Monday, March 4, 2013

Pollinators of the American Chestnut

American Chestnut Pollen Stalks: Photo by Timothy Van Vliet
 "And the blossoms gave up one of the best honey crops we ever had." said Noel Moore of Rabun County, Georgia, in 1980 when asked about his memories of the American chestnut. He added mournfully: "We've never had a honey crop like we did since the chestnuts died, because there's not that much nectar in the wild now. Whenever chestnuts bloomed, in the morning, early, the trees looked like just the whole tops were alive with honeybees working on getting the nectar."

The American chestnut, Castanea dentata, became virtually extinct as a species, almost entirely gone from the Eastern United States by the 1950s. Before that time it had represented 25% to 30% of the forest canopy from Maine to Georgia. An infestation arrived from Asia, probably introduced with Asian chestnut trees brought by horticulturists. A fungus, first noticed in 1904, but probably already present in several areas of this country, had become a killer of native chestnut trees.

The Asian chestnut species has coexisted with the fungus for millions of years and has developed ways to withstand its attacks; but the American chestnut is helpless. To make things worse, the fungus spores travel easily through air and show great resistance to weather changes. So, what came to be known as the American chestnut blight spread through the entire range of this species killing nearly all the trees in less than half a century.

Many species that depended on the nuts for food, from chipmunks to bears, experienced significant losses. In fact, populations of squirrels and wild turkeys crashed. Squirrels bounced back on their own, but turkeys needed human assistance. The effect traveled up the food chain. Goshawks and other predators felt the impact. Gone with the chestnut trees, although less noticed by most, were some insects that fed on this tree; probably seven species of moths became extinct.

Humans also suffered the loss grievously. Not only the nuts but the timber were valued and now they were gone. Dedicated scientists and agronomists of the Department of Agriculture fought for years to slow down or stop the march of the parasitic fungus, resorting to a variety of methods to heal the trees or to introduce pests that weakened the fungus. All were futile.

What happened to the pollinators of this species? Pollination research was almost non-existent when the trees were abundant and now that they are almost gone nobody is gathering such information. That is why I was glad to read the observations of one old enough to remember what things were like when the tree was still alive. His comments show that non-native honey bees visited the flowers in huge numbers. But, what about the native pollinators, the ones that had lived for thousands of years with the tree and its flowers?

For lack of American chestnut trees, we can use proxies, the closest relatives of the deceased, to gather some indirect information on their pollination . The flowers of species of chestnuts are strongly scented and attract numerous insect visitors, not just honey bees, but also many species of solitary bees, butterflies, flower flies, and beetles. Specialist honey producers sell a monofloral honey made from blossoms of the European chestnut, Castanea sativa. It is strong tasting and darker than other honeys. Chinese chestnut trees, Castanea mollissima, grow in some gardens and parks where I regularly go for walks. The scent of their flowers is almost overpowering in the months of May and June.  

We can assume that American chestnuts were also visited by many insects and that they were pollinated by them. They too must have suffered from the near disappearance of the species. Some of them may have been able to adapt and resort to pollen and nectar from other plants although no other trees bloom as heavily as chestnuts in late spring and early summer. The American chestnut disappearance must have been a severe blow to all of them. Perhaps some of them, too, went nearly or totally extinct. Probably we'll never know.

The struggle to bring the chestnut back from its near extinction continues. Horticulturists are resorting to a variety of approaches, from controlling the virulence of the fungus to breeding new varieties of the tree in an attempt to restore the American chestnut to its native geographic range. Some strategies look more promising than others. All require enormous effort. Let us hope that they succeed.