From out of the north, it came. Inanimate, inexorable and indifferent: Nothing could withstand its cancerous growth. With the patience of inevitability, it slowly consumed North America. It chewed up the terrain, pulverized granite and left nothing alive in its wake. When it had grown to maximum size, some seven million square miles, it broke the back of the continent. Nothing can withstand…The Ice Age!

No, this is not a preview for another post-apocalyptic Hollywood survival movie. This catastrophe is a major part of Earth’s recent history and is best known for cavemen, mammoths and, well, ice. Lots of ice. It is less well-known as the engine of change in North American forests for the last two and a half million years: Forests were consumed, driven south, rearranged and reordered. As unfortunate as this sounds, without the ice age, the forests of today would not be as they are.

If you draw a line from Washington, D.C., to Washington state on one of those multi-colored vegetation or ecoregion maps, you will see that it crosses a dozen or so different kinds of forest from the broadleaf deciduous forests of the East to the temperate rainforests of the Pacific Northwest. Unfortunately, reality does not exactly match the map. More than 99 percent of old-growth, eastern forests have been cleared for agricultural and urban environments or have been replaced by second-growth forests devoid of the once-dominant American chestnuts. The oak savannas of the Midwest have been reduced to tiny remnants crowded by corn fields. Diverse, ancient forests of the Pacific Northwest have been clear-cut and replaced with a monoculture of skinny, young trees in a checkerboard pattern right up to national park boundaries.

Each of these views is but a snapshot in time, and none is sufficient for a proper understanding of our forests. Our present time is marked by rapid and profound change. But change from what? How can we know the degree to which our activities are changing the long-term health, composition and extent of North American forests if we don’t understand what they once were and how they became what they are today? We can’t answer these questions with snapshots. What we need is, well, something more like a Hollywood movie. In this first of a two-part series, we’ll see that the natural baseline is a moving target.