“Marcescence” is the retention of leaves through the winter by deciduous trees — those that normally shed their leaves each year — shown here in a Washington apple orchard
How the unusual winter of 2022-23 affected the Methow Valley’s trees
By Sandra Strieby
Deciduous trees in the Methow Valley usually put on a glorious color display in the fall and then drop their leaves in preparation for winter. Last year, though, cold and snow arrived before that happened, and many trees did not release their foliage.
Some leaves fell during the winter, nudged by the weight of snow or blown off by the wind; others are still holding on.
Delayed leaf drop can have a couple of consequences for trees.
Mechanical damage
In cold, snowy locales like ours, snow that accumulates on leaves can add enough weight to break limbs and even cause trees to fall, as we saw throughout the valley last winter. Most deciduous trees — those that (usually) lose their leaves in the winter — are structured differently than conifers, most of which retain their needles all year round.
Conifer limbs tend to bend under the weight of snow. The trees’ conical shape allows lower limbs to support the upper ones as they are weighed down, and the overall form helps the tree shed snow when sunshine and warmer weather follow storms.
The branches of deciduous trees angle upward, reaching for light. While the forms of different species vary, their geometry does not leave them well suited for supporting extra weight. Leaves have a lot of surface area that can catch and hold snow, and letting them go at the end of the growing season is one strategy for keeping wood intact.
Without its leaves, a tree is better able to withstand the snow’s weight — especially the heavy loads we experienced late in 2022. Growing new leaves every year takes a considerable amount of energy, but, to the tree, it’s a worthwhile expense to protect a long-term investment in developing a trunk and branches.
Tree nutrition
Another consequence of holding onto leaves is nutrient loss. During the growing season, trees accumulate nutrients in their leaves. As the season draws to a close, shortening days and cooling temperatures signal the tree to stop photosynthesizing, and nutrients flow back into the tree. The tree then cuts off the flow between leaf and branch, creating an abscission layer — similar to a callus or scab — between the two. When the abscission layer is fully formed, the leaves are free to drop under the influence of wind, rain, snow or gravity without leaving an open wound on the branch.
Last fall, though, many trees didn’t have a chance to complete that process. Temperatures remained high throughout September and October and then plummeted early in November. Leaves froze before they’d had a chance to deliver their loads of carbohydrates, nitrogen and minerals to the trees’ trunks and roots. Without those nutrients, the trees won’t have as much to draw on as they develop new leaves and flowers to support this year’s growth and production.
How big a problem is that? Probably not very big, although orchardists and other tree caregivers may want to add some extra nutrition to keep their trees on track this year.
According to a February, 2023, paper by WSU Tree Fruit Extension Specialist Bernardita Sallato and others, some nutrients had probably moved back into trees by the time the leaves froze, so the impact of the early freeze is likely to be minor. Sallato and her co-authors cite a similar situation in 2014, which did little to affect tree health and productivity. They do recommend monitoring nutrient levels and applying supplemental nitrogen and micro-nutrients if necessary.
Cold damage
In addition to the challenges posed by persistent leaves, trees may also have been affected by last year’s temperature regime. According to John Richardson of Booth Canyon Orchard, “Leaves aren’t the real issue — they fall off or the new leaf pushes them off.” What may cause long-term damage, he said, is the sudden shift from warm to cold weather.
Typically, trees prepare for winter not just by shedding leaves but also by changing their internal chemistry to protect woody tissues from freezing. High temperatures in September and October followed by below-zero readings during the first week of November left trees vulnerable.
“Depending on tree species and variety, the problem wasn’t the -28 near the end of December, but the -2 we had in the first week of November after the snowstorm,” said Richardson.
Trees rely on their cambium — cells just under the bark — to insulate the stems, trunk and roots, and to transport water and nutrients within the tree. Closest to the bark, cork cambium cells help protect the living tissues within the trees and, when mature, contain fatty polymers that provide additional insulation. Beneath the cork cambium is the tree’s vascular cambium, a network of cells that conducts carbohydrates, minerals and water between leaves and roots.
As temperatures fall, sugars and salts concentrate in the vascular cambium to make the cells more resistant to freezing.
The sudden drop in temperature last November left trees no time to prepare for the cold and in some cases killed or damaged the cambium, Richardson said. That means the tree’s circulatory system can’t function, because stored nutrients aren’t able to reach the tree’s buds to support leaf, flower and fruit development. At Booth Canyon, some apple and pear trees look OK; others may not reveal damage “until fruit set or later,” said Richardson. Still other trees have clearly sustained damage, as have the orchard’s peach and wild cherry trees, he said.
Among trees that did survive, blossom buds may be dead or damaged. According to Richardson, blossoms may appear, but if they’ve been damaged by cold, they won’t be receptive to pollen and so will not set fruit.
Overall, the valley’s trees have fared better than they did during the record-cold winter of 1968-69, when many of the apple trees in the middle and upper valley were killed. Only as the growing season advances will the full story unfold.
For more information
• Washington State University Okanogan County Extension, (509) 422-7245.
• Tri-County Horticultural Pest & Disease Board, (509) 667-6677.
