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 forests contribute to their own irrigation. The source of this self-generated watering is the fog that forms over the ocean and then drifts inland, enveloping the forest. Here, the huge canopies of Douglas-fir trees harvest moisture through condensation of water droplets on their needles. Given that a single large coastal Douglas-fir supports millions of needles, what may seem like an insignificant “drip-drip” in a hike through the woods turns out to be a significant contributor to the forest’s annual water budget. In the Bull Run Watershed that serves as the water source for Portland, Oregon, fog drip contributes an average 35 inches of moisture, or about one-third of the total annual precipitation.
Scientists continue to delve into the genetic, physiological, and architectural anomalies that make Douglas-fir unique. Increasingly sophisticated measuring instruments and analysis techniques should help shed light on myriad remaining questions and reveal more secrets in the future. The increased knowledge and insights that come from this work, along with the special partnerships Douglas-fir forms with countless other forest organisms, further underscore its status as a world-renowned tree.
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