Story by Virginia Barlow
Illustrations by Adelaide Tyrol
Hasn’t almost everybody who has looked at a hawthorn had the same reaction that
I have, which is to wonder what on earth any plant around here is doing with such
ridiculous-looking, huge thorns? They clearly don’t work to keep the tree’s twigs and
small glossy leaves safe from our commonest browser, the white-tailed deer. Mangled
hawthorns often pepper the same neglected pastures where browsing deer have
pruned and distorted the apple trees that try to grow there. And it’s not only deer at
fault: mules, sheep, goats, cows, and horses browse hawthorns along with the apples.
Thorns, spines, and prickles all come at a cost, and the question of why a plant
would invest so much in armament that does nothing to deter hungry animals
piqued the interest of ecologist Dan Janzen about 40 years ago. He had what he called
a “screwy idea” that widely spaced thorns, often borne to great height on a tree,
evolved in response to feeding, not by whitetails, but by the much larger mammals
of the Pleistocene – 1. 8 million to 11,700 years ago. A hawthorn’s thorns would still
work today against the huge muzzles of mastodons and ground sloths – elephant-sized mammals that could reach to the top of most hawthorns – that is, if those
and many other giant animals hadn’t become extinct soon after humans colonized
North America. Janzen’s “screwy idea” has since caught on with many ecologists. As
Connie Barlow writes in Ghosts of Evolution, “Hawthorn looks more like an African
plant sculpted by an arms race with rhinos and kudus than a companion of America’s
white-tailed deer.” When I took a tape measure out one day, I found that often there
are more than 10 inches between the sharp tips. Maybe giant sloth extinction wasn’t
such a terrible tragedy for us.
Hawthorns have flourished in this region since the Wisconsin glaciation, at first
because the glacier-scoured landscape offered the fully sunlit conditions that hawthorns love. It is presumed that they diversified during this time. Then, some 11,000
years later, when a new batch of humans arrived on the continent to repeat the
deforestation – this time with axes and ploughs – hawthorns again took advantage
of the situation. Forests are too shady for hawthorns, but hedgerows are perfect for
them, and those that succeed in getting above the browse line are, in turn, perfect for
many of today’s wild animals. It’s not typically the fruits, but rather the dense, tangled,
thorny thickets formed by hawthorns that are sought by birds, especially for nesting.
Mammals also appreciate their protective cover.
Though the thorns make identifying a hawthorn a breeze, they are not of much
use in distinguishing the 100 to 1,000 hawthorn species from one another. That range
in species number gives you an idea of how confusing this genus is, even to botanists.
More than 50 hawthorn species are native to New England, and countless more cultivars are available from nurseries. Most are native to North America, but some come
from Europe, Asia Minor, China, and Japan.
Most hawthorn species have a few things in common besides their hallmark thorns.
The winter buds are dark red and round. The usually small leaves have sharp teeth
(Jefferson’s ground sloth)