We know that trees lose their leaves in autumn. Some trees lose their leaves earlier in the season, while others keep their green foliage throughout. How do we explain this disparity?
Trees lose their leaves at different times because each tree is subject to unique conditions in terms of temperature, water availability, and sunlight duration. The rate of decrease in water reserves, sunlight hours, and temperature determine the time a tree loses its leaves.
In this article, I’ll highlight the primary factors that cause trees to lose their leaves and explain why trees lose foliage at different times. I’ll also discuss possible reasons a tree could be shedding leaves when it shouldn’t be.
Why Some Leaves Turn Faster Than Others
Turning refers to the change in color and texture you typically see right before a leaf dies and falls from its tree.
While walking outdoors in autumn, you’ll notice various leaf colors. Many will be orange, but you’ll also see brown, yellow, and the usual healthy green. Most leaves will eventually turn, but they will do so at different times.
Generally, we can attribute the difference in timing to the trees’ genetic and environmental differences. Trees of different species have different types of leaves, which turn at different times based on genetic instructions. At the same time, environmental factors play a major role in determining when leaves turn.
Why Trees Lose Their Leaves in Autumn
Before discussing the factors involved in leaf loss, we need to look at why trees lose leaves in the first place. Most trees are deciduous, meaning they lose their leaves seasonally, and the main reason trees lose their leaves, in autumn particularly, is the decreasing daylight hours and the sun’s intensity.
Trees Use Leaves To Make Food
Trees need sunlight to carry out photosynthesis, which is a vital, life-sustaining process that converts nutrients gathered from the soil into sugar, a readily-usable source of energy.
Without sunlight, trees can’t carry out photosynthesis and can’t make food, which leads to malnourishment over the long term. Trees can absorb sunlight for food because of a green pigment in their leaves called chlorophyll, which gives leaves their signature green color.
Leaves can therefore be thought of as food production machines. As long as they receive sunlight, nutrients, and water, they’ll keep churning out sugar, which the tree can use to feed itself.
Leaves Make Less Food as Autumn Advances
Things change in autumn, and with the decrease in sunlight intensity and exposure duration, food production gradually declines.
Since leaves are living organs that use energy, it gets to a point where the leaves use more food than they produce. That’s the tipping point that usually happens sometime during autumn, although it can happen as late as winter, depending on how much sunlight a tree needs.
Remember that sunlight conditions only worsen as winter rolls in, so keeping leaves around beyond autumn becomes pointless.
Trees Extract the Chlorophyll From Leaves as Winter Nears
When the energy requirements of a tree’s leaves become unsustainable, the tree cuts off the food supply to its leaves to reduce resource expenditure and prepare for the upcoming winter. But before doing so, it extracts all the chlorophyll inside the leaves.
Though trees can produce chlorophyll naturally, they store existing chlorophyll in autumn to save the time and resources it would take to make fresh chlorophyll once winter ends.
As the leaves lose the green chlorophyll, they change color and turn yellow, then orange, and eventually, red or brown. Once the tree has extracted all the chlorophyll from its leaves, it cuts off its food supply.
The turned leaves lose the moisture content they have left and become crispy.
Eventually, those “crispy” leaves fall to the ground, making a nice little mess for you to clean up. However, depending on how many leaves you have in your backyard, you might be able to leave them on the ground throughout the winter. They will disintegrate and turn into fertilizer for your lawn the next spring.
The Connection Between the Tree and Its Leaves Weakens
After cutting off the food supply, the tree lowers the amount of the hormone auxin that it supplies to its leaves and raises the supply of ethylene.
This change in chemical concentrations causes certain cells near the base of the leaf to expand, weakening the connection between the leaf and the tree. As a result, if the leaf doesn’t fall on its own, it’s usually blown off by the wind.
The tree, now without any foliage, hunkers down in preparation for the winter. We describe it as “dormant,” which is a state similar to hibernation. As the days revert to their usual length in spring, the tree comes out of dormancy and grows new leaves. The cycle repeats in the coming year.
Factors That Influence When a Tree Sheds Its Leaves
Leaf loss is an adaptive behavior meant to conserve resources during harsh times. It follows that poor environmental conditions can encourage a tree to lose its leaves.
