12/16/2009

When A Tree Falls In The Forest...It Becomes Large Diameter Woody Debris

When I taught grade six science my favorite unit to teach was Trees and Forests which was all about the importance of forest eco-systems. While discussing old growth forests I had to teach my students about major characteristics of these ancient places, such as Large Diameter Woody Debris. Even though this term never failed to elicit giggles and snickers from students, they soon came to see how it holds great importance for the forest environment.

I often scour the forested hills surrounding my home for large diameter woody debris. LDWD is a tell-tale sign of an older, undisturbed forest. These large trees can be standing dead trees (snags), or fallen dead trees, or healthy trees downed by windthrow. They provide irreplaceable habitat for a host of forest life. On one recent trip I followed the Galloping Goose trail into the Sooke Hills Wilderness.

Just before Sooke the western section of the Galloping Goose trail turns north and heads into the Sooke River watershed . The trail traverses a 1918-built CNR rail line that once saw wood-powered Shay locomotives hauling massive trees out of the pristine forest. Now it provides access for people recreating, including those seeking out big tree wilderness.



The trail's western terminus, about 12km from where it leaves the coastal plain below, is 14 hectare Kapoor Park Reserve. This undeveloped park contains the remains of Leechtown, a 20th century gold rush and lumber town. Anything that remains has been reclaimed by the ceaseless growth of the forest, not to mention the invasive Scotch Broom. The Galloping Goose trail ends here, but the old line pushed north through the forested hills to Cowichan Lake.


Along this section of the trail there is a high frequency of scattered Douglas fir veterans that soar over 45 m/150 ft and are hundreds of years old. The tops of these trees emerge from the smaller forest around them, and announce their presence. Douglas fir typically live about 750 years with documented cases of well over one thousand years old.


The old tree's trunks stand out from the background of toothpicks of 2nd or 3rd-growth trees. Often the trunks show evidence of fire across the deeply furrowed, fire-resistant bark. Before fire suppression started, wildfires occurred on a regular basis. Such fires kept down smaller trees and underbrush, leaving an open forest dominated by 122 m/400 ft ancients.


Every winter storms slam into this region, and their power and fury is recorded in old tree's twisted limbs and broken tops. Occasionally storms topple trees and the big ones come down with a crash... if there is someone in the forest to hear it, that is. Sometimes they are uprooted (windthrow or blowdown), and sometimes trees snap off up the trunk (windsnap).

Imagine standing next to one of these 10 story tall trees while it is oscillating back and forth during a gale-force wind. Soon you hear wood snapping and cracking, then during a strong gust, the dangerously leaning tree keeps on going until it crashes to the ground at your feet. You hear it. You feel it, too. This tree, perhaps 400 years old, has just become large diameter woody debris, a very important part of this ecosystem.



Standing veteran trees are ecosystems unto themselves over the hundreds of years they live and provide habitat for a variety of plants and animals. From the mosses and lichens growing on their bark to the marbled murrelet in its branches or the heartwood decaying fungus slowly hollowing out the inside of the trunk, such a tree harbours a complex self-sustaining system. When such trees die and/or fall to earth the micro-hoards of decomposing creatures begin the relentless process of recycling these giant columns of biomass back into the system.


Many species of birds, mammals, amphibians, and reptiles use LDWD for a variety of purposes. Birds such as the Pileated woodpecker (the largest type of woodpecker, used as a model for Woody Woodpecker), visit downed Douglas firs to hunt for carpenter ants or excavate cavities to live in. Unlike headbangers, Pileated woodpeckers have a cushion built into their brain to reduce the damage from all that beak slamming into solid wood. Ancient snags are hollowed out as nesting sites, and after primary cavity nesters move out owls, other birds and small mammals move in. The shedding wrinkly bark also gives shelter to bats.


Black bears, martens, fishers and bobcats all use suitable downed LDWD for denning sites. Sharp-tailed snakes, endangered in B.C., are residents of the vanishing coastal Douglas fir forest and live in and around downed trees. A salamander can live its whole moist life in a single log; everything it needs is there.

Over decades, as the log melts into the forest floor, new trees take advantage of the rich nutrients it provides. One day one of these small trees may fill the hole left by its fallen ancestor. The same ancestor that nursed the young tree to its youth.

The whistle of the logging locomotives is not heard any longer in these parts, but veteran trees continue to fall. From the old rail bed I can see where recent clear cut logging has taken out some of the remaining 1% of original low elevation Douglas fir forest on Vancouver Island.

Such clear cuts raze the forest right down to the ground. Extra woody debris (and there is often a lot of it) is burnt in large piles, or is salvaged by outfits further down the chain, much like the vultures that pick on the remains of dead animals. But as vultures prefer the carcasses of herbivores, the same with post-clear cut salvage outfits. They prefer the high value cedar more than anything else.

Large diameter woody debris may be funny, but it is of supreme importance to the things that live in and around it, including humans. You can educate your family and friends AND provide humour at the same time - just mention LDWD.

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