Influence of growth form on FST
We found that trees have populations with significantly lower
FST than both shrubs and non-woody plants (Fig. 1b).
Even though most trees are outcrossing in our dataset, our results show
that growth form contributes to the variation in FSTindependently from mating system, contrary to the findings of Duminil et
al. (2007, 2009). The inherent difference in scale between growth forms
may contribute to this pattern: a given geographic distance between
subpopulations may restrict gene flow much more for an herb than for a
tree. In fact, neighborhood size, i.e. the spatial extent of closely
related individuals, is larger in trees than shrubs and herbs (Vekemans
& Hardy, 2004). Furthermore, trees usually have greater longevity than
shrubs and non-woody plants (Duminil et al., 2009), which may increase
the chances of gene flow between tree subpopulations, more than for
other growth forms. Finally, the fact that growth form and habitat are
tightly linked may also contribute; many non-woody plants and shrubs in
our dataset occur in the forest understory, while many trees reach the
canopy. Givnish (2010) and Theim, Shirk, and Givnish (2014) hypothesized
that the understory imposes more limits to gene flow than the canopy
because of the sedentary lifestyle of animal mutualists in the
understory.