Photo: Wikimedia CC
The American elm (“Ulmus americana”). Illustration from d'”History of forest trees in northern”, by François André Michaux.
Since always, they serve to build as much as dream… In this summer series, The Duty attempts to identify what wood they are made out of trees that surround us. Today : the American elm.
Jean Lamontagne plant the tip of his tape measure in the bark covered with cracks oversized, and then crosses the root of the tree, kinds of legs of Titans, to work around the trunk. Except that her ribbon is too short for the enormity of the beast. The nurseryman will point a finger at the end of the cord, graduated, and then complete the revolution by returning to the zero of his tool.
Verdict : six metres circumference, that is to say a diameter of nearly two meters. “Look at me, majesty,” says an expert on trees, who cares for four decades. “He is lucky : he has probably 200 years old, but he has never been attacked by the disease. It survives, even if it is surrounded by the dead… “
The colossus, regarded as the largest and oldest elm of Quebec, is overshadowing the tombstones in the cemetery of St. Matthew, in the district of Saint-Jean-Baptiste. Inaugurated in 1772 in order to receive the remains protestant in the aftermath of the conquest of New France, this cemetery today offers a haven of freshness to the walkers of the city, in particular through the immense canopy of his king leafy.
It is often said of the American elm that it is the tree perfect urban. It is resistant to compaction of the soil and survives drought episodes. Such a huge umbrella, he deploys his canopy height, leaving the bottom of a trunk naked. Its branches grow horizontally, curving to the periphery. Its leaves, shape of the eye, are jagged and rough.
So beautiful as it is, Ulmus americana lives of hard times. The Dutch elm disease (MHO), who saved the specimen from the cemetery of St. Matthew, has killed tens of millions of elm trees in North America since its introduction in 1928. In Quebec, we have noticed its presence for the first time in 1944, near Sorel. In the decades that followed, the disease decimated including the trees living on the island of Montreal. She took also, little by little, the elms mature that stood solitary in the middle of fields. In nature, there is now only very rarely in large specimens.
Death and resurrection of the perfect tree
In urban areas, the capital of quebec, however, makes an exception. The agglomeration of Quebec has approximately 15 000 American elms on the set of his land, public or private. We see them everywhere : on the main Alley, they overlook the terraces ; on the plains of Abraham, they mix with the flowers.
In front of the new pavilion for the Musée national des beaux-arts du Québec, inaugurated in 2016, rises another specimen remarkable. Lovers of the trees are concerned (and remain concerned) about the impact of this construction on the tree at least 150 years. As an architectural element, door-to-false advances toward the crown of the elm tree, it has carved out a part of its branches. And such as sidewalks surround its base, more than half of its root system has been sacrificed, according to John Lamontagne, who handles the work to keep the tree alive.
These pressures, due to urban development, in addition to that exerted by the MHO, which still poses a threat to strike.
Both fungi cause the disease : Ophiostoma ulmi (causes of the first outbreaks on the continent) and Ophiostoma novo-ulmi(a sub-species more virulent, now widespread). These enemies move in the conductor vessels of the sap, where they release toxins. To protect itself, the tree secretes substances that kill fungi, but also its own tissues.
The MHO is also in need of vectors. Two species of small beetles transmit the fungus from one elm to another. It is a beetle native and beetle european. These insects breed in elm trees are diseased or dying. Their offspring, when it burrows under the bark of elm trees healthy, deposits spores of a fungus deadly. The disease can also travel from one elm tree to another by way of root, when the trees are sufficiently close together.
Laval University has its passionate elms. Guy Bussières, a pathologist forest, toured the campus each summer to identify diseased trees. In a small wooded area near the university, and some specimens boast just for a few days leaves yellow or brown on their top. This sign does not lie : these trees bear the MHO. They will have to be slaughtered to prevent the spread, as mandated by the City of Quebec.
“I went here two days ago, and this elm, there had not yet symptom-free,” explains the specialist to the hiring of the institution since 1987. It goes down the ditch leading to the new tree is sick, where is also a small rod which he had already marked. Stunted leaves of the latter form a kind of horns. A cut, Mr. Bussières cut a branch to expose the brown coat immediately under the bark, typical of the vessels of the sap are killed by the reaction of excessive self-defense caused by the disease.
Treatments exist to prevent the disease. At the end of the 1970s, the death of a thirty trees on the hill of Quebec, decided the municipal authorities to put in place a program to combat the MHO. During the 1980’s and 1990’s, Jean Lamontagne won many contracts to combat the parasite and fungus. Or attacked it in the first one, thanks to an insecticide applied to the base of the tree, or second, by injecting a fungicide into the vessels of sap.
At the turn of the millennium, the arrival of the beetle in europe in the capital has poisoned, disturbed the fight. (The cold winters of Quebec had probably delayed the arrival of this insect, found in Montreal since a few decades.) Against this beetle hibernating in the top of the elm, the application of insecticide became ineffective. In addition, bad experiences with the fungicide, which can cause cracks along the trunk when it is wrongly applied, have made it so that the injections have stopped.
But for the past five years, the death of a few elms symbolic has convinced the City, the Commission des champs de bataille nationaux and the University of Laval to provide funding for a new injection preventive fungicide in the trees deemed irreplaceable. The treatment of a single individual, good for three years can cost more than $1000.
Mr. Bussières attributes the recent resurgence of the epidemic in climate. “When the temperature is ready, the beetle european can make two generations in a summer. It usually comes out of the tree in early June to feed and lay eggs. When the autumn is very warm, the insect has the opportunity to complete a second reproductive cycle, ” says the pathologist forest.
What future awaits the American elm, Quebec city, and elsewhere on the continent ? Despite the obvious threats, Louis Bernier, a professor of forestry who is trying to understand what gene ofOphiostoma novo-ulmi are the most devastating, says he is optimistic. The maintenance of a programme to combat the MHO, combined with the search for elms resistant to the disease we found a few in Ontario, note-t-he — will ensure that the elms should remain among us.
“In 200 years, I don’t know if there will be still human, but at least there will still be the elms “, lance-t-il joke.