Add one more category of victims from this year’s flood – trees and shrubs lining Manitoba’s riverbanks and shorelines.
Trees drown if they stand in water too long and certain species are past that point already, says a forester with the Manitoba Agriculture Food and Rural Initiatives’ Manitoba Agro Woodlot Program.
“It’s almost a form of suffocation for them, when they’re not able to draw oxygen out of the soil,” said Carol Graham, who is based in Souris.
“Significant losses in some areas are likely. It might be very similar to what Dutch elm disease did to the riverbanks.”
The prognosis is not good for trees with now-wilting or yellowed or brown leaves, or those completely defoliated, while shrubbery along riverbanks that’s been completely submerged doesn’t stand a chance.
It will take two to five years to assess the toll this year’s standing water has taken. But riverbanks and lakeshores will likely look very different in the years ahead.
In a worse-case scenario, large numbers of trees, particularly older ones, will begin, over the next year or two, to collapse into rivers and streams, creating deadfall and clogging watercourses. So far, conditions vary from area to area – some seem all right while others are already in significant decline, said Graham.
But since it can take two years for a tree to show signs of impact, it’s very much a wait-and-see situation, she said.
Some trees, such as green ash, can handle watery conditions.
“It’s been observed where they’ve been under water an entire growing season and they’ve been OK,” said Graham.
On the other hand, poplar can’t take standing water at all and oak trees only tolerate having their feet wet for about 30 days. Maples tend to be more tolerant.
“In those areas that didn’t have the diversity or the range of tolerant species, then you’re going to probably be seeing quite a bit of decline,” she said.
A tree’s current state of health also affects survival. As with humans, it’s the elderly and those with some sort of pre-existing health problem that are most affected.
But even the survivors pay a price. Trees that survive the flood conditions
will be susceptible to root rot, fungi, and other defoliating diseases. There is evidence of blight and fungi now, Graham said, adding that there may be a rise in cankers, which attack the main stem of trees.
Equally gloomy is the prospect of Round 2. With water levels expected to remain high, there’s a greatly elevated risk of ice battering trees and shrubs during next spring’s breakup.
“I think that’s a very significant concern,” Graham said.
Combined with this year’s losses, losing more trees next spring increases the risk of riverbank and shoreline erosion.
“They act like a wall,” noted Graham. “These big trees prevent the ice from coming up the bank and act as a protection for all the younger trees and shrubs behind them. What’s probably going to happen as the ice rips out these trees is we’re going to see that much more damage farther up along the banks because there’s nothing now protecting those smaller, younger trees in the understory.”
And, of course, a repeat of flooding in 2012 will mean more trees drowning or succumbing because of frail health.
Woodlot staff will be closely monitoring the situation.
There is some good news, though – Mother Nature has ways to undo the damage caused.
Trees will naturally regenerate by producing large seed crops under these kinds of conditions, Graham said.
“Particularly with ash and oaks, you’ll start to see significant seed production,” she said. “A normal response of a tree that’s dying is that it will put all its energy to reproduction.”
“Significantlossesinsome areasarelikely.Itmightbe verysimilartowhatDutchelm diseasedidtotheriverbanks.”
– CAROL GRAHAM