Pines Need Help With Evolution

Human-assisted evolution may be the only way to save some subalpine ecosystems from the ravages of an invasive fungus.

As white pine blister rust (Cronartium ribicola) spreads across high elevations of western North America, it can take out almost entire forests. The ecological consequences could be disastrous, unless someone intervenes.

The deadly introduced rust continues to invade new areas of mountain forests in the United States, where five kinds of five-needled pine are vulnerable: whitebark, foxtail, limber and two bristlecone species. In 2003 blister rust was discovered in Rocky Mountain bristlecone pine for the first time, leaving only native forests of Great Basin bristlecone pine untouched.

At lower elevations over the last hundred years, the rust has killed off 90 percent of species such as western white pine. Other conifers have replaced these trees in forests.

On high ridges and mountainsides, however, only pines like bristlecone and whitebark can survive the harsh climate. There'll be no substitutes once they're gone.

An absence of pine will drastically impoverish mountain ecosystems. In some areas, whitebark and limber pines are keystone species, feeding birds, squirrels and bears.

A small proportion of pines have an innate genetic makeup to resist the rust. Over several generations, natural selection will cause these resistant strains to dominate the gene pool, enabling the pines to eventually prevail.

Natural selection will be a long process, though, for the high-elevation species because they are slow-growing and long-lived. These pines don't produce seed until they're 30 to 50 years old. Limber pines commonly live over 1,000 years and the oldest bristlecone pine is 4,500 years.

To prevent collapse of high-elevation pine ecosystems as the rust invades, researchers from the US Forest Service suggest we hasten natural selection of rust-resistant trees, thereby speeding up the process of evolution. A helping hand could maintain tree cover as mountain forests adapt to the new disease.

Intervention in evolution begins by finding the trees that tolerate blister rust. Screenings for rust resistance are already underway for some pines. Then seedlings from rust-hardy parents would be planted amid the ancient subalpine stands. Ideally, inter-planting of trees begins before white pine blister rust moves into a forest.

Blister rust most rapidly kills young trees. So when a new area becomes infested, resistant trees get selected for first among saplings. Those that survive will grow to replace the mature trees that are gradually dying.