495
context, the combination of threshold models with ecological studies along an altitudinal gradient provided a useful approach for the prediction of seed behavior of
N. obliqua, N. alpina, and N. pumilio in the forest, from dispersion to germination
(Arana et al. 2016). This conceptual framework provided insights into the plasticity
of species to changes in the natural thermal environment.
Threshold models allow to predict changes in phenology in response to changes
in the environment, and by incorporating population variance, they can investigate
mechanisms of intraspecific variation or population synchronization (Donohue
et al. 2015). For the particular example of temperature, thermal time models are
threshold models that describe changes in phenology in response to the thermal
environment (Bradford 1996). A brief description of thermal time models can be
found in Chap. 3.
For the particular case of N. obliqua, N. alpina, and N. pumilio, experiments
under controlled conditions established that the thermal time model that showed the
best fit with the germination behavior in response to different temperatures was
based on a constant base temperature (Tb) and variable lower limit temperature for
germination (Tl). In this case, Tl of the population is characterized by the population parameters Tl (50) that is the mean lower limit temperature for germination of the
seed population and its standard deviation θ Tl . Tl means the minimum temperature
at which seeds are able to accumulate developmental units (thermal time), and
therefore germination is possible. The rate of accumulation of developmental units
will be defined by the differences between the soil temperature and Tb (Arana et al.
2016). By using thermal time models, it was possible to determine that the germination of the three Nothofagus species showed specific adaptations to the thermal
environment of their preferred ecological niches. This was evidenced by the fact
that once dormancy was lost after a period of 110 days of cold stratification, the
species that were more abundant at higher (i.e., colder) positions of the gradient
showed lower Tb (Tb N. obliqua = 4.5 °C > N. alpina = 0.5 °C > N. pumilio = 0 °C).
Similarly, non-dormant seeds of species inhabiting the colder positions of the gradient showed lower values of the mean lower temperature limit for germination
(Tl (50) ). This means that species of the higher altitudes are able to germinate at very
low temperatures (around 0 °C), and this behavior is not present in the species of the
warmer (i.e., lower) zones. In this particular case, the estimation of the population
thermal time parameters Tb and Tl (50) allowed to quantify the germination sensitivity to the temperature and suggest the existence of specific adaptation of the different species to temperature. Both Tl (50) and σ (Tl) are high at the moment of seed
dispersal and decreased with the cold stratification in the range between 0.5 and
6 °C (Fig. 18.5).
These laboratory observations allowed to set up a model that could be tested in
nature, in ecological experiments along an altitudinal gradient. In this model, it was
proposed that seeds of the three Nothofagus species are dormant at dispersal time,
and their level of dormancy is broken during overwintering, when soil temperatures
are around or below 5 °C. During spring, interspecific variation in Tl (50) , in combination with natural gradients of soil temperature across altitudes, would strongly regu18 Questions, Perspectives and Final Considerations of Planting Native Species…
context, the combination of threshold models with ecological studies along an altitudinal gradient provided a useful approach for the prediction of seed behavior of
N. obliqua, N. alpina, and N. pumilio in the forest, from dispersion to germination
(Arana et al. 2016). This conceptual framework provided insights into the plasticity
of species to changes in the natural thermal environment.
Threshold models allow to predict changes in phenology in response to changes
in the environment, and by incorporating population variance, they can investigate
mechanisms of intraspecific variation or population synchronization (Donohue
et al. 2015). For the particular example of temperature, thermal time models are
threshold models that describe changes in phenology in response to the thermal
environment (Bradford 1996). A brief description of thermal time models can be
found in Chap. 3.
For the particular case of N. obliqua, N. alpina, and N. pumilio, experiments
under controlled conditions established that the thermal time model that showed the
best fit with the germination behavior in response to different temperatures was
based on a constant base temperature (Tb) and variable lower limit temperature for
germination (Tl). In this case, Tl of the population is characterized by the population parameters Tl (50) that is the mean lower limit temperature for germination of the
seed population and its standard deviation θ Tl . Tl means the minimum temperature
at which seeds are able to accumulate developmental units (thermal time), and
therefore germination is possible. The rate of accumulation of developmental units
will be defined by the differences between the soil temperature and Tb (Arana et al.
2016). By using thermal time models, it was possible to determine that the germination of the three Nothofagus species showed specific adaptations to the thermal
environment of their preferred ecological niches. This was evidenced by the fact
that once dormancy was lost after a period of 110 days of cold stratification, the
species that were more abundant at higher (i.e., colder) positions of the gradient
showed lower Tb (Tb N. obliqua = 4.5 °C > N. alpina = 0.5 °C > N. pumilio = 0 °C).
Similarly, non-dormant seeds of species inhabiting the colder positions of the gradient showed lower values of the mean lower temperature limit for germination
(Tl (50) ). This means that species of the higher altitudes are able to germinate at very
low temperatures (around 0 °C), and this behavior is not present in the species of the
warmer (i.e., lower) zones. In this particular case, the estimation of the population
thermal time parameters Tb and Tl (50) allowed to quantify the germination sensitivity to the temperature and suggest the existence of specific adaptation of the different species to temperature. Both Tl (50) and σ (Tl) are high at the moment of seed
dispersal and decreased with the cold stratification in the range between 0.5 and
6 °C (Fig. 18.5).
These laboratory observations allowed to set up a model that could be tested in
nature, in ecological experiments along an altitudinal gradient. In this model, it was
proposed that seeds of the three Nothofagus species are dormant at dispersal time,
and their level of dormancy is broken during overwintering, when soil temperatures
are around or below 5 °C. During spring, interspecific variation in Tl (50) , in combination with natural gradients of soil temperature across altitudes, would strongly regu18 Questions, Perspectives and Final Considerations of Planting Native Species…