On the desk here, a cone from a single-needle piñon pine. The pine’s species is a matter of some debate, either Pinus monophylla or something else, according to the author of the field guide I have in hand, which author showed me the tree from which she picked my cone when I visited Prescott. There were pine nuts in the cone when I opened the box. They were still edible, if a little stale. There are three or four nuts still in the cone.
I camped at Walker Pass about ten years ago, and families drove up from Ridgecrest and Bakersfield with rakes and garbage bags, walked off into the pinyon and juniper. It took me a bit to figure out what they were doing. “¿Se buscan piñones?” I asked one grandfather, and he smiled and held up a gallon ziplock bag full of the pine nuts he’d gathered. It was a mast year. Nut pines put a lot of resources into setting seeds, enough to sap (so to speak) the vitality of the tree. A tree needs a season or two to recover, typically, between heavy nut years. Whitebark pines, those stoic denizens of the sub-alpine forests, put more effort into their seeds than about any other pine: each pound of whitebark pine nuts holds about 2700 calories. In Yellowstone, and probably in other subalpine settings as well, whitebark pine seeds provide most of the body fat that gets grizzlies through their cold winter sleep. Not that a bear would turn down carrion, mind you, but pound for pound the pine nuts are the most economical way to gorge.
There is some thought that griz maulings may coincide, roughly, with lean pine nut years. Only when the crop fails do the bears turn to their less-preferred meal, the American Tourist.
Bears don’t necessarily take the cones off the trees themselves. They’re more likely to raid middens collected by red squirrels, a.k.a. chickarees, who put a lot of effort into stacking pine cones conveniently at ground level. Chickarees harvest cones from the whitebarks, though at times they themselves may raid caches of seeds collected by another animal, the Clark’s nutcracker (no relation), Nucifraga columbiana.
Without the nutcracker, the whitebark would eventually die out. Without the whitebark, the nutcracker would have trouble surviving. Clark’s nutcrackers can eat other pine seeds, but they don’t do well on them: either the nuts are too small and collecting a calorie more inefficient, or the other pines’ cones open up early to let their seeds flutter away on the wind. Whitebark cones stay closed until a nutcracker — or a chickaree, or a grizzly — pries them open to get at the seeds.
Many other species of pine have seeds that grow little membranous wings. The Pinus sabiniana I napped under this weekend, alive with Steller’s jays, opens its huge baroquely recurved cones in season and its seeds flutter out like maple samaras. The whitebark pine’s seeds have wings as well. They simply grow attached to a Clark’s nutcracker. The cone holds its crop tightly. A Nucifraga must shoehorn the seeds out of the cones with its sharp beak. The bird then tests the seed by clacking it in its beak, discards undeveloped or rotten nuts, and flies the good ones off to cache them in the ground.
Clark’s nutcrackers are well known among behaviorists for their astonishing memories. They birds can recall the location of hundreds of individual caches for more than a year. Researchers have demonstrated that the birds use visual cues to relocate their food stores. A nutcracker might cache a few seeds a meter south of a distinctively shaped rock, and it will look for them a year later a meter south of that rock, despite the meddling scientists having moved the rock a meter north. Prodigious memories allow the Clark’s nutcrackers to eat archived seed through years of sparse seed set. And they archive a lot of seed: up to 100,000 seeds per bird per year.
But some seed caches are, inevitably, forgotten. Alone among eaters of whitebark seeds, the Clark’s nutcracker buries pine nuts at a depth conducive to germination and long-term survival — an inch or so deep. In fact, the birds may use pine seedlings as a cue to locate forgotten, unsprouted seeds in that same cache. As each cache generally contains a number of seeds, and as whitebark pines do not have their seeds dispersed successfully other than by way of nutcrackers, the trees tend to grow in clumps. This itself aids in survival of the trees at altitude: the clump helps blunt the blasting, drying wind. Hike up to timberline in the west, and where you see a small grove of whitebark pines, there an ancestral nutcracker cached and eventually forgot some seeds, a century or five ago. The birds seem to seek out marginal locations for their caches, away from most competitors for the seed and — coincidentally — away from most things that would compete with the new trees for sunlight and water. As the trees mature and modify the local climate, other trees, subalpine firs and such, often germinate in their lee.
Which brings us to the image that’s been haunting me the last few weeks.
There are pines with a long, distinguished ancestry in North America. There are corvids, mainly the jays, whose roots in the Western Hemisphere run deep. Neither the whitebark pine nor the Clark’s nutcracker can make this claim. Both species have numerous close relatives in Eurasia. Both have relatively uniform genomes: the kind of thing one would expect to see of populations that have expanded dramatically from a small founding group. Neither can reproduce without the other. The birds plant trees in marginal locations, often some miles from the parent tree. After some years, fifty or a hundred, the new trees produce seeds.
The Bering land bridge opens as ice sheets usurp the water supply, lowering sea levels. The ancestors of Clark’s nutcrackers work their way into the area from Siberia, planting whitebark groves on land now buried under 50 meters of seawater, and move yet farther into the Yukon basin. Thousands of years pass. The great ice sheets begin to recede. As the strait floods behind them, the birds are suddenly presented with a corridor between two major receding ice sheets. The Laurentian ice sheet, over the Eastern two-thirds of the continent, and the Cordilleran sheet over the western mountains have parted, and a band of alternating dry land and muskeg and tundra stretches from the Mackenzie Delta to Montana. It is about 12,000 years before the present day, perhaps 13,000.
A slow tsunami of bird and forest rolls south from the Arctic Ocean to the Rockies. That is the image that has flitted through my consciousness this month. Birds plant trees, which grow and make seed for the birds to plant farther on, and other species follow in their wake. Subalpine firs sprout, and chickarees, and in the growing leaf litter sprout grassses, and hay-harvesting pikas.
And now, of course — of course! — that partnership is threatened. An imported rust fungus has weakened white pines, including the whitebark, throughout North America. Increasing alpine temperatures spur fatal outbreaks of bark beetles in whitebark groves. The silvicologists were among the first to warn of climate change two decades ago, bark beetles uppermost in their minds. The world is coming undone, and yet the nutcrackers still scold people away from their precious remembered treasures.