The hope of trees, cont’d.  Guided evolutionary rescue of the American chestnut

We humans have been altering the genes of animals and plants for thousands of years.  We have created dogs from wolves, developed corn from teosinte, bred cows from aurochs, transformed Asian jungle fowl into chickens, and on and on.  These alterations have involved selective breeding; the careful choice of traits by generations of humankind with consistent goals. These efforts represent a major augmentation and direction of evolution.  A decades long project is now underway by the American Chestnut Foundation to bolster evolution enough to breed a blight-resistant American chestnut.  This work is now advancing with the use of genomic selection methods to target selections.  Also involved is an endeavor to find surviving American chestnut trees and preserve their DNA. 

Why this effort?  The American chestnut was a majestic and useful tree.  Its wood was light, strong, and rot resistant.  The tree grew fast, and it sprang better than most trees from a cut stump, making new telephone pole-sized trees in a few decades.  Its nuts supported legions of wildlife and people.  But the tree is now functionally extinct, effectively wiped out by the chestnut blight.  This disease was carried to America by imported chestnuts that were relatively immune.  Once here, the blight spread like wildfire among the susceptible American trees.  Hike in the Appalachian Mountains, and you may see little chestnut trees.  It’s still sporadically present there and in other parts of its original range, but almost always as a “stump sprout” that will be killed back to the roots by the blight before it gets more than 10 or 15 feet tall.  It may sprout again from the roots (actually the root crown) but in time it will fade away. The chestnut has to have full sun to bloom, and it needs another chestnut close by for pollination. If it can’t reach the canopy, and doesn’t have a blooming neighbor nearby, it can’t reproduce.  It has now disappeared from most of its former environs, which once included most of the Eastern U.S.

There are still a few large survivors here and there. Such trees can be found especially at the northern periphery of the original range.  These areas probably represent reservoirs of blight resistance.  The blight is weaker in colder regions, and isolated pockets of American chestnut trees can live long enough to produce nuts. Genes that convey some degree of blight resistance in these trees can survive. But even so, the threat of extinction looms.  Evolution can still operate, but it needs help.

I’ve been lucky enough to find around a dozen nut-producing trees in the high elevation forests of northern Pennsylvania.  About 150 progeny of these trees are growing at my farm in Potter County. Some of these in one of my orchards are pictured. These trees are participants in what could be a warp speed acceleration of evolution.  At least they may be experiencing a boost sufficient enough to give evolution a chance.

How so? Consider the extravagance and the merciless indifference of nature.  Mature American chestnuts were known to produce over 6,000 nuts per year, and to live 200 years or more.  200 years of 6,000 nuts a year is more than a million nuts.  Yet, in a forest with a stable population of these trees, on average just one of these nuts would grow into a replacement for each tree that died.  That’s a nut survival rate in the range of one in a million.  

The nuts I gather from surviving chestnuts have a better survival rate.  I aid their survival by holding them at the right temperature and moisture in a refrigerator over the winter, planting them carefully in the spring, minimizing competing weeds, fencing out voles, rabbits, and deer, and watering the seedlings in dry spells.  I estimate at least one in ten of these nuts will grow into a tree old enough to bear nuts of its own.  At that point, I’ll do what countless plant-breeding forebears have done; select the best trees for further reproduction.  Other nut collectors and breeders are taking similar steps.  In so doing, we are augmenting the power of American chestnut’s evolution by thousands, perhaps tens of thousands of times.  This may be enough to help get the tree, as one of my colleagues in the American Chestnut Foundation has put it, beyond the “cusp of extinction.”