Life and Death of a Western Hemlock

The most common species of trees found in the Opal Creek watershed are Douglas fir (Pseudotsuga menziesii), red alder (Alnus rubra), bigleaf maple (Acer macrophyllum), vine maple (Acer circinatum), western red cedar (Thuja plicata), Pacific yew (Taxus brevifolius) and western hemlock (Tsuga heterophylla). The oldest known trees in the watershed are about 1,500 years old.

The cycle of life in the forest is dependent on the trees at every stage of life and death.

Hemlock seedlings, copyright Trygve Steen.

Hemlock seedlings, copyright Trygve Steen.

It begins with a seed . . . or does it?

When the western hemlock seed cone was knocked out of the tree by a Douglas squirrel (Tamiasciurus douglasii), it landed and germinated on a large downed log of Douglas fir. This log is called a nurse log because its decomposing nutrients help young hemlocks and other plants in their struggle to germinate and survive. The forest floor is very dark and crowded; there are a lot of plants competing for space and light. Our hemlock has gotten a boost up into the higher light at the top of the log. This position is also beneficial because the log holds water better than the surrounding ground and during the dry months of the summer the thirsty young hemlock needs all of the water it can find.

As the roots grow, they gather nutrients from the decaying nurse log and make their way to the ground. The hemlock grows slowly until its roots reach the ground. Eventually the nurse log will fully rot away and the maturing hemlock will stand on its own. At this stage the roots of the tree are establishing an intimate relationship with many fungus species. This lifelong relationship is discussed further on the fungus page, but for now what is important is that this relationship is providing the tree with essential nutrients and water.



As the years pass, our tree has grown and is now about 50 years old. Our hemlock has a new neighbor: Douglas-fir dwarf mistletoe (Arceuthobium douglasii). This plant species tricks the tree into feeding it by mimicking its growth hormone. The tree sends building block sugars to the area experiencing ‘growth’ and the result is a witch’s broom-like abnormal growth on the affected branch. Eventually this parasite may kill our tree, but in the meantime the large flat areas produced by the infestation are great nesting sites for many canopy-dwelling species. After a few years the branches have become a home for a family of red tree voles (Phenacomys longicaudus), which feed on the needles of the nearby Douglas fir tree.

The tree continues to grow and every year the trunk grows ever so slightly to the eventual diameter of three feet. At 150 years old, our tree is 100 feet tall, and its top is broken off in a lightning strike; but this middle-aged tree, already weakened by mistletoe, will grow for another 50 years before it dies. In that time it will see 15 generations of spotted owls (Strix occidentalis) born on its branches. Yearly generations of flying squirrels (Glaucomys sabrinus) will live in the tree and eat the underground truffles (Tuberales spp.) that grow among its root system. Many songbirds will forage and nest on our tree.

In its two hundredth year, our hemlock succumbs to fungus, insects, lightning, and this spring it does not produce any green needles and dies.

But the story does not end there . . .


Carpenter ant

Carpenter ants (Camponotus spp.) invade! Thus begins the long process of decay that will eventually turn our tree into soil to be used by future generations of trees. The ants dig their nests into the tree and create large hollows to house their larvae. Both the larvae and the adult ants are food for the pileated woodpecker (Hylatomus pileatus). This woodpecker excavates huge holes in search of its dinner. Twenty years after its death our tree has become host to so many ants that the woodpecker decides to build a nest in it. The following year the woodpecker nest becomes a flying squirrel nest. Fifty years after its death, with some of the bark still clinging to the tree, a family of silver-haired bats (Lasionycteris noctivagans) takes up residence in the crack between the bark and the wood and live there for the summer.

Eventually the ants and the fungus weaken the snag and in the seventieth year of its ‘after-life’ the snag falls to the forest floor in a winter windstorm. Now two hundred and seventy years after its germination our tree is finally back to the ground. In the spring our tree will again become habitat for many animal species including small mammals such as voles and moles. After a few years of rain the log will become moist enough for salamanders and termites to invade. At this point there are more living cells in the log than at any point in its life or death.

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