Some of the Disjunct Species Seen at Quarryhill
In biology, a disjunct distribution is one that has two or more groups of species that
are related but widely separated from each other geographically.
As we all know, the Quarryhill Living Collection is comprised largely of trees and
other flora grown from seeds gathered in Eastern Asia, from the provinces of Sichuan
and Yunnan in China and from Japan and other islands off the coast of Eastern Asia,
and this perhaps makes it inevitable that several specimens we have here in the garden
are examples of the disjunction of species of flora between the eastern United States and
the eastern edge of Asia, including Japan.
The prevailing body of evidence today suggests that during the tertiary period 10 to
15 million years ago, there was a widespread forest across the landmasses in the
northern temperate zone. The Americas, Europe, and Asia were all connected on
multiple occasions by land bridges across the north Atlantic and the Bering Sea. As the
earth cooled, extensive glaciation occurred in Europe and North America, and together
with the rising Rocky Ranges altering the North American climate, the once continuous
forest was fragmented and became disjunct as seen today.
Examples of the disjunction
There are many examples of this disjunction present at Quarryhill. We have all
three species of Sassafras extant in the world today.
Sassafras albidum is from the Eastern North
Americas while Sassafras tsumu and Sassafras
randaiense are from Eastern Asia.
The picture shows Sassafras albidum and the
spicebush swallowtail, a butterfly host plant.
The tulip tree, Liriodendron tulipifera, well
known on the eastern US coast for its straight
trunks and for being one of the tallest trees in the
forest, has leaves shaped like tulip silhouettes
similar to Liriodendron chinense, growing at
Quarryhill. Like its Eastern North American cousin, L. chinense is also a tall tree with
tulip shaped leaves. These two species are native nowhere else in the world and look
like twins of the forest to the untrained eye; yet, they evolved thousands of miles apart.
Liriodendron tulipifera with its tulip
shaped leaves like Liriodendron chinense,
seen at Quarryhill.
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Nyssa chinensis, with beautiful autumn leaves growing on the Dam Face area just
inside the Garden, is a close relative of Nyssa aquatica, the tupelo tree (loosely
translated from Seminole Indian as swamp tree, which
grows in wet areas of the southeastern United States.
Its wood has been used extensively for carving,
particularly duck decoys, and for other furniture
components.
Tupelo (Nyssa chinensis) leaves in early autumn,
showing early color.
Similarly, we have Cornus kousa, or Chinese flowering dogwood, with white bracts
that appear later in spring than the white
bracts on its Eastern American cousin, Cornus
florida, but otherwise is very similar in
conformation and leaf characteristics.
White bracts with the central green flower
that becomes a red raspberry-like fruit in late
summer and fall.
Hamamelis japonica, or Japanese witch hazel, has as its Eastern American cousin
Hamamelis virginiana, a plant that has been extensively cultivated for its rich yellowto-orange-red flowers in autumn and into winter.
Hamamelis japonica in bloom.
Catalpa ovata has two American
cousins, C. bignonioides and C. speciosa,
the southern and northern catalpas. C.
ovata has white flowers that look like the
lavender flowers of Paulownia.
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Liquidambar formosana, from Eastern Asia, also has closely related Eastern
American species. The Eastern American Liquidambar or sweet gum, in addition to the
Eastern American-Eastern Asian
disjunction, also has a north south
disjunction with similar liquidambars
growing in Mexico.
Our eight species of Lindera, all
from Eastern Asia, have three Eastern
American cousins. Lindera benzoin,
sometimes called the northern spice
bush, is one of the American species
along with the common spicebush and
bog spicebush. All of the Lindera are dioecious, with male and female flowers on
separate bushes that grow in light shade. Because of this, they depend more on
pollinators and are more affected by habitat loss with large distances between male and
female plants. Pollination is done by bees and other insects, and the various Lindera
species are food plants for the larvae of the spicebush
swallowtail butterfly. Because of this attraction for
butterflies, they may be used in butterfly garden
plantings.
History
Western botanists walking through the forests of
Eastern Asia had a déjà vu sensation, as if they were
in the Eastern North American forest. They were
disjunct genera rather than species, since most of the species within these genera were
very similar but nevertheless separate species.
It was Asa Gray, the preeminent American botanist of the nineteenth century, who
focused the attention of the scientific community by writing a series of articles that
spanned forty years. Gray applied the hypothesis that a once extensive temperate forest
belt with many species of shrubs and trees grew across Central Europe, all the way to the
shores of East Asia, and across a land bridge to the North American continent. It had
been broken up by geological and climactic causes. This forest was perhaps at its peak in
the Middle Miocene, approximately ten million years ago. When the ice ages dropped
temperatures across Europe, the East-West aligned southern mountain ranges
prevented southern migration of the forest species causing the forests to disappear
except for those in Eastern Asia and Eastern North America, where they could extend
sufficiently far south to survive. Separated by these climactic and geological barriers,
the many species evolved gradually until they became separate species but members of
the same genus.
Source material and suggested further reading.
http://arnoldia.arboretum.harvard.edu/pdf/articles/2012-69-3-land-bridge-travelers-of-the-tertiary-theeastern-asian-eastern-north-american-floristic-disjunction.pdf
http://www.annualreviews.org/doi/abs/10.1146/annurev.ecolsys.30.1.421
Submitted by Philip Wilkinson11/2014
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