Parameter Estimation
The parameter estimates obtained in our analyses were in units of
coalescent generations and for Thuja plicata and Tsuga
heterophylla generally fit with most of our expectations based on the
history of the bioregion. For both species, the population sizes
estimated for the coastal population are slightly larger than those of
the ITR (Table 3), consistent with their current distributions. For both
species, the median divergence time estimates between the coastal and
inland rainforests were approximately 252,000 generations ago (Table 3).
The timing of the population bottleneck event for both species was
estimated to be between 50 and 90 Kya (Table 3). The time of the
population expansion for Thuja plicata was ~1,050
generations ago, whereas the time of population expansion forTsuga heterophylla was nearly twice that value (2,020 generations
ago). The magnitude of the Thuja plicata coastal bottleneck was
apparently slightly larger than that of the inland bottleneck. The
opposite was true for Tsuga heterophylla , where the bottleneck in
the inland was more severe than that on the coast (Table 3). Not
surprisingly, the populations with the most severe bottleneck also had
the largest population expansion rates (Table 3). Note that this
expansion rate was modeled backwards in time; a negative rate indicates
the population is getting smaller towards the past, thus expanding
forward in time. In both species, migration rates from the coast to the
ITR were larger than migration rates from the ITR to the coast (Table
3).
In order to interpret our time estimates, which in coalescent analyses
are expressed in units of generations, we need to consider the
generation length of each species. The generation length is essentially
the average amount of time between consecutive generations in a
population. For western redcedar, estimates of trees reaching maturity
typically range from 20-30 years (Turner 1985), however trees can reach
maturity as early as 10 years in some open grown areas (Minore 1990).
The same is true for western hemlock, where most estimates suggest
maturity is reached between 25-30 years (Owen et al. 1984) but with
trees reaching maturity earlier in some cases (Tesky 1992). To be
conservative, we assumed a relatively short generation length of 10 yrs
/ generation and a longer generation length of 25 yrs / generation for
both species. In doing this, we can convert our estimates of the timing
of the events from generation into years while accounting for
uncertainty in generation length amongst the populations (Table 4). From
these generation lengths, we calculate median divergence times between
inland and coastal populations between 2.5 - 6.3 Mya (Table 4), where
each estimate has an associated 95% CI (Table 4). This indicates the
divergence between inland and coastal populations, for both species, wasbefore the onset of the Pleistocene. Similarly, given the overlap
in 95% CI for the estimates for both species, we cannot reject that
these species’ disjunct populations diverged in concordance.