is a tree named Hyperion, measuring 381 feet. Inland Douglas-fir grows more slowly than its coastal kin due to frigid winters and drought-plagued summers exacerbated by daytime relative humidity of 15 percent or less. Still, in narrow valleys and canyons they can attain heights of 150 to 200 feet.
Centuries-old trees are another noteworthy feature of coastal Douglas-fir forests. Trees greater than five hundred years of age are fairly common, and occasionally trees a thousand years and older have been reported for both Douglas-fir and a number of its associates. The oldest coastal Douglas-fir was believed to be a more than 1000-year-old monarch in Lynn Valley in British Columbia. That trees and forests of this species and its associates have survived for centuries may give the impression that they have been a centerpiece of the Pacific Northwest landscape extending back into antiquity. Massive trees, coupled with giant moss-covered logs slowly decaying into the forest floor, seemingly provide visual evidence of sustainable “ancientness.” But tree pollen found in sediment layers underlying small ponds across the coastal Northwest paints a different picture. Today’s Douglas-fir-dominated old-growth forests first appeared in similar form only about six thousand years ago—a mere blip in the calendar of geologic time. Prior to that time, forest vegetation in the Northwest varied greatly depending on rapidly changing temperatures that occurred 10,000 to 6000 years ago.
Douglas-fir’s longevity plays a key role in current efforts to determine climatic patterns (particularly precipitation) over the past centuries and even millennia, providing a baseline for assessing modern-day climate trends. Here it is not the coastal giants but dwarfed, bonsai-shaped inland Douglas-fir trees that have special value.
Growing in the cracks of lava flows at El Malpais National Monument in New Mexico, tiny water-stressed trees survive to amazing ages and their year-to-year growth is highly sensitive to precipitation. Tree rings counted on increment cores show that the oldest known living Douglas-fir at El Malpais dates back 1280 years; another old tree on the flow began life about 960 years ago. These and other centuries-old trees serve as nature’s version of solar-powered data recorders, archiving tree-ring widths through good times and bad. Scientists have used the tree-ring patterns (chronologies) to quantify relationships between ring widths and precipitation over the period for which weather records are available.
After establishing such relationships, scientists use tree-ring patterns to estimate annual precipitation in the more distant past. Ancient (subfossil) logs lying on the barren lava flows provide a record of year-to-year tree growth that extends back even further than the living trees. Tree-ring patterns from relict logs that overlap those of old, living trees have allowed scientists to reconstruct a continuous tree-ring chronology—and thereby estimate precipitation—dating back more than 2100 years. Their work shows a period from 1566 to 1608 to be the driest during the last two millennia. Insights provided by the El Malpais tree-ring data suggest that twentieth-century precipitation trends are not outside long-term norms for that area.
Farther south, the Douglas-firs of central Mexico also produce a sensitive, reliable, and long chronology of climatic history. A stand located about 10 miles northwest of 18,500-foot Pico de Orizaba (Citlaltépetl), southeast of Mexico City, is the largest and the least disturbed by woodcutters and by grazing sheep and goats. Some of the trees are five hundred years old, 100 feet tall, and more than 3 feet in diameter. Their growth-ring patterns have been found to correlate well with regional records of spring rainfall and annual yields of maize, the major food crop. A 528-year climatic chronology developed from Douglas-fir tree rings has been used to estimate annual food production from the pre-Hispanic era to modern times.
This tree-ring chronology was also employed in a study of historic typhus epidemics in central Mexico. Typhus, a deadly disease caused by a bacterium transmitted by body lice, occurs where living conditions are crowded and unsanitary. Mexico has written records of typhus epidemics going back to 1655, and the disease historically accompanies famine and war. Documents describe how drought induced large numbers of subsistence farmers to flee to towns for food relief. There, they lived in densely crowded, squalid camps. Historical records were compared with tree-ring reconstructions of growing-season moisture conditions. Below-average tree growth, indicating drought and poor crop yields, occurred during nineteen of the twenty-two typhus epidemics, the most recent of which began in 1915 during the Mexican Revolution.
Detailed analysis of tree-ring chronologies from Douglas-fir and other trees, notably bristlecone pine (Pinus aristata and P. longaeva), provide a remarkably clear record of climatic variation in many regions of North America. These records are a window to the distant past and are routinely used to accurately date prehistorical events, such as the volcanic explosion that destroyed Mount Mazama and created Crater Lake. They also correlate with ancient records such as the warm climate in Greenland a millennium ago, and centuries later the “Little Ice Age” that devastated European agriculture and resulted in massive famine. Tree-ring records are a living chronology that allows analysis of past and current climatic trends in many areas of the world.
One of coastal Douglas-fir’s secrets to long life and exceptional height is that it doesn’t go it alone. Behind the scenes it gets help from numerous organisms, most of them obscure, and a few even from the animal kingdom. An example that adds further intrigue to Douglas-fir’s story is the role salmon play in linking this tree’s cycle of life and nutritional needs to the sea.
A nitrogen isotope known as 15N, which uniquely identifies it as coming from the sea, provides a stealth source of this nutrient for the coastal Douglas-fir forest and involves a complex biotic web that resembles a relay team. Salmon from the Pacific Ocean travel upstream along the west coast of North America to spawn in the many rivers and streams that drain the coastal uplands. Bears, coyotes, eagles, osprey, ravens, and other raptors and scavengers are drawn to the easy pickings offered by spawning salmon or carcasses of the spawned-out fish. Gorged on salmon and often dragging or carrying dead fish, these animals move widely throughout the forest, leaving fish scraps and their nitrogen-rich urine and scat to fertilize the soil. Trees, shrubs, and understory plants thrive on the enriched soil and further distribute the nitrogen in the form of fallen needles, leaves, and twigs. This novel animal-plant nitrogen web developed over millennia, with most of the animal contribution moving upstream from the ocean to provide the coastal Douglas-fir forest with a component of its annual nutritional needs in the form of marine-sourced nitrogen. Douglas-fir also obtains some nitrogen from terrestrial sources, made available by soil microorganisms that break down woody material. Another source of this nutrient is rainfall, which sometimes contains nitrogen converted from the atmosphere by lightning. Fires and other disturbances also periodically stimulate nitrogen-fixing plants such as red alder (Alnus rubra), Sitka alder (A. sitchensis), buckbrush (Ceanothus), and lupine (Lupinus).
Douglas-fir acquires nitrogen from another kind of biotic partnership, this one with a lichen high up in the tree canopy. Lettuce lichen (Lobaria oregana) is a light green fungus resembling garden lettuce that grows in the crowns of large older trees. Lichens are bizarre organisms typically comprised of two fungi and an alga (photosynthetic cyanobacterium) that allow them to process nitrogen gas from the air, which is otherwise unavailable to plants, and convert it into ammonia nitrogen, a form usable by trees. But the trees cannot acquire this nitrogen directly from the lichens. Instead lichens growing in the canopy must be blown or knocked to the ground by wind or snow, where precipitation gradually leaches nitrogen into the soil as the fungi decompose. This pathway sometimes takes a detour when deer or elk eat the fallen lichens, and the associated nitrogen moves into the soil wherever the animals urinate or defecate. The collective contributions are substantial, as lichens can supply an old-growth Douglas-fir forest with a significant portion of its annual nitrogen needs.
Douglas-fir not only survives but flourishes from associations with other organisms in the forest. Some of these biotic partners are belowground, such as two species of false truffle (Rhizopogon)— fungi that form mutually beneficial relationships exclusively with Douglas-fir. (Approximately two thousand species of fungi have been identified as potential partners with Douglas-fir.) The fungi attach themselves