Trees growing on poor-quality soil, for example, will lose their leaves faster than those on fertile land. Also, trees under shade are more prone to the effects of decreasing day length than trees that receive unobstructed sunlight.
Let’s discuss the factors that influence the timing of leaf-shedding in trees.
Reduced Exposure to Sunlight
Decreasing day length, which equates to lower time under the sun, is the major motivator of seasonal leaf loss.
In areas where sunlight exposure reduces at a high rate, trees drop their leaves early. Trees in areas where the rate of sunlight exposure reduction is lower will keep their leaves longer.
Even in areas that receive more or less the same exposure to sunlight, the first trees to lose their leaves are likely to be the ones under shade.
Freezing Temperatures
Decreasing temperatures caused by the approaching winter signal trees to go into dormancy. Although trees are more resilient to the sheer cold than other plant life, water acquisition becomes an issue, particularly in sub-zero climates.
Low temperatures restrict chlorophyll production, which is a major problem, but it gets worse.
Sub-zero temperatures are a serious threat to plant cells, leaf cells included. Such temperatures cause the water inside the cells to freeze and expand, creating ice crystals that tear through fragile cell walls and cause massive internal damage.
Trees in frost-ridden locations have evolved to drop their leaves early in autumn, which helps them secure the chlorophyll in their leaves instead of losing it to frost damage. They lose their leaves earlier than trees in areas that are less affected by freezing temperatures.
The Strength of the Wind in an Area
Trees should always grow straight, which is especially true for larger ones.
It’s hard enough to stay stable against fast-blowing winds as it is, and unfortunately, leaves can make this problem worse. They significantly increase the overall surface area of a tree, thereby increasing how much force the wind can exert on the tree.
A tree with leaves will have to fight harder to stay upright than a tree without leaves, which is not a luxury that trees can afford during harsh winters when trees are at their weakest because of the low supply of sunlight, nutrients, and water.
Also, winds are markedly stronger during the winter because of the increased temperature differences. Trees that regularly face windy winters are likely to drop their leaves before the winds pick up to avoid being blown over or uprooted.
Extremely Hot Temperatures
A smaller class of deciduous trees actually lose their leaves during the summer, a phenomenon we call dry-deciduousness. It’s not that they can’t withstand the heat. Their thick, sturdy barks can protect them against the heat just as well as they do against the cold.
The problem is the lack of water, which is an essential reactant in photosynthesis, without which the chemical process cannot occur. In desert-like regions, the extreme heat means any water in the top layers of the soil, at least those accessible to the trees, dry almost immediately.
As trees under these conditions cannot carry out photosynthesis until the rainy season begins, they drop their leaves and go dormant.
This is similar to waiting for more sunlight in the winter but is a much less commonly seen occurrence and is mainly seen in parts of Africa. It’s not very common in the US, where temperatures are much more moderate.
Other Environmental Stresses
While robust trees can cope with environmental stress, long-term chronic stress will eventually take its toll on them in one way or another.
Trees suffering from the following stresses may lose their leaves prematurely:
- Overwatering
- Sun deprivation
- Air pollution
- Unsuitable soil pH
- Infertile soil
Air pollution, in particular, likely causes city trees to be unhealthier than their forest-based varieties and shed their leaves earlier.
Genetics
As you may expect, genetics dictate, to a great degree, when a tree will lose its leaves. Trees of different species naturally lose their leaves at different times as they have adapted to best suit their personal needs.
Even among trees of the same species, there can be great genetic variance. You may see the same type of trees losing their leaves at different times, even if they’re very close to each other.
Why Some Trees Don’t Lose Their Leaves at All
While many trees are deciduous and will lose their leaves at one point or another, some are evergreen, which means that they’ll always stay green.
Evergreen trees usually have thin, needle-like leaves with an excellent capacity for water retention. Since trees lose their foliage due to a lack of water, this leaf adaptation in evergreen trees reduces water loss via transpiration, allowing the tree to retain its water for much longer.
Evergreen trees also have more branches with more foliage than deciduous trees, allowing them to absorb sufficient amounts of sunlight even in winters when intensity is low and exposure is short.
