"Look! A bristlecone graveyard!" shouts U.S. Forest Service ecologist Connie Millar. Millar and her colleague, Bob Westfall, have just reached the western slope of a high ridge overlooked by California's 14,246-foot-high White Mountain Peak. Rounding a bend in the trail, I see them standing amid the remains of long-dead trees. In some cases, all that's left is a short stump or a toppled-over log. A few trunks stand oddly upright, with limbs that stretch skyward, as if in prayer. How old these relicts are is uncertain, but given the slow decay of bristlecone wood, some may have started out as seedlings more than four millennia ago, making them as old as the Great Pyramid of Giza.

But Millar is even more intrigued by the young trees that are starting to colonize this spectral grove. Some are 20-something-year-old bristlecones, easy to identify by their upright growth habit and dark green needles. Others are shrubby-looking limber pines that stand no more than thigh-high. The youngsters, Millar says, did not creep upslope; they've leapt up here, no doubt aided by pine-nut-loving birds. Their parent trees live more than a thousand feet below. "Striking, isn't it?" says Millar, who works out of the Forest Service research center in Albany, Calif. "Now we've got all these little trees coming in, and all these big old dead bristlecones, but absolutely nothing in all those years (between)."

And it's not just here, Millar says. West of us, in the Sierra, and eastward, across the Great Basin, young pines are also on the move. In a few spots, seedlings and saplings are charging uphill; elsewhere they're simply filling in the spaces between the sparsely distributed adults. Mature trees, too, are kicking up their heels. At the highest elevations, thick-waisted bristlecones are growing faster than before, laying down annual layers of wood that are noticeably wider than in centuries past. It's a growth spurt without parallel in the past 3,700 years, say scientists at the University of Arizona Laboratory of Tree-Ring Research — a sign, they think, that these ancient conifers are responding to the warming of our world.

In itself, that comes as no surprise. Biology, after all, has been dancing to climate's beat for hundreds of millions of years. But given the rate at which heat-trapping gases are streaming into the atmosphere, the pressure on organisms this time around promises to be extreme. In coming decades, ecologists say, we might well witness a sequence of botanical arabesques and grand jetes not seen since the end of the last Ice Age, when spruce and fir pirouetted across boreal lands, bristlecones high-stepped up Great Basin ranges and oaks jumped from isolated pockets to waltz across the hills of California.

It's in anticipation of the drama to come, in fact, that about a dozen scientists, including Millar, are spending the week in this high place, operating out of the University of California White Mountain Research Station. This summer, as in past years, much of the activity centers around meticulous plant surveys taken for the Global Observation Research Initiative in Alpine Environments — commonly known as GLORIA — a long-term ecological monitoring effort launched in 2001 that presently encompasses some 80 sites on five continents. The participating scientists have added other climate change surveillance projects to the mix, ranging from an annual census of butterflies and other insects to Millar's ongoing observations of the changing growth patterns of pines.

This wide-angle perspective is proving instructive. In the White Mountains — White Mountain Peak is the high point of a 60-mile-long range — young trees are doing more than moving up in elevation, Millar has found. They are also diving downslope, into newly opened microclimate niches in steep-sided valleys and ravines. Below us, young limber pines are advancing down the sides of the Crooked Creek Valley, which lies at an elevation of 10,200 feet. Farther down, they are pushing into the Owens River Gorge, venturing as low as 6,700 feet. "It seems counterintuitive," says Millar, "until you try to understand how mountain climate works."

Mountain climate is patchy, she explains. The American West is warming more rapidly than other parts of the U.S., but that's just on average. Within the 11-state region lie tens of thousands of cooler pockets, due, in large part, to the presence of mountains. Biologically, this translates into resilience. On the flat, for example, an organism might have to travel several hundred miles to find a suitable niche, whereas in the mountains, a hop, skip and jump will often do the trick. "Because mountains are so topographically rough, so heterogeneous, they provide incredible opportunities for movement," Millar observes.

Thanks to scientists like Millar, formerly simplistic ideas of how highland species will respond to climate change are giving way to a more nuanced view, one that is raising a raft of new questions. What species are relocating, and in what direction are they heading? How many will end up in climatological cul-de-sacs? How many will keep pace with the rate of change? The future is going to present species with opportunities as well as challenges, Millar observes: Witness the young pines that are currently colonizing so many different elevations.

Several years ago, Millar started seeding the subalpine zone with thermal sensors called iButtons in order to study these newly opened habitats. I watch as she climbs partway up a dead bristlecone to retrieve one. For over a year, this small but durable device has nested inside a protective sleeve of PVC pipe, recording the temperature at four-hour intervals. Millar deftly wrests the iButton free and hands it to Westfall, who downloads the data into a battered laptop computer. Then they repeat the exercise in reverse, putting the iButton back in the tree.

A short time later, we head back to Crooked Creek, site of the field camp where Millar and the other scientists are staying. I'm quickly reminded of the fact that Millar hikes well over 1,000 miles each summer in pursuit of her ecological research. As I take small, careful steps, wary of stumbling, Millar bebops her way downhill, a supple, sinewy 56-year-old crowned by a halo of ginger-colored curls.