The Tree That Made Yellowstone

The giant volcano that erupted 640,000 years ago left most of what’s now Yellowstone National Park blanketed in rhyolite, a volcanic rock that makes for very poor soil. It’s acidic, lacks the nutrients most trees require, and it holds water poorly.

Yellowstone, moreover, is high-altitude — most of my recent trip there was spent above 7,500 feet — and so it is cold much of the year: Some parts of the park can get snow year-round, and in late August we found most every night it dropped down to near freezing.

Oh, yes — and the park is regularly swept by fire.

So, as the park ranger at the overlook for the Lower Falls of the Yellowstone River told us, evolution served up the lodgepole pine. It’s an evergreen, so it can photosynthesize even through the long winters and cold springs and autumns. And it does well in quick-draining, acidic soil with minimal nutrients.

Most intriguingly of all, though, is how this tree — whose long, straight trunk grows with a distinctive, near-barber’s-pole twist to the grain — responds to fire.

In short, lodgepoles thrive on it. Some of its female cones are serotinous, meaning they are sealed in a kind of wax that’s dissolved only at very high temperature — the kind of temperature you get only with wildfires.

Because such fires release the seeds even as they take down the mature trees, lodgepole forests begin regenerating immediately after big fires like the 50 or so that cumulatively burnt a third of the park in 1988. On my visit this summer, the charred husks of mature trees stood like mausoleum spires knee-deep in bright-needled pines anywhere from 4 to 15 feet tall.

In fact, because lodgepoles need so much direct sun, they actually can’t propogate terribly well without fire. However, in some parts of the park they have made way for other species that can thrive in the soil the lodgepoles themselves have enriched.

Lodgepoles dominate more than 80 percent of the park. Without them, said the ranger, the place might never have been revegetated at all.

Photos by Renee Rosensteel

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