Leaves falling from tree

Plant Dropping Leaves – Why A Plant May Be Losing Leaves

When leaves drop, it can be quite disheartening, especially if you don’t know why it’s happening. While some leaf loss is normal, there may be many reasons for a plant losing leaves, and not all of them are good. In order to pinpoint the likely cause, it helps to thoroughly examine the plant and take note of any pests or environmental factors that may be affecting its overall health.

Common Reasons for a Plant Dropping Leaves

Leaves drop for many reasons, including environmental stress, pests and disease. Listed below are some of the most common causes for leaves falling off.

Shock – Shock from transplanting, repotting or dividing, is probably the number one reason for leaf loss in plants. This can also be true of plants going from an indoor environment to an outdoor one and vice versa. Fluctuations in temperature, light, and moisture can have an adverse effect on plants, especially as they are transitioning from one environment to another—often resulting in the loss of foliage.

Weather and Climate – As with environmental changes that can lead to shock, weather and climate play a huge role in causing leaves to fall. Again, temperatures can greatly affect plants. A sudden change in temperature, be it cold or hot, can lead to foliage turning yellow or brown and dropping off.

Wet or Dry Conditions – Many plants will drop their leaves as a result of overly wet or dry conditions. For example, overwatering commonly results in leaf yellowing and the dropping of foliage. Dry, compacted soil can have the same outcome, as roots become restricted. To conserve water in dry conditions, plants will oftentimes shed their foliage. Overcrowded container plants may drop leaves for the same reason, giving a good indication that repotting is necessary.

Seasonal Changes – The changing of the seasons can lead to the loss of leaves. Most of us are familiar with leaf loss in fall, but did you know that it can also occur in spring and summer? It is not uncommon for some plants, like broad-leaf evergreens and trees, to shed their oldest (often yellowing) leaves in spring to make room for regrowth of new, young leaf tips. Others do this in late summer/early fall.

Pests and Disease – Finally, certain pests and disease can occasionally cause leaf drop. Therefore, you should always examine the leaves carefully for any signs of infestation or infection whenever your plant is losing leaves.

No, autumn hasn’t come early. The leaf fall is mostly due to the lack of rain during the past few months.

In the autumn, trees begin their preparation for the extremities of winter. “There is a net cost to maintaining a canopy full of leaves,” says Nick Brown, a plant biologist at the University of Oxford. When temperatures drop to below 5C, photosynthesis is so difficult that it’s not worth having leaves.” By letting the leaves fall, trees slash their energy and nutrient expenditure.

Temperature and day length are the two main cues trees use to work out when to shed their leaves. They sense day length with two chemical photoreceptors: phytochrome measures red light and cryptochrome measures blue light. They can detect changes in day length of as little as half an hour.

As soon as the temperature and light levels fall below a certain level, the tree knows it is autumn and starts to recycle the nutrients in its leaves.

“There’s lots of things in a leaf that a plant really needs and can easily recycle,” says Brown. As the leaves lose their nutrients to the tree, they return to their natural colour. “In a sense, the leaves aren’t turning yellow, the green colour is going away,” says Brown. When the leaf dies, it turns brown.

Of course, this normally happens in late September. The recent abnormal leaf fall has its roots in last year’s winter, which was wetter than normal. “As a result, trees’ root and shrub systems were impaired by drowning,” says Guy Baker, an adviser at the Royal Horticultural Institute. They have been under more stress due to the recent dry summer.

To conserve water, the trees have sucked out whatever is in their leaves. With no water, a leaf will die. The result is dead brown leaves in August.

Brown says trees can also be tricked into losing leaves by extreme environmental conditions. Volcanic dust, bad ground frost or storms can make trees shed their leaves.

Another factor is climate change – some species simply can’t keep up with the current rapid changes in global weather patterns. “Climate is changing rather more rapidly than plants are getting an opportunity to adapt to,” says Brown.

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  • In nature, most trees will lose their leaves during fall and overwinter and everyone knows you should expect this, however, there are times when leaves will drop off when you don’t expect them to and there are a number of reasons for this.

    The exception to the rule for leaves falling off trees is, of course, evergreen trees and just like the name suggests this variety of tree should stay green forever, if it doesn’t there is a big problem.

    In this article I am going to look at reasons why leaves fall off trees at times you don’t expect and what if anything you can do to fix this problem.

    Why Do Leaves Fall Off Trees In Summer

    So anyone that knows anything about trees knows that during the summer months the leaves should be lush and green and definitely not falling off, that supposed to happen in fall.

