Raven Glacier (Photo credit: Whit Bronaugh)
It’s All About Glaciers
Around three million years ago, South America ended its long isolation and joined North America at Panama. This blocked ocean currents between the Atlantic and Pacific and, with the Arctic Ocean nearly walled off by North America and Eurasia, helped usher in the Pleistocene ice age which has been going on for the last two and a half million years. At the moment, we are enjoying a warm period between glaciers — just the latest of many periodic lulls. The Pleistocene ice age is really a succession of 17 or more glacial periods separated by warmer times, called interglacial periods, when the ice retreats. These episodes of glaciation are thought to be caused by cyclical changes in the Earth’s orbit, tilt and orientation, collectively known as Milankovich cycles.
At the peak of glacial periods, ice can cover 75 percent of North America, nearly two miles thick in places and weighing so much that it can depress the continent by as much as 1,000 feet. On the front lines, trees withdraw behind the advancing strip of tundra or are buried and pulverized beneath surging lobes of the glacier. Of course, the individual trees don’t move. They die on the front line, but the species’ geographic range shifts southward. Because North America extends to the lower latitudes, there has always been room for most tree species to retreat. Throughout the Pleistocene, the forests advanced and retreated with the glaciers.
About 18,000 years ago, when the ice was last at its peak, it extended to the Ohio River in the east and just south of Canada in the west. Most of the ice front was bordered by a strip of tundra a few to 100 miles wide. South of the tundra, a broad band of boreal forest, dominated by white spruce and jack pine, stretched from Pennsylvania to Tennessee and westward to cover nearly all the Great Plains. The Gulf Coastal Plain, which was up to 200 miles wider because of lowered sea levels, was mostly oak-hickory and southern pine forest. Florida, then twice its current width, was mostly sand dunes and shrub lands.
Our knowledge of when and where different trees once grew comes mostly from taking core samples of bogs, lakes and other wetlands, and identifying fossil pollen grains at different levels. Such sites only cover a tiny percentage of the continent, and it took some trial and error to find the right ones. Radiocarbon dating provides the timeline, and fortunately, pollen can usually be identified to specific species, so we get a good picture of the forests that once covered our continent.
The Rocky Mountain forests of pine, fir and spruce were pushed 2,000 feet downslope and squeezed against the edge of the dry valleys. The lower-elevation ponderosa pines were practically eliminated from the Rockies altogether, and sought refuge in the mountains of the Southwest and Mexico. The Coast Range in the Pacific Northwest was subject to colder, dryer weather that banished the range’s now iconic Douglas-fir and red alder to the Puget lowlands. The southwestern deserts escaped to Mexico and were replaced by juniper, pinyon pine and oak forests.