Douglas Fir. Stephen F. Arno
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 to Douglas-fir’s roots and extract sugars manufactured by the tree to meet their entire energy (food) needs. But the tree also benefits. Fungi that colonize the tree’s roots form an expansive mycorrhizal web, or network, that greatly increases the tree’s ability to access nutrients and water. Early-twenty-firstcentury research has uncovered the sophistication of mycorrhizal networks and how they share characteristics with computer information networks. The network connects Douglas-fir trees of different sizes and ages and allows them to exchange food resources, transfer chemicals needed to fend off insects and disease, and share strategies for tolerating drought. Big trees serve as major connection points in these networks, illustrating their importance in forest structure.
Douglas-fir stumps created by logging sometimes provide visual evidence of belowground networking via root-grafting with surrounding live trees. This phenomenon occurs occasionally in the coastal variety of Douglas-fir and can also be found in moister regions of the inland West. Root-grafting is typically indicated when a layer of pitch-like callus tissue forms on the flat top of the stump. The callus tissue acts as a natural sealant that largely protects the dead stump from decay. Because the stump cannot photosynthesize the food (sugars and starches) it needs for callus formation, it has to obtain them from an external source—live, grafted trees. The trees also benefit, as the stump’s roots are still alive and physiologically active, extracting nutrients and water from the soil and transporting them to the living trees.
If the grafting relationship lasts more than a year, photosynthate (sugars) from the live trees transported back to the stump may be used to grow a new layer (or ring) of wood around the outside of the stump. The dead stump is somewhat analogous to the dead heartwood of a living tree: both serve as the core around which a new ring of live wood is added each year. A half-dozen such root-grafted Douglas-fir stumps have been documented in the Plumas National Forest in Northern California. The stumps vary in size, age, and time since thinning, but the stump that has so far supported the widest layer of post-thinning growth came from a tree that was 120 years old and 33 inches in diameter at the time of cutting. It had grown a 2.3-inch-wide layer (on average) of live wood around the stump in the eighty-seven years since cutting, illustrating the potential magnitude and longevity of this novel networking relationship. Older root-grafted stumps may even grow a rounded mushroom-like cap that is covered with bark, morphing into a bizarre forest inhabitant seemingly more at place in the domain of elves and goblins than in a Douglas-fir forest.
Another intriguing aspect of Douglas-fir relates to a basic life need: water. Besides living in a region that receives copious precipitation, coastal Douglas-fir