only about 3000 years ago (Villagran 1991; Heusser et al. 1999).
The current structure and fragmentation of Araucaria forests is also due to the
presence of humans, which in the region began some 11,000 years ago with the
settlement of the continental migrations (Montané 1968). However, the real impact
was more significant during the twentieth century, when growing agricultural and
livestock activities led to a higher frequency and intensity of intentional forest fires.
On the eastern border of the Araucaria native range, in Argentina, the situation is
aggravated by a combination of extreme environmental conditions that increase
drought stress in association with a highly intense human impact. This situation
reduces natural regeneration of forests, which is the most obvious sign of Pewen
forest degradation. Moreover, marginal xeric populations of A. araucana are characterized by a higher proportion of clonal growth, as found in other forest trees with
usually less extent of clonality (e.g. Aparicio et al. 2009; Wilmking et al. 2017).
In this scenario of high habitat fragmentation, the species autoecology and
related ecological processes are altered, and different genetic processes can be
affected (Hartl and Clark 1989; Hanski 1998). The isolation between populations in
a fragmented landscape might increase genetic differentiation, inbreeding and levels of genetic drift (Templeton et al. 2001), while gene flow can be favoured due to
opening of the landscape (Robledo-Arnuncio et al. 2004). With the help of molecular markers, it is possible to evaluate the impact of habitat fragmentation on the
genetic diversity and its distribution among populations.
Organelle DNA markers proved to perform best in studies on historic biogeography or phylogeography (Petit and Vendramin 2006). In Araucariaceae, chloroplast
and mitochondrial DNA are paternally inherited according to cytological evidences
(Kaur and Bhatnagar 1984), as well as mother-progeny comparisons with molecular
markers (Marchelli et al. 2010). Therefore, both plastid genomes move with the pollen grain. With the aim of studying if the species persisted throughout glacial times
in scattered and fragmented populations located towards the east of the glacial margins (“rear-edge” populations), a set of 16 populations covering the fragmented area
were studied with organelle markers (Marchelli et al. 2010). Rear-edge populations
are thought to be stable relicts usually isolated and much older than any other population from the rest of the range (Hampe and Petit 2005). Considering the long life
P. Marchelli et al.