Biology Reference
In-Depth Information
Figure 9.1
Distribution of
Clintonia
in eastern Asia and in eastern and western North America.
From Ying 1983. The center of distribution is indicated by the shaded area. Used with permis-
sion from the Missouri Botanical Garden Press, St. Louis.
individual elements present since the Late Cretaceous (e.g.,
Nordenskioel-
dia
; Wang et al. 2009). The fossil genera include virtually all those men-
tioned above, in addition to
Sequoia
, presently confi ned to the west coast of
the United States, but widespread in Europe and Asia during the Tertiary,
and the present eastern Asian
Ginkgo
,
Glyptostrobus
,
Metasequoia
,
Eucom-
mia
,
Pterocarya
,
Platycarya
, and others known from the Tertiary of North
America and Europe. The reason, based on the modern distribution of these
and other temperate deciduous plants, is the same—a belt of deciduous
forest across the temperate latitudes of the Northern Hemisphere. The dis-
tribution maps for
Carya
,
Clintonia
,
Euphrasia
, and others all speak of an
ecosystem formerly extending across the region, and now existing in more
restricted areas because of alterations in climate and landscape. Although
it is possible to emphasize migrations in the explanation, a balanced model
must still stress a prominent role for range fragmentation. This is evident
from the congruence in phylogenies of several extant genera based on DNA
data sets (
Adiantum pedatum
, sect.
Aralia
of
Aralia
,
Calycanthus
,
Cornus
,
Boykinia
,
Tiarella
,
Trautvetteria
): “The data are in agreement with the long-
standing hypothesis that this well-known fl oristic disjunction represents
the fragmentation of a once continuous Mixed Mesophytic Forest Commu-
nity” (Xiang et al. 1998, 178). So the next question is, what were those
alterations in climate and landscape that fragmented the forest's once more
extended range?
The fi rst fragmenting event had begun by the Eocene, when the Rocky
