120
consist of the natural falling of over-mature trees; both involve a massive recruitment related to canopy opening (Veblen et al. 1996; Donoso 2006).
Temporal overlapping of flower maturity during reproductive stages favors the
occurrence of natural interspecific hybridization between these species, which is a
widely reported phenomenon among South American Nothofagus (see Chaps. 3 and
4). The natural hybrid between N. pumilio and N. antarctica presents intermediate
characteristics between both species (i.e., bark roughness and color, stem straightness, crown form). It has been suggested that N. antarctica acts predominantly as
pollen receptor (Acosta and Premoli 2010). The introgression of the chloroplast
genome from N. pumilio to N. antarctica, due to repeated interspecific crossings
and/or hybrid-parent backcrossing, would have occurred more frequently during
unfavorable climatic periods (e.g., glaciations, Palmé et al. 2004; Heuertz et al.
2006) or during forest recovering (e.g., postglacial recolonization), constituting an
additional source of variation.
The climatic changes that occurred during the Quaternary modified the areas
covered by forests in Patagonia and have strongly impacted in the population
dynamics, possibly restricting gene flow, isolating them geographically, or even hindering their chances of regeneration. This regional disturbance left a trace on the
distribution of genetic variation. At a narrower level, other processes could have
shaped the population structure, such as the mating system or, more recently, anthropogenic disturbances. The genetic variability of these species, shaped along their
evolutionary history, should be preserved in order to ensure their adaptability and,
finally, their persistence.
5.2 Phylogeography: Inspecting Nothofagus Evolutionary
History Through Chloroplast DNA
Climatic changes during the Quaternary imposed a great selection pressure along
the distributional range of forest species. After ice advance, the remaining patches
of forests withstood adverse climatic conditions. In Patagonia, glaciations occurred
from the Late Miocene to the Pleistocene, being the Great Patagonian Glaciation
(GPG; about 1 million years before present [M years BP]) the maximum expansion
of ice in extra-Andean Patagonia (Flint and Fidalgo 1969; Rabassa et al. 2005). The
Last Glacial Maximum (LGM) occurred about 18,000–20,000 years BP (Porter
1981), when the ice covered the Patagonian plains beyond the mountain range
(Rabassa and Clapperton 1990; Glasser et al. 2008). However, several ice-free areas
remained (Markgraf et al. 1995) and constituted refugia for vegetation, and some
became the center of expansion of the biota after the ice retreated. Multiple evidence suggests a latitudinal trend in the type of glaciations during the LGM: valleytype glaciations characterized the north, whereas continuous ice layers covered the
southern region (Glasser et al. 2008). A transitional zone at mid-latitudes (42°
S–44° S) was established. The predominance of westerlies determined intermediate
C. Soliani et al.
consist of the natural falling of over-mature trees; both involve a massive recruitment related to canopy opening (Veblen et al. 1996; Donoso 2006).
Temporal overlapping of flower maturity during reproductive stages favors the
occurrence of natural interspecific hybridization between these species, which is a
widely reported phenomenon among South American Nothofagus (see Chaps. 3 and
4). The natural hybrid between N. pumilio and N. antarctica presents intermediate
characteristics between both species (i.e., bark roughness and color, stem straightness, crown form). It has been suggested that N. antarctica acts predominantly as
pollen receptor (Acosta and Premoli 2010). The introgression of the chloroplast
genome from N. pumilio to N. antarctica, due to repeated interspecific crossings
and/or hybrid-parent backcrossing, would have occurred more frequently during
unfavorable climatic periods (e.g., glaciations, Palmé et al. 2004; Heuertz et al.
2006) or during forest recovering (e.g., postglacial recolonization), constituting an
additional source of variation.
The climatic changes that occurred during the Quaternary modified the areas
covered by forests in Patagonia and have strongly impacted in the population
dynamics, possibly restricting gene flow, isolating them geographically, or even hindering their chances of regeneration. This regional disturbance left a trace on the
distribution of genetic variation. At a narrower level, other processes could have
shaped the population structure, such as the mating system or, more recently, anthropogenic disturbances. The genetic variability of these species, shaped along their
evolutionary history, should be preserved in order to ensure their adaptability and,
finally, their persistence.
5.2 Phylogeography: Inspecting Nothofagus Evolutionary
History Through Chloroplast DNA
Climatic changes during the Quaternary imposed a great selection pressure along
the distributional range of forest species. After ice advance, the remaining patches
of forests withstood adverse climatic conditions. In Patagonia, glaciations occurred
from the Late Miocene to the Pleistocene, being the Great Patagonian Glaciation
(GPG; about 1 million years before present [M years BP]) the maximum expansion
of ice in extra-Andean Patagonia (Flint and Fidalgo 1969; Rabassa et al. 2005). The
Last Glacial Maximum (LGM) occurred about 18,000–20,000 years BP (Porter
1981), when the ice covered the Patagonian plains beyond the mountain range
(Rabassa and Clapperton 1990; Glasser et al. 2008). However, several ice-free areas
remained (Markgraf et al. 1995) and constituted refugia for vegetation, and some
became the center of expansion of the biota after the ice retreated. Multiple evidence suggests a latitudinal trend in the type of glaciations during the LGM: valleytype glaciations characterized the north, whereas continuous ice layers covered the
southern region (Glasser et al. 2008). A transitional zone at mid-latitudes (42°
S–44° S) was established. The predominance of westerlies determined intermediate
C. Soliani et al.