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Lorsqu’un arbre tombe dans la forêt

Un arbre tombe dans la forêt acadienne du Parc national de Fundy. Fait-il du bruit? A-t-il un impact écologique sur son milieu?

Les arbres, qu’ils soient moribonds, morts ou en décomposition, sont la pierre angulaire de la structure et de la fonction des écosystèmes. Nous comprenons maintenant qu’un arbre commence seulement à réaliser son contrat écologique lorsqu’il meurt. Nous avons surnommé ce processus "le cycle du bois mort". Suivons le cycle de vie d’une épinette rouge, de sa naissance comme semis jusqu’à ce que sa dépouille mortelle décomposée s’effrite et tombe sous forme de pluie détritique dans les bas-fonds médio-atlantiques.

Si nous voulons maintenir les nombreuses relations écologiques critiques, il nous faut instaurer les meilleures pratiques de gestion forestière possibles afin de s’assurer que le cycle du bois mort puisse continuer.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Pileated woodpecker
(photo: Fundy National Park)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

" 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. "

       

When a Tree Falls
in the Forest
  

Renee Wissink,
Park Ecologist, Fundy National Park
October 2001

 

tree falls in the Acadian forest of
Fundy National Park. No one is there. Does it make a sound?


(photo: Fundy National Park)

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.