What a marvellous sight trees give us in autumn, changing their leaves from green to yellow, orange and red! It’s so beautiful and supernatural, it’s like we’re in a dream! But when one of your trees loses its leaves in summer, it’s more like a nightmare…
Why Do Trees Lose Their Leaves?
Before we look at why trees lose their leaves prematurely, let’s examine the process by which they lose them as winter approaches. It’s not so different after all!
As winter approaches, it’s normal for deciduous trees to drop their foliage. This is a mechanism known as leaf abscission, a hormonal process that causes leaf detachment, usually triggered by changes in light levels, day length or temperature.
Leaf abscission is a mechanism that allows the tree to conserve water. The photosynthesis process causes transpiration, and since the ground is frozen in winter, water resources are limited. The loss of leaves protects the tree from dehydration. In any case, the cells in the leaves would explode due to the cold winter temperatures. You might as well let them go! The nutrients contained in the leaves return, in part, to the branches and roots, to be used when the warm weather returns.
Premature Leaf Loss
The reason why trees sometimes lose their leaves in summer, when it’s still hot and the days are long, is that they’re trying to protect themselves. In a way, it’s the same process they go through in autumn, and it has many causes.
Meteorological factors such as drought, excessive rain, extreme heat or sudden cold can sometimes lead to leaf loss. The tree seeks to conserve its resources and protect other parts of its anatomy.
Pest or Disease
Many insects, fungi and diseases can attack a tree’s leaves and cause them to fall, but they can also weaken the tree to the point where it feels the need to protect itself by dropping its leaves.
Like all plants, trees need basic nutrients (nitrogen, phosphorus and potassium), as well as other minerals, to carry out their physiological processes. A major deficiency of these nutrients can lead to premature leaf loss.
A tree’s roots transport water and nutrients to its leaves. When these are damaged, they lack resources, which can cause them to fall. Common causes of injury include landscaping and construction work, soil compaction or root rot caused by excess water or poor soil structure.
Have you ever heard that you shouldn’t prune more than 10 or 20% of a tree? I can’t confirm the exact figure, but when a tree loses too much of its mass, the remaining leaves can’t supply the nutrients needed by the remaining roots, and this puts stress on the remaining leaves. Whether through over-pruning, accident or weather event, this type of unbalance can also be detrimental to leaves. Injuries to the bark on the trunk are particularly harmful, as it is the bark that is responsible for transporting water and nutrients between the leaves and the roots.
Like all living things, a tree will reach an age when its body is no longer as strong as it once was, and it may begin to lose leaves prematurely. Poplars, birches and Manitoba maples, for example, often live only 30 to 50 years. It’s common to see these trees suffering from defoliation.
…Do you think this is why I’m losing my hair?
It’s perfectly normal for some tree species to lose their leaves before others. These include maples, birches and poplars. For those who live in warmer climes, trees of northern origin may also shed before those native to these regions. They are genetically programmed to lose their leaves at this time of year, having evolved in a colder climate. This problem is often encountered in Europe with trees of North American origin.
What to Do?
First thing: don’t panic! A tree can survive premature leaf loss just fine. It will have to draw on its reserves, of course. On the other hand, if the situation is repeated year after year, it will eventually run out.
Next, take the time to observe your bare tree and see if any of the problems mentioned above apply to yours. It’s possible that it’s only temporary. Think about what your tree has been through in recent years. Perhaps you’ve had work done on your property recently. The repercussions may not be visible for several years.
Long-Term Tree Care
To keep a tree healthy for a long time, think before you plant. Choose a tree suited to the conditions on your land, as you would for shrubs or perennials. Then, when planting, dig a hole 2 to 3 times the width of the root ball and the same depth as the root ball. Keep the original soil, but enrich it by adding compost to the surface. A decomposable organic mulch, such as ramial fragmented wood, is recommended. Depending on the size or caliber of the specimen chosen, it may be necessary to water it for one or even two years after planting.
You can continue to add mulch once it has decomposed, or you can leafcycle by leaving its leaves on the ground in autumn. The best food for a plant is its own leaves! If your tree is in a lawn, you can herbicycle it by leaving the grass clippings on the ground. These will decompose and feed your tree and lawn. You can also add a thin layer of compost (5 mm or 1/4 inch) to your lawn once a year or every 2-3 years.
Above all, avoid deep soil compaction by driving heavy machinery close to your trees, especially when the soil is soggy, as in spring.
If you’re not sure about the condition of a tree or how to care for it, don’t hesitate to contact a certified arborist for an examination. Tree planting is a long-term relationship with multiple benefits for human health and well-being. They’re worth the investment!