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On a steep, rocky alpine slope outside the northeast corner of Yellowstone National Park, scientists examine what they call the "dead trees standing," a copse of ancient whitebark pine trees [Pinus albicaulis] that have thrived for thousands of years on these snowy Montana peaks. But lately the once-hardy trees, whose cone seeds account for a crucial part of the local grizzly bear's autumn diet, have been suffering. Their evergreen needles have turned dark red, and their distinctive silver-grey trunks are flecked with wounds oozing dried sap — evidence of attacks by predatory insects and fungi.

The whitebark pine has long survived in the Northern Rockies at elevations too high and too cold for the mountain pine beetle [Dendroctonus ponderosae] or blister-rust fungi. But scientists say these beetles and exotic fungi are now surging into whitebark pine stands because temperatures at these once-frigid heights have risen with global warming, making the trees tragically vulnerable. Unlike pines at lower elevations, the whitebark did not co-evolve with the beetle, and so has never developed adequate protection against it. The lodgepole pine, for example, which grows up to 4,000 feet below the whitebark, has a remarkable defense, emitting bursts of sticky pitch to thwart the beetle's movements. But when the whitebark pine tries to defend itself, it usually expends so much energy that the tree dies.

The scientists wander the mountainside to diagnose the stricken trees. In this stand, at about 10,000 feet above sea level, there are signs of grizzly activity: a pile of scat, a clawed excavation of rocks and dirt in search of ants. The bear is still around, but for how much longer?

Bears that normally gather whitebark pine nuts on high, isolated mountainsides are less likely to wander to lower, settled areas where they run the risk of mortal conflict with people. If they don't get the cone seeds here, they don't get them at all. Scientists and bear advocates, such as the Natural Resources Defense Council (NRDC), say that the federal wildlife agency that de-listed the Yellowstone grizzly from threatened status in 2007 failed to take into account documented threats to its food supply — and thus to its long-term survival. A bad seed-crop year equals fewer bear cubs.

"A disrupted climate is the single greatest threat to ever face our Western national parks," says NRDC science director Daniel A. Lashof. He cited the loss of ice bodies in Montana's Glacier National Park from 38 in 1968 to 26 today, as well as Yellowstone's warmer summers and winters causing ever-earlier shedding of the diminishing mountain snowpack. Warmer temperatures also create ripe conditions for destructive invasive plants to displace natives. Warmer river waters kill fish, shrinking yet another grizzly food supply. The grizzly bear, NRDC argues in its legal effort against de-listing, is an "umbrella species," whose well-being predicts the health of the entire ecosystem that surrounds it.

Dr. Jesse Logan chips at a whitebark pine trunk with a hand hatchet, revealing beetle damage beneath. He says that when the U.S. Fish & Wildlife Service drew up its grizzly de-listing plan, it failed to consult him, even though he was then the head of the U.S. Forest Service mountain pine beetle team; nor did Fish & Wildlife consider his research findings, which revealed an alarming growth in whitebark infestations. In the Northern Rockies, he says, the warming trend has shortened the mountain pine beetle's maturation life cycle from two seasons to one, a "catastrophic threshold."

According to Fish & Wildlife, only 16% of whitebark pine in the Greater Yellowstone area has experienced some level of beetle-caused mortality, a statistic Logan, now retired, disputes — he points to the huge beetle outbreak on Yellowstone's Avalanche Peak, where the forested slopes are rapidly growing red with infestation. It is mainly the female grizzlies that rely on whitebark seeds; the rich, fatty proteins they contain help the bears gain weight in the fall, going into winter, when they hibernate and birth their cubs. Logan charges that the federal grizzly de-listing document "was based on misleading science" and should have been rejected.

Despite such discouraging evidence, there are still traces of hope. Dr. Diana Six of the University of Montana peels back some bark with her hatchet, showing how one whitebark pine has survived a beetle attack, albeit a light one. The resin-soaked lesions on the tree are evidence of a pitch response — the tree's way of exuding natural pesticide. To further help protect the trees, Six says, small bags of synthetic pheromones can be nailed to them; the bags, which last one season, ward off beetles by tricking the insects into thinking the tree is already occupied.

At her research plot lower on the mountain, an area burned over by the massive 1988 Yellowstone fires, Dr. Diana Tomback of the University of Colorado shows whitebark pine seedlings in various stages of growth, evidence of forest recovery after the fires. She says the careful study of how whitebark pine grow in these unique microsites can help identify the tree's optimum survival conditions. Researchers are now painstakingly developing whitebark strains that are genetically resistant to blister rust, and when the time comes, it's crucial that they're planted in the right place. "We can't waste them," says Tomback. "We need to plant them in sites that give the highest chance of survival."

• Pat Dawson
• Because of global warming, whitebark pine trees are dying. And that is threatening the food supply of grizzly bears