    If any significant amount of leaves are falling off your tree during the summer it suggests that there is an underlying problem as it is not meant to happen.

    If trees become stressed out during the summer due to excessive heat or drought this can be one of the reasons he will start to drop its leaves, this is the tree going into survival mode as it puts all of its energy into keeping its core (the roots and trunk) alive.

    When the leaves are falling off your tree during the summer another problem that could cause this would be if your tree is suffering from a fungal or bacterial infection and your tree could be putting all of its energy into fighting the infection rather than keeping its leaves green and healthy.

    A bug infestation could have a similar effect to a bacterial infection as in it could be the tree is putting all of its energy into fighting the infestation and not so much energy into keeping its leaves green which could result in leaves dropping off.

    Why Do Leaves Fall Off Trees In Spring

    If you have a tree and you notice the leaves are falling off in the spring there could be a number of reasons for this, some are perfectly natural like a late frost being the cause and some are more of a cause for concern.

    Have you recently fed your tree with fertilizer as this could be a reason for the leaves to fall off in spring as if you’ve over-fertilized your tree they will sometimes produce more leaves than it can actually support and this could result in some of them falling off.

    Some trees actually naturally drop their leaves during the spring so if you have a Hackberry, Hickory, Holly or Live oak tree you have nothing to worry about leaf drop during the spring as it is the time these varieties of tree are supposed to shed leaves.

    If you don’t have a tree variety that is supposed to drop its leaves during spring and you haven’t over-fertilized yet leaves are falling off you have a problem.

    Spring leaf drop is common if your tree is infected by a fungal disease and these diseases thrive in damp cool weather that is common in many places during spring.

    Why Are Leaves Turning Yellow And Falling Off

    There are lots of reasons that could be turning the leaves on your tree yellow and making them fall off.

    One of the most common reasons for leaves to turn yellow this is either underwatering overwatering your tree identify this as the problem all you have to do is change your watering routine and it should be problem solved.

    Another common reason for leaves to turn yellow on a tree especially in container trees is a nutrient deficiency, this is because your tree constantly feeds off nutrients in the soil so that means they will eventually run out and you have to replace them by using a fertilizer of some sort.

    One other reason that may cause your leaves to turn yellow and fall off is if the pH in your soil has changed to the wrong level for your tree, to diagnose this problem efficiently the only way to do it is to use a pH test and find out what the levels are then adjust them if need be.

    Bugs may be the reason your leaves are turning yellow and falling off, you really should inspect the underside of the leaves to make sure you don’t have some sort of bug infestation as this could be a reason for leaves to turn yellow and fall off.

    Why Do Green Leaves Fall Off Trees

    If your tree has leaves falling off that are still green, there could be a number of issues that are affecting your tree.

    If there has been a late cold snap this could cause your tree to go into shock and shed some of its leaves.

    Some trees like a hackberry tree are prone to shedding leaves at times you don’t expect, maybe the variety of your tree you have will just naturally shed leaves when you don’t expect it to.

    If you are in an area that has been suffering from drought this can cause a tree to get stressed and this may result in leaves falling off, if this is the case, you should not panic just keep your tree well-watered and it should recover.

    Causes Of Leaf Drop In Container Plants

    If you have plants in a pot or container and the leaves start to drop off it can be a real cause of concern.

    It is a nightmare situation when you put lots of time and effort into growing a plant or tree and the leaves start falling off at a time of year they should not be. There are lots of things that can cause leaves to fall off, some are self-inflicted and some are misfortune.

    Here is a list of some causes of a container plant to suffer leaf drop:

    • Overwatering a plant can cause the leaves to drop off
    • Not watering a plant enough can cause the leaves to drop
    • Watering with water that is too cold
    • Sudden change in temperature i.e moving a plant indoors/outdoors
    • Repotting a plant can cause stress and leaf drop
    • Sudden change in the amount of light your plant receives
    • If your plant has outgrown its container
    • Some diseases or pests can cause leaf drop
    • Not enough nutrients in the soil
    • Children or pets touching the plant too much

    That is only some of the more common reasons that can cause the leaves on plants or trees to fall off.

    Most of these reasons are from plants suffering from stress or shock, this happens when you change the environment your plant is growing in suddenly by moving it indoors or outdoors for example.

    What To Do If Your Container Plant Starts Dropping Leaves

    If your container plant starts to drop its leaves you have to consider whether you have drastically changed the conditions it is growing in.

