Acool summer and unusually late fall may have affected Manitoba’s largest perennial crop: trees.
Leaves stayed green far longer than usual this year and some scientists suggest trees may go into winter in a weakened state as a result.
It was common last week to see trees still with green leaves, which would normally have turned yellow and fallen to the ground long ago.
The leaves were probably dead, killed by frost but retaining their green colour, just as vegetables do when you put them in the freezer.
Because leaves froze early, the trees didn’t have a chance to take back their nutrients. This left them in “a famine state” going into winter, according to Janice Cooke, a University of Alberta tree biologist.
“This is the dire consequence of having these green leaves on the trees that are now fading to brown. The nutrients that were tied up in those leaves, that the tree invested in those leaves, are now lost to the tree,” Cooke was recently quoted as saying in a newspaper article.
This will hurt trees in spring because they were counting on those nutrients for growth, she added.
Manitoba industry officials agreed 2009 was highly unusual for trees. Many were late to leaf out because of the cold spring. Some ash trees weren’t fully in leaf until July. Trees generally have been slow to shut down for the winter.
But the late presence of greenery isn’t necessarily bad news for a tree’s health, said Ken Fosty, a Manitoba Forestry Association extension officer.
The function of a leaf is to capture sunlight and convert it into energy for the tree. But leaves don’t store energy, said Fosty, who is also a certified arborist.
“I don’t think it’s going to have a major effect because the leaves had already done their job as far as making energy for the tree,” he said.
“It happened so late in the year that, in my opinion, the leaves had already done their function and the vast majority of the energy had already been converted and sent into the tree.”
Fosty said his main concern is that leaves remaining on trees create a mass for snow or freezing rain to settle. This could result in a lot of broken branches. The late season is “more of a physical concern than a biological concern” for trees, he said.
Martha Barwinsky, Winnipeg’s city forester, said when leaves drop off in autumn, the tissue at the base of the leaf (called the abscission layer) normally seals and closes up. That hasn’t happened this year, setting the stage for possible twig die-back in spring.
“The tissue hasn’t hardened off properly for the winter, so basically the trees really haven’t shut down in the normal way that they would,” said Barwinsky.
But Fosty said as soon as a leaf is formed, a new bud forms behind it. Even if leaves blow away or break off, buds are already there waiting for spring.
Shane Tornblom, who manages the Manitoba Agro-Woodlot Program, said comparisons between Manitoba and Alberta aren’t all that valid this year because Alberta experienced drought, which gave leaves less potential for photosynthesis. Then came an early frost, which Manitoba avoided.
“I think the circumstances between Alberta and Manitoba are fairly significantly different,” Tornblom said.
The good news is that trees in much of Manitoba put on “a phenomenal amount of growth” this summer because of abundant rainfall, said Barwinsky.
Too much, in fact, because excess water in the soil can prevent tree roots from getting enough oxygen, resulting in root die-back, she said.
But trees are hardy, long-term perennials with complicated defence systems enabling them to withstand occasional stresses, said Fosty.
“Trees have been here longer than we have so, I think they’re good to go.” [email protected]