Life and Death of a Western Hemlock
There are many plant species found in the Opal Creek watershed; to discuss them all is beyond the scope of this page. So for now we will just focus on trees, and in particular the Western hemlock. While we focus on one species, keep in mind that every species of tree found at Opal Creek is essential for the natural functioning of our forests; each one supported by and supporting a distinctive suite of organisms, from fungi to bugs to birds.
The most common species of trees found around the Opal Creek education center are Douglas-fir (Pseudotsuga menziesii), Red alder (Alnus rubra), Bigleaf maple (Acer macrophylum), Vine maple (Acer circinatum), Western red cedar (Thuja plicata), Pacific Yew (Taxus brevifolius) and Western hemlock (Tsuga heterophylla). The trees of Opal Creek are obviously very important in the ecosystem, and many trees in the watershed are over hundreds of years old. 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. To illustrate this concept we will follow one tree through its life in Opal Creek and explain its importance to the forest and the creatures that call this forest home.
It begins with a seed . . . Or does it?
We will begin the cycle with the seed cone of a Western hemlock. When this cone was knocked out of the tree by a Douglas squirrel (Tamiasciurus douglasii), luckily it landed and germinated on a large downed log of Douglas fir. This log is called a nurse log because it helps out the young hemlocks and other plants in their struggle to germinate and survive. This is helpful for the hemlock because 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 present 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 get to the ground. Eventually the nurse log will fully rot away and the maturing hemlock will be required to 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 all we need to know 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. It is now that our tree becomes a host for mistletoe (Arceuthobium douglasii). This plant species is a parasite and 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 mean time the large flat areas produced by the infestation are great nesting areas 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. It is at this stage when the tree is 100 feet tall and 150 years old that its top is broken off in a lightning strike. This hastens the dying of this middle-aged tree, already weakened by mistletoe. But our tree is still very much alive and 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, which are now perfect nesting sites. Yearly generations of flying squirrels (Glaucomys sabrinus) will live in the tree and eat the underground truffles (Tuberales spp.) that are supported it. Many songbirds will forage and nest on our tree.
Eventually fungus, insects, lightning ,and parasites finally have their way with our tree and in the two hundredth year since its original germination on the nurse log, our tree does not produce any green needles, and it dies.
But the story does not end there . . .
It as this point that carpenter ants (Camponotus spp.) invade and begin 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 hollow large areas to house their larvae. Both the larvae and the adult ants are food for the Pileated woodpecker (Dryocopus 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 its nest in it. Twenty-one years from the birth of our snag the woodpecker nest becomes a flying squirrel nest. Fifty years after its death some of the bark falls off, and some stays on. At this point a family of Silver-haired bats (Lasionycteris noctivagans) take up residence in the crack between the bark and the wood of our tree 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 has 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.
It is now that our cycle becomes complete, as a young Western hemlock lands on the log and germinates. This cycle is only one of countless cycles that are played out in our forests, all of which are interrelated and interdependent.
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