Each fall in Eastern Canada, shorter days and cooler temperatures trigger hormonal changes in woody plants. These changes activate hardening processes that prepare trees and shrubs for winter. This physiological adjustment is known as cold acclimation.
For deciduous trees, leaf drop is one of the key survival strategies during winter. However, in some cases, leaves do not fall. Is this linked to climate change? A disease? Can it harm the tree? Should we be concerned?
In this article, we explore these questions while explaining how leaf physiology and cold acclimation work in woody plants.
Timeline of Leaf Drop in Deciduous Trees in Eastern Canada
Leaves: Nature’s Energy System
The most active growth period in plants coincides with the longest days of the year. This is no coincidence—plants synchronize their metabolism to capture solar energy efficiently, supporting growth and storing reserves.
This process takes place in the leaves, which act as highly efficient energy systems. At the centre of this process are chlorophyll molecules—tiny green pigments that capture sunlight during photosynthesis.
Leaves also contain other important compounds. Carotenoids, responsible for orange tones, act as antioxidants. Anthocyanins, which produce red and purple colours, help protect leaves from UV radiation.
During the growing season, chlorophyll dominates—giving foliage its characteristic green colour.
Leaf Colour Change and Leaf Drop
As summer ends, shorter days and cooler temperatures trigger cold acclimation. Plant metabolism slows, allowing trees to store energy for the next growing season.
While bark protects branches and trunks, and soil insulates roots, leaves are not adapted to withstand harsh winter conditions. Trees must shed them—but not before recovering valuable nutrients.
To do this, trees form a corky abscission layer at the base of the leaf petiole. This layer gradually restricts sap flow to the leaves.
As chlorophyll breaks down, its green colour fades, revealing carotenoids and anthocyanins. This is what creates the vibrant yellows, oranges, and reds of fall.
Eventually, sap flow stops completely. Without water and nutrients, leaves dry out, die, and fall with the wind.
Does Climate Change Affect Leaf Drop in Eastern Canada?
In some regions, the growing season is becoming longer due to milder conditions. Spring arrives earlier, and the first fall frost comes later.
During warmer-than-usual autumns, leaf colour change and leaf drop may be delayed. Fall colours may also appear less intense.
Some introduced species from warmer climates may retain their leaves longer and continue limited photosynthesis. Because the abscission layer forms less completely, leaves may fall while still green.
Why Do Some Trees Keep Their Leaves in Winter?
In some native Eastern Canada species, dead leaves remain attached throughout winter. This natural phenomenon is called marcescence.
In these cases, the abscission layer does not fully develop, which explains why leaves stay attached. This trait is species-specific.
It is also observed in some herbaceous plants, such as hydrangeas, where dried flowers persist through winter.
Although marcescence is well documented, its exact ecological role remains unclear. It is often more noticeable in young trees and tends to occur on lower branches in mature trees.
Marcescence is generally harmless, although heavy snow or ice may occasionally cause branch breakage. There is no need to remove these leaves—they will fall naturally in spring when bud break occurs.
Marcescent Tree Species in Eastern Canada
Eastern Canada Native species that may show marcescence include American Beech and several oaks such as White Oak, Red Oak, and Bur Oak.
Among introduced species, Norway maple and fastigiate English oak may also exhibit this trait.
Evergreen Trees and Shrubs
Evergreen plants do not lose all their leaves in fall. Instead, they shed them gradually as part of a natural renewal cycle.
Because this process is continuous, they remain green throughout winter. Their leaves are typically thicker and often coated with a waxy layer.
Conifers are the most familiar evergreen trees. Several generations of needles coexist on the same tree.
Although they look different, needles are modified leaves—smaller, wax-coated, and less efficient at capturing light. However, this adaptation makes them highly resistant to cold climates and browsing by wildlife.
Ultimately, these are survival strategies shaped by environmental conditions.
Article co-written by Audrey Rondeau, Biol., M.Env., and Suzanne Simard, horticultural writer and technologist.
References
Marcescent trees: An intriguing in-between state – Zoom Nature (2023)
The programmed death of leaves – Zoom Nature (2019)
When fall leaves don’t drop – Le Jardinier Paresseux (2023)
