163
and humid populations did not discriminate the carbon isotopes in a differential
way, i.e., we found no evidence of adaptation. On the other hand, the two arid populations, very close to each other, proved to be very different in terms of genetic
variation, which could be a consequence of genetic drift by isolation.
As a conclusion to this second stage of analysis, we found no evidence of genetic
differentiation between populations from the humid and xeric extremes of cypress
in traits linked to the early light capture and water stress tolerance. This is not a
lesser information: the populations of the arid margin would be globally similar to
the humid ones in their gene pool, but in some cases, high intra-population variance,
as well as differences between them, indicates their evolutionary importance and
potential for breeding in a context of climate change towards higher regional aridity.
6.3.3 Variation Along the Arid Marginal Edge
In trees that do not form seed banks, seed germination and plantlet emergence are
precisely synchronized with the current environmental conditions. Modeling of its
dynamics in common garden trials can be useful for assessing inter- and intrapopulation variance of key traits for breeding. We have analyzed the variation in
cypress plantlet emergence, using seed samples from 177 trees randomly selected
within 10 arid marginal populations (Table 6.4) (Pastorino et al. 2013). With the
data of emergency accumulated over time, we fitted sigmoid curves (Gompertz
model), from which we derived six parameters describing plantlet emergency
capacity, energy, and timing. On average, for the emergency capacity, energy, and
timing of cessation and duration of the process, the variance components analysis
showed a larger effect of the mother tree (V F = 55.4%) than of population (V P = 22%),
suggesting microevolutionary potential (although to clarify this, the additive genetic
variance should be measured). For the initiation and timing to the maximum emergence rate (energy period), the variance component of population was larger than
the family component (V P = 41.1% vs. V F = 34.9%, on average), which may indicate
differentiation between populations.
The genetic variation of seedling architecture between humid and arid edge populations has already been discussed above. In this stage, Aparicio (2013) deepens
the analysis in the arid edge of cypress range (ten populations, Table 6.4), with focus
in the immediate branching degree. This architectural trait expresses the number of
branches of second order (A2) formed during the first year of life, in relation to the
vigor of the main axis (A1). In this research, it was observed that the genetic differentiation between populations was low (Q ST = 0.08), virtually equal to the degree
of neutral differentiation. Despite the low Q ST value, a linear trend (r
2
= 0.53) of
ecotypic variation was detected, in which the mean population branching degree
decreased with altitude. The intra-population genetic variance (additive variance
coefficient: CV A ) of the branching degree was very high (CV A = 24.4% on average),
and its heritability was moderate (h
2
= 0.26 on average). This could reflect processes
of heterogeneous selection within populations given high spatial variation of fine
6 Patagonian Cypress (Austrocedrus chilensis): The Cedarwood…
and humid populations did not discriminate the carbon isotopes in a differential
way, i.e., we found no evidence of adaptation. On the other hand, the two arid populations, very close to each other, proved to be very different in terms of genetic
variation, which could be a consequence of genetic drift by isolation.
As a conclusion to this second stage of analysis, we found no evidence of genetic
differentiation between populations from the humid and xeric extremes of cypress
in traits linked to the early light capture and water stress tolerance. This is not a
lesser information: the populations of the arid margin would be globally similar to
the humid ones in their gene pool, but in some cases, high intra-population variance,
as well as differences between them, indicates their evolutionary importance and
potential for breeding in a context of climate change towards higher regional aridity.
6.3.3 Variation Along the Arid Marginal Edge
In trees that do not form seed banks, seed germination and plantlet emergence are
precisely synchronized with the current environmental conditions. Modeling of its
dynamics in common garden trials can be useful for assessing inter- and intrapopulation variance of key traits for breeding. We have analyzed the variation in
cypress plantlet emergence, using seed samples from 177 trees randomly selected
within 10 arid marginal populations (Table 6.4) (Pastorino et al. 2013). With the
data of emergency accumulated over time, we fitted sigmoid curves (Gompertz
model), from which we derived six parameters describing plantlet emergency
capacity, energy, and timing. On average, for the emergency capacity, energy, and
timing of cessation and duration of the process, the variance components analysis
showed a larger effect of the mother tree (V F = 55.4%) than of population (V P = 22%),
suggesting microevolutionary potential (although to clarify this, the additive genetic
variance should be measured). For the initiation and timing to the maximum emergence rate (energy period), the variance component of population was larger than
the family component (V P = 41.1% vs. V F = 34.9%, on average), which may indicate
differentiation between populations.
The genetic variation of seedling architecture between humid and arid edge populations has already been discussed above. In this stage, Aparicio (2013) deepens
the analysis in the arid edge of cypress range (ten populations, Table 6.4), with focus
in the immediate branching degree. This architectural trait expresses the number of
branches of second order (A2) formed during the first year of life, in relation to the
vigor of the main axis (A1). In this research, it was observed that the genetic differentiation between populations was low (Q ST = 0.08), virtually equal to the degree
of neutral differentiation. Despite the low Q ST value, a linear trend (r
2
= 0.53) of
ecotypic variation was detected, in which the mean population branching degree
decreased with altitude. The intra-population genetic variance (additive variance
coefficient: CV A ) of the branching degree was very high (CV A = 24.4% on average),
and its heritability was moderate (h
2
= 0.26 on average). This could reflect processes
of heterogeneous selection within populations given high spatial variation of fine
6 Patagonian Cypress (Austrocedrus chilensis): The Cedarwood…