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6.3 Quantitative Genetic Variation in Key Adaptive
Juvenile Traits
As an introduction to the quantitative genetic variation of the Patagonian cypress,
we summarize some key adaptations of the species that, together with the patterns
of neutral variation, have guided us in the design of our study systems. During its
juvenile life stage, Austrocedrus (together with Libocedrus and Papuacedrus) is
distinguished from the other Cupressaceae by its aerial architecture. During the
Paleogene, a context of change towards a warm and humid climate, and of transition
from conifer-dominated to mixed evergreen forests, would have had a profound
impact on the quality of the understory light. The flattening of the photosynthetic
organs is a trait evolved in that context, which would have allowed Austrocedrus (its
ancestor Libocedrus) to establish and compete with angiosperms (Hill and Brodribb
1999). This capacity would have been facilitated by the evolution of longevity, a
characteristic that gives shade-tolerant species enough time to reach the canopy and
to find suitable events to recruit seedlings. More recently, in the context of relative
aridity of the Quaternary, the capacities to avoid desiccation and prevent xylem
cavitation have been key adaptations for the persistence of many Cupressaceae
(e.g., Brodribb and Cochard 2009). Under strong atmospheric demands, the cypress
prevents cavitation by strict control of stomata closure; this adaptation allows it to
persist in very arid environments. At the same time, cypress maintains physiological
characteristics that are typical of shade-tolerant species, e.g., the ability to keep
stomata open under low photosynthetic fluxes (Gyenge et al. 2007). At the end of
the Pleistocene, while many tertiary species in the Northern Hemisphere became
extinct during the glaciations (Sakai and Larcher 1987), in the Southern Hemisphere
less rigorous conditions allowed the persistence of relicts, thus species evolving
limited cold tolerance (Sakai 1971). The maximum cold tolerance of cypress would
be between −6.7 and −12.1 °C (Bannister and Neuner 2001). In summary,
Patagonian cypress has evolved adaptations to inhabit varied environments, from
mixed rain forests to arid steppe environments (Fig. 6.4).
At the same time, it would be a species with moderate cold tolerance, which
would explain its habitat location in deflated, protected rocky substrates and high
slopes, at latitudes such as that of Bariloche (Anchorena and Cingolani 2002). This
capacity for growing in ecologically contrasting situations suggests that there could
be different genetic pools among populations, because natural selection may have
“improved” certain functions in each environment. On the other hand, it could be
postulated that this ability to inhabit contrasting environments is due to the development of a generalist strategy based on plasticity. In our study system, we intended
to disentangle these questions at three scales of analysis: (1) at the whole regional
scale, (2) between humid and arid extremes, and (3) along the arid border of the
cypress’ distribution.
The focus of our research was on juvenile quantitative trait variation. Natural
selection, and therefore adaptation, intervenes along tree life, but seedling establishment is one of the most sensitive stages (Brubaker 1986; Green 2005). In addition,
6 Patagonian Cypress (Austrocedrus chilensis): The Cedarwood…
6.3 Quantitative Genetic Variation in Key Adaptive
Juvenile Traits
As an introduction to the quantitative genetic variation of the Patagonian cypress,
we summarize some key adaptations of the species that, together with the patterns
of neutral variation, have guided us in the design of our study systems. During its
juvenile life stage, Austrocedrus (together with Libocedrus and Papuacedrus) is
distinguished from the other Cupressaceae by its aerial architecture. During the
Paleogene, a context of change towards a warm and humid climate, and of transition
from conifer-dominated to mixed evergreen forests, would have had a profound
impact on the quality of the understory light. The flattening of the photosynthetic
organs is a trait evolved in that context, which would have allowed Austrocedrus (its
ancestor Libocedrus) to establish and compete with angiosperms (Hill and Brodribb
1999). This capacity would have been facilitated by the evolution of longevity, a
characteristic that gives shade-tolerant species enough time to reach the canopy and
to find suitable events to recruit seedlings. More recently, in the context of relative
aridity of the Quaternary, the capacities to avoid desiccation and prevent xylem
cavitation have been key adaptations for the persistence of many Cupressaceae
(e.g., Brodribb and Cochard 2009). Under strong atmospheric demands, the cypress
prevents cavitation by strict control of stomata closure; this adaptation allows it to
persist in very arid environments. At the same time, cypress maintains physiological
characteristics that are typical of shade-tolerant species, e.g., the ability to keep
stomata open under low photosynthetic fluxes (Gyenge et al. 2007). At the end of
the Pleistocene, while many tertiary species in the Northern Hemisphere became
extinct during the glaciations (Sakai and Larcher 1987), in the Southern Hemisphere
less rigorous conditions allowed the persistence of relicts, thus species evolving
limited cold tolerance (Sakai 1971). The maximum cold tolerance of cypress would
be between −6.7 and −12.1 °C (Bannister and Neuner 2001). In summary,
Patagonian cypress has evolved adaptations to inhabit varied environments, from
mixed rain forests to arid steppe environments (Fig. 6.4).
At the same time, it would be a species with moderate cold tolerance, which
would explain its habitat location in deflated, protected rocky substrates and high
slopes, at latitudes such as that of Bariloche (Anchorena and Cingolani 2002). This
capacity for growing in ecologically contrasting situations suggests that there could
be different genetic pools among populations, because natural selection may have
“improved” certain functions in each environment. On the other hand, it could be
postulated that this ability to inhabit contrasting environments is due to the development of a generalist strategy based on plasticity. In our study system, we intended
to disentangle these questions at three scales of analysis: (1) at the whole regional
scale, (2) between humid and arid extremes, and (3) along the arid border of the
cypress’ distribution.
The focus of our research was on juvenile quantitative trait variation. Natural
selection, and therefore adaptation, intervenes along tree life, but seedling establishment is one of the most sensitive stages (Brubaker 1986; Green 2005). In addition,
6 Patagonian Cypress (Austrocedrus chilensis): The Cedarwood…