    If you have moved your tree it may be in shock from having less or more light or from a hotter or colder temperature.

    Similarly, if you have repotted your tree it may have gone into transplant shock and this is not uncommon.

    If there are no leaves left on your tree you will have to figure out whether it is in shock or if it is dead, to do this just snap a branch and if it is green and moist underneath then your tree is in shock and can be recovered.

    To recover your tree I would recommend adding a layer of mulch to the surface of the soil as this should add some natural nutrients back into the soil.

    Make sure you keep your soil moist but at the same time you don’t want to be overwatering it as if you do that you could finish your tree off.

    The main thing you will need to save your tree is patience, it can take between 1-5 years for a tree that has gone into transplant shock to fully recover.

    Just care for your tree as normal and it should fall back into its natural routine as the seasons change.

    November 2017 on Maple Street in Johnson City, Tennessee. Image via Teri Butler Dosher.

    In temperate forests across the Northern Hemisphere, trees shed their leaves during autumn as cold weather approaches. In tropical and subtropical forests, trees shed their leaves at the onset of the dry season. Many types of trees shed their leaves as a strategy to survive harsh weather conditions. Trees that lose all of their leaves for part of the year are known as deciduous trees. Those that don’t are called evergreen trees.

    Common deciduous trees in the Northern Hemisphere include several species of ash, aspen, beech, birch, cherry, elm, hickory, hornbeam, maple, oak, poplar and willow. In tropical and subtropical regions, deciduous trees include several species of acacia, baobab, roble, ceiba, chaca and guanacaste.

    Image via Tosca Yemoh Zanon in London

    Photo via Daniel de Leeuw Photography

    Most deciduous trees have broad leaves that are susceptible to being damaged during cold or dry weather. In contrast, most evergreen trees either live in warm, wet climates or they have weather-resistant needles for leaves. However, there are exceptions in nature, such as tamarack trees that shed their needles every autumn and live oaks that retain their broad leaves for the entire year even in relatively cool climates.

    Shedding leaves helps trees to conserve water and energy. As unfavorable weather approaches, hormones in the trees trigger the process of abscission whereby the leaves are actively cut-off of the tree by specialized cells. The word abscission shares the same Latin root word as that in scissors, scindere, which means “to cut.” At the start of the abscission process, trees reabsorb valuable nutrients from their leaves and store them for later use in their roots. Chlorophyll, the pigment that gives leaves their green color, is one of the first molecules to be broken down for its nutrients. This is one of the reasons why trees turn red, orange, and gold colors during the fall. At the end of the abscission process, when the leaves have been shed, a protective layer of cells grows over the exposed area.

    Layer of abscission cells separating a leaf from its stem. Image Credit: U.S. Forest Service.

    The shedding of leaves may also help trees to pollinate come springtime. Without leaves to get in the way, wind-blown pollen can travel longer distances and reach more trees.

    Autumn leaves. Image Credit: Tracy Ducasse.

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    Bottom line: Many types of trees shed their leaves as a strategy to survive cold or dry weather.

    Deanna Conners is an Environmental Scientist who holds a Ph.D. in Toxicology and an M.S. in Environmental Studies. Her interest in toxicology stems from having grown up near the Love Canal Superfund Site in New York. Her current work is to provide high-quality scientific information to the public and decision-makers and to help build cross-disciplinary partnerships that help solve environmental problems. She writes about Earth science and nature conservation for EarthSky.

    In many parts of the country, Fall is a time when leaves turn brilliant colors and make for a spectacular sight to see! Surprisingly, most of these amazing colors have been in the leaves all Summer long, however, we don’t see them because they have been masked by the dominant green color from chlorophyll, a photosynthetic pigment that turns sunlight into food. As temperatures become cooler in the Fall, the leaves are not able to produce as much food for the tree and the tree begins to rely on stored sugars to last through Fall and Winter. As this happens, the chlorophyll breaks down and the green leaf color goes away revealing new, amazing colors!

    Why are there different colors of leaves?

    In the Spring and Summer months, most trees display their beautiful green leaves. This green color comes from chlorophyll, a pigment found in plants that helps convert sunlight into food energy. When temperatures become cooler in the Fall and chlorophyll breaks down, other colors in the leaves appear.

    This colors visible in the Fall include the following:

    Red: The red color in leaves is due to anthocyanin. Unlike other leaf colors such as orange and yellow that are always present in the leaves (but hidden by the dominant green color most of the year), anthocyanin is produced by the tree when the chlorophyll breaks down and the green color goes away. Brilliant red colored leaves can be found in the Fall in Red Maples and Scarlet Oaks, among other special trees.