Now, to be clear, these needle-like leaves aren’t immortal. They have a natural life cycle of about 3 or 4 years, after which they die and fall off. But this is a constant process that runs simultaneously with new leaf generation, so the overall structure and density of the dense green foliage are preserved.
Possible Reasons a Tree Loses Leaves When It Shouldn’t
Is a tree on your property losing leaves at an alarming rate? Some occasional leaf loss isn’t a cause for concern because of how resilient trees are. They survive for decades in the toughest of conditions without any human assistance.
However, sometimes, a tree might lose its leaves in the late summer, which might seem odd to you. It could be possible that your tree’s genetic code tells it to cut off the food supply to the leaves early, which could be normal.
But if your tree is losing a noticeable amount of its foliage when it shouldn’t be doing so, such as during the spring or summer, you may have a problem that needs attention.
Drought Stress
You usually don’t have to water your trees the way you water the plants in your garden, as trees have roots that extend deep into the soil and can draw water from reserves conventionally inaccessible to regular plants.
In most cases, rainfall will take care of your trees’ water needs. However, an unexpected dry spell during the summer can leave your tree thirsty and dehydrated. Most trees survive the typical dry spell and do so by taking steps to reduce their water expenditure. For most trees, the most effective step is getting rid of their leaves.
In addition to leaf loss, here are other indicators that a tree isn’t getting enough water:
- Curled or wilted leaves
- Brown leaf tips and edges
- Bone-dry soil
While trees can survive drought independently, they can still benefit from watering. Consider watering your tree if it hasn’t rained for a while.
Overwatering
Overwatering can also cause abnormal leaf loss. If you’re in a region that receives regular rainfall, watering your tree could result in overwatering.
Overwatering in trees is rare, but when it occurs, it’s usually due to unique factors like compacted soil or an underground waterway. Younger trees are more at risk of being damaged by overwatering.
When tree roots are submerged in water, it’s more difficult for them to get oxygen, which they need to survive. Continued lack of oxygen suffocates the roots, making them incapable of transporting water and nutrients up the tree.
Other than falling leaves, here are some signs of an overwatered tree:
- Fungal growth near the base
- Root rot
- Wilting leaves
Inadequate Sunlight
Inadequate sunlight triggers leaf loss in trees. While seasons affect the amount of sunlight a tree gets, the tree’s location can be a significant factor.
Unfortunately, there’s not much you can do if a tree’s location prevents it from getting enough sunlight, even in summer when the days are long. An exception is if the problem tree is overshadowed by a larger one, in which case you can cut some of the larger tree’s foliage to let sunlight through.
Interestingly, trees can bend and grow toward sunlight automatically, so this problem may resolve itself in time.
A Pest Problem
Bugs and insects such as caterpillars, moths, worms, and beetles naturally feed on leaves. When some insects feed on trees, the effect is insignificant.
Sometimes, however, you’ll run into more damaging pests. Sap-sucking aphids and scale insects can infest a tree and cause moderate leaf loss. These pests are so small that they may slip by you unnoticed unless you pay careful attention.
To determine whether pests are to blame for your tree’s leaf loss, check the underside of leaves, as pests tend to hide there for cover. The good news is that insecticidal treatments for most pests are readily available and easy to use.
Infections and Disease
Diseases also cause leaf loss and are, unfortunately, more difficult to deal with than pests. I recommend involving a professional, especially if it is an adult tree.
The best thing you can do to help your tree fight off the disease on its own is to take care of fundamental aspects like proper watering. Also, rake away and safely dispose of any diseased foliage that falls from the tree so that you don’t allow the disease to return the next year.
Final Thoughts
Deciduous trees drop their leaves in the fall because keeping them becomes unsustainable. The species of a tree determines when it will lose its leaves, but environmental factors have a major role to play as well.
Light, temperature, wind, humidity, air quality, and soil quality all influence when a tree lets go of its leaves. Trees under environmental stress will likely drop their leaves sooner to reduce energy expenditure.
If a tree on your property is losing leaves when it shouldn’t, check for water-related problems, sun deprivation, pests, and diseases.
