How is Witch Hazel Pollinated?
While
I was preparing my recent column on witch hazel, Dick Christensen, president of
the Niagara Frontier Botanical Society, called my attention to an inquiry by
Donald W. Stokes in his delightful book, The
Natural History of Wild Shrubs and Vines. Stokes describes the two species of witch hazel in eastern
North America: ³One blooms in late fall after the leaves have shed; the other
[more southern species], sometimes known as Springtime Witch Hazel, blooms in
late winter and early spring before the leaves appear.² He then asks, ³Who pollinates the
flowers? Both species bloom at
times when very few insects are out collecting food. I have watched the flowers when they are in bloom and the
only visitors I have seen are ants.²
That
question intrigued me, because it is hard to imagine a plant blooming at a less
hospitable season for pollination.
By the time witch hazelıs delicate yellow blossoms appear in
mid-November, all the other fall flowers — the asters and goldenrods, the
everlastings, the burdocks, teasels, and thistles — have faded to
brown. More important, the
plummeting temperatures are killing off the few insects remaining abroad. For example, all male wasps have
already died, and only the few queens are now quietly aestivating in
subterranean passages.
In
The Book of Forest and Thicket John
Eastman nominates as witch hazel pollinators fungus gnats, parasitic wasps,
hover flies, and tachinid flies; Arthur Craig Quick in Wild Flowers of the Northern States and Canada suggests late flying
bees. But Stokes has watched and
never seen these insects; neither have I.
Now
by quite remarkable chance unrelated reading has led me to a different answer
to Stokesı question. It turns out
that he and I have been looking at the wrong time of day.
Bernd
Heinrich is one of my favorite authors.
His most entertaining books are Ravens
in Winter, about his experiences with these crow relatives in the sub-zero
New England winter woods, and Bubo,
about his fierce pet, a great horned owl.
But Heinrich is also a serious scientist and like the very best of them,
he asks significant questions, addresses those questions in creative ways, and
provides evidence for interesting answers. One of his central research concerns is how insects adapt to
temperature.
In
a 1987 Scientific American article,
he describes how a few species of owlet moth remain active when winter
temperatures drop to near freezing.
When they are at rest, the mothsı body temperature reduces to near that
of their surroundings and they enter a state of torpor. In daytime and when temperatures are
below freezing they hide on the ground under leaves that provide excellent
insulation.
But
when night comes, these moths have to fly to find food and that requires them
to raise their body heat as much as 50° to activate their flight muscles. To do this, they shiver, the same
response to cold that our human body applies in far less efficient form as what
we call chills. When the moths
fly, they rapidly lose this necessary 86° temperature: on long flights they
must repeatedly stop and shiver to warm up again.
It
is in this interesting article on the thermoregulation of owlet moths that
Heinrich provides the definitive answer to our question about witch hazel
pollination: ³Adult winter moths generally feed on the sap of injured trees,
although on late-fall nights a few years ago I saw many of them feast on the
blossoms of witch hazel, Vermontıs latest-blooming plant.² And as they fed, the moths
inadvertently fertilized the flowers.
He concludes, ³Until that time no one knew just how the plant was pollinated.²