Decomposition of trees represents a vital link in ensuring life for streams, rivers and oceans. In fact, the remarkable relationship between the land, its fresh waterways and tidal estuaries along the West Coast of North America depends upon a constant source of big, dead trees.
Known as driftwood, the trunk, roots and large branches of dead trees form dams and pools, creating backwater areas along streams and rivers. Driftwood provides nutrients and becomes the foundation for biological activities that spread energy throughout the water, trapping newly created sediments.
These big trees may topple along the stream bank due to undercutting. They might enter the waterway from areas of wind throw, along ridge tops, or be carried by landslides and massive stream erosion during major floods. Other trees are felled by beavers, adding thousands of tons of wood to coastal streams annually.
Once driftwood enters fresh water, fungus, bacteria and spineless insects called invertebrates go to work. They shred, gouge, scrape, graze, collect and even devour the decomposing wood. In the process, their fecal content feeds the sediment beneath the water's surface. Larger insects like caddis and stoneflies eat wood infested with nitrogen-rich fungus. In turn, the streamside trout devour these flies.
Driftwood controls water flow and temperature, and maintains trout and salmon egg-laying habitats and habitat for survival of young fish. Pools are created by slowing currents that cause deep depressions to be formed along the banks. This provides secure habitat where fish can hide from predators. Deep pools allow different species as well as different ages of the same species to coexist. These pools are crucial over-wintering habitat for stream-residing fish. Driftwood creates critical diversity.
Every 20 years or so, a massive flood event relocates large amounts of driftwood downstream onto flood plains. The driftwood then becomes essential habitat for small mammals including deer mice, voles and chipmunks, and soon draws predators like weasels and minks. Mergansers, harlequin ducks, spotted sandpipers and night hawks use the driftwood for nesting, while song sparrows, violet green swallows, robins and cedar waxwings use it for perches. Bald eagles and blue herons use it as feeding grounds.
Eventually driftwood moves once again to the mouth of the river where the ocean's arm will meet it. Vast amounts of stored carbon — collectively called wood — is broken down and serves as the basis of the saline aquatic food pyramid.
Tiny gribbles, huge shipworms
There are two main ocean wood borers that undertake this initial decomposition process. Gribbles are tiny and begin to overtake the driftwood by sheer numbers. However, as the first wave of the invasion they are able to penetrate only a few millimeters into the driftwood. The second and final wave is undertaken by the gruesome shipworms, some of which can attain lengths in excess of 5 feet. Captain James Cook had good reason to demand that his crafts always had double hulls, for shipworms could easily plow through a single hull, like a hot knife going through butter.
Because shipworms bore to grow, much of the wood is not digested but rather flushed directly out of their tunnel as fecal pellets. These pellets serve as an important food source for flatworms, roundworms and predatory snails.
Larger mammals rely upon driftwood, too. Harbor seals use it as rafts to climb out of the water and rest, while spotted skunks prefer to excavate their dens under large, buried and beached driftwood.
Much of the driftwood in the North Pacific remains inshore due to oscillating currents. However, some large western red cedar, Sitka spruce and Douglas fir enter the open ocean and the great North Pacific gyre, a vast circular vortex, which carries driftwood as far as the Hawaiian islands, and beyond.
Driftwood at sea is heavily populated with plants and animals including the only known insect to successfully invade the open ocean: the ocean strider. It lacks wings so it alternately floats and skates on the surface of the sea and attaches its eggs to floating driftwood.
Popular item at sea
An individual piece of driftwood can have in excess of 100 species of invertebrates, and some 130 species of fish are known to congregate on and around it. Having collected plankton and attached first small fish and then larger predatory fish, such as dorado and tunas, the combined weight of its associated tunas alone may reach as much as 94 tons. The tuna and other predatory fish deplete their available prey quickly and move away, but use the driftwood as a point of reference and return to it.
Tuna time their migration to the North Pacific continental shelf for spawning to coincide with the onset of monsoon rains from December to March. The resulting floods carry new driftwood to the ocean arriving as the young tuna hatch from their eggs.
Large driftwood behaves essentially as tree-drifting islands or reefs. Hundreds of years ago, tuna fishermen discovered this phenomenon and ever since are constantly on the lookout for driftwood. Exactly how tuna know where to find driftwood is not known, though surely it must be a combination of visual, olfactory and/or sonic detection.
Large driftwood can float for up to two years. Eventually it sinks and meets its final destiny with three species of deep-sea wood borers, which convert wood into fecal pellets and feed numerous bottom-dwelling animals.
Overharvesting of big trees and deliberate removal of large driftwood by harbor patrols has significantly reduced tuna habitat. Environmentally, what needs to be understood is that dead trees inexorably link the forest to the sea, sustaining thousands of unique species in many ecosystems.