Many of us have pondered this popular philosophical conundrum. Fewer,
however, have considered a much more interesting question, "When that
same tree falls, does it have an ecological impact?" Whether it makes
a sound or not, ecologists are beginning to understand the role that dead
wood plays in ecosystems. Much of what we are learning is startling. Trees
- moribund, dead and decomposing - are cornerstones of ecosystem structure
and function. We now understand that a tree only begins to fulfil its
ecological contract once it dies. We’ve dubbed the process the dead wood
cycle. As ecologists, we are listening!
Let’s listen to one such tree in the cycle. Our tree is an old red
spruce. It stood for 300 years on a steep bank of the Point Wolfe River.
Starting as a 15 centimetre high seedling, it grew very slowly for the
first 50 years of its life, suppressed by deep under-storey shading. In
this state of suspended animation, it waited for a gap. Finally, that gap
came when several of its shallow rooted conspecifics in the over-storey
were thrown by the winds of a failing hurricane funnelling up the Fundy
Coast. Revived by the flush of sunlight, our tree bolted 25 metres for the
heavens.
Over the next 250 years, our tree saw a lot of both human and
ecological history. It was a young, vigorous tree when the Acadians were
expelled in the mid 1700s and was middle aged when, in 1822, a Loyalist,
John Ward, built a mill at the mouth of the Point Wolfe River. Luckily, it
resided on a bank so steep that it was spared the saw even though up to
60% of what is now Fundy National Park would be harvested. Its location on
the cool, damp Fundy coast spared it the worst of a number of cyclical
insect episodes, primarily Spruce Budworm. Its roots, well intertwined
with its neighbours, held it fast as high winds blew and heavy rains fell.
Hundreds of generations of red squirrels dined on its cones and eruptions
of crossbills came and went. But as old age approached, the senescent tree
became more susceptible to the increasing tempo and crescendo of this
centuries spruce budworm outbreaks. Weakened, bark beetles found the
spruce’s cambium to be a perfect canvas for their artistic galleries and
decay fungi began to digest the rotting heartwood. Carpenter ants moved in
and a pair of Pileated woodpeckers came to banquet on them. Smaller
black-backed woodpeckers followed their larger cousins, adding to the
trees shotgunned look. Boreal chickadees, cavity nesters, were only too
glad to further excavate the hollows.
(photo: Fundy National Park)
Death followed with hardly a change in ecological pace. As decades
passed the tree still stood defiant, despite the fact that it had lost its
crown and bark. It now stood as a snag, a sharp spire looking like a
bleached bone fractured half way up its length. A yellow-shafted flicker
now used the snag for its nest and a black bear made its den in a cavity
under the decaying root system. Much debris now lay scattered on the
forest floor at it base providing additional structure for small mammals,
particularly in winter when this debris, combined with a blanket of snow,
created ideal subnivean habitat. A marten, a specialist hunter of small
mammals, found refuge up the snag one cold winter day when it was in turn
pursued by a red fox.
Eventually, gravity defied the snag and it toppled down the bank and
into the Point Wolfe River where it lodged. Here the process of decay
continued but was somewhat arrested. The snag now added structure to the
riverbed and provided shelter for both aquatic invertebrates and Brook
Trout. It altered the system’s hydrology. For years, the log resisted
the vernal and autumnal floods, slowing the rivers flow and depositing
sediment in the pool on its downstream side. Atlantic salmon would rest in
this pool before continuing on upstream while a few found its gravel
bottom the ideal material in which to build a redd.
In late September of 1999, a hurricane struck the Fundy coast with a
climate change vengeance. Over 180 millimetres of rain fell in two days
and rivers responded with incredible floods. Our snag, lodged for so long,
was now washed downstream into the Point Wolfe River estuary and out into
the Bay of Fundy. For months, it remained within the bay, moving with the
currents created by the highest tides in the world.
Snag in River
(photo: Fundy National Park)
But upon entering saltwater, it came under a new assault. While
freshwater invertebrates, which cannot digest wood, were happy to shelter
in its shadow, marine invertebrates began to burrow into the wood for both
food and shelter. The snag now began to take on a honeycombed appearance
as it made its way out into the open Atlantic. Algae and barnacles began
to accumulate on its remaining mass and small schools of fish formed a
constant entourage. Somewhere off the Atlantic Coast of Nova Scotia, a
tuna fisherman sank an anchor for a beacon into the log. The fishermen
knew that tuna are attracted to these floating ecosystems and future
fishing success for this highly prized fish may depend upon finding it
again. As the log became increasingly reticulated, its remains crumbled
and fell as detrital rain into the extreme depths of the mid-Atlantic.
Even here nothing goes to waste as the bacteria and invertebrates of the
deep ingest the fragments through weird and wonderful energetic pathways.
Wolf River
(photo: Fundy National Park)
So when a tree falls in a forest, ecosystems hear its fall.
Unfortunately, that sound is becoming less frequent. Outside of protected
areas, many forest landscape managers manage forests for wood volume, not
snags per hectare or coarse woody debris on the forest floor. If we are to
keep many critical ecological relationships, we must institute best
management practises in forestry, which will ensure that the dead wood
cycle continues. To truncate the cycle will be to lose species. It will be
in the second and subsequent cuts that maintaining the dead wood cycle
will be most difficult unless we change our forestry management practices
today. Managed forests must have quotas not only for snags left per
hectare but also healthy trees left to form future snags. If we are to
avoid the disappearance of certain species from large areas through the
phenomena mortifiedly dubbed by conservation biologists as ‘relaxation’,
we must ensure an ongoing supply of wood to the dead wood cycle. With
Annual Allowable Cuts so finely tuned to predicted future volume per
hectare, the task of convincing government and industry may not be an easy
one. Everyone needs to listen and to act.
tree
falls in the Acadian forest of 