    Orange: The orange color seen in leaves is due to carotene. This spectacular color is one of many things that makes the Sugar Maple tree so special (carotene is also the same chemical responsible for giving carrots their unique orange color). The dominant brilliant orange displayed by the Sugar Maple tree fills the hills of the Northeast in the Fall and is a beautiful sight to see!

    Yellow: The yellow color seen in leaves of certain trees such as ash, beech, birch, aspen, and some oaks comes from xanthophyll, a yellow pigment that occurs widely in nature. This brilliant yellow color goes on full display throughout the Rocky Mountains in late September and into early October and can also be found in other places throughout the U.S. In addition to being present in trees, xanthophyll is also found in squash and corn!

    Why do trees drop their leaves?

    Deciduous trees, such as ash, beech, elm, aspen, cherry, oak, maple, willow, poplar, and birch trees, shed their leaves each Fall to prepare for the Winter. Evergreen trees are different and keep their green foliage year-round. Evergreens can do this because most live in warm or wet areas, or they have narrow, weather resistant needles rather than large leaves that are present on deciduous trees. These large leaves require a significant amount of energy to maintain.

    As the weather changes and there is less daylight, hormones in deciduous trees let it know it is time to start the process to shed its leaves for the Winter. At the beginning of this process, the tree will absorb some of the nutrients from the leaves and store them so they can be used later. Then, tree hormones trigger a process whereby the tree uses its cells to form a bumpy line at the place where the leaf stem meets the branch. The leaves are then actively “pushed” or “cut” off the branch. After the leaves are removed, most trees will grow a protective cell layer over the exposed area.

    Below are three main reasons why trees drop their leaves in the Fall:

    • To Survive Upcoming Harsh Weather Conditions. The main reason why trees shed their leaves is part of a strategy to survive upcoming harsh weather conditions. Most deciduous trees display broad leaves that can be damaged with cold or dry weather. Alternatively, evergreen trees keep their leaves year-round with their hardy, weather-resistant needles as leaves.
    • Conserve Water and Energy. Dropping its leaves in the Fall helps a tree conserve water and energy. This helps the tree focus its energy on root growth in the Fall and helps it survive harsh Winter weather.
    • Helps with Pollination. After trees shed their leave and when springtime comes around, it is much easier for pollen to be carried by the wind and reach more trees. This helps the pollination process!

    Even though deciduous trees become dormant and are without leaves, they are still active – efficiently using their energy and resources to expand their root systems. A strong root system will better position the tree to grow in the Spring. Also, dropping its leaves creates a layer of mulch that covers the surface of the growing environment, which insulates the soil and keeps it moist.

    Why is Fall a great time to plant trees?

    Many arborists and other tree experts consider Fall to be one of the best times of the year to plant trees. But why? The reason is simple – in the Fall, trees have less to do and can spend their energy on becoming established. Deciduous trees, such as maples, oaks, cherry, poplar, birch, elms, and willows, shed their leaves in the Fall and use their energy during this time on growing and expanding their root systems. With a larger and more expansive root system, the tree will have greater access to water and nutrients when it is ready to add leaves and mass above ground in the growing season. Moreover, in the Fall and Winter, a newly planted tree can devote its energy to its roots, whereas if it was planted in the Spring or Summer, the tree would need to divide its energy and resources to also support its leaves and branches, which can add stress.

    In addition, the cooler temperatures present in the Fall promote and encourage underground root growth, compounding the effect. And, as the weather cools, trees no longer have the stress that can occur with extreme heat and dry conditions and the Fall window presents a great planting opportunity before potentially harsh Winter conditions set in.

    In summary, Fall is a great time of year to plant a tree! If you had a loved one (or pet) pass during the year, Fall is also a great time of year to plant a living tree memorial with The Living Urn’s bio urn and planting system. This patented system allows you to grow a beautiful and enduring memory tree from the special tree urn containing cremated ashes. With The Living Urn, families can honor a loved one and give back with a special living memorial.

    Why Leaves Fall from Trees in Autumn

    Fall color enriches a road near Tully, New York. Yellow and gold pigments are present in tree leaves throughout the growing season but are masked by green pigments.

    French author Albert Camus tells us, “Autumn is a second spring when every leaf is a flower.” And what opulent bouquets the leaves of autumn do create—whole mountainsides of dazzling color. But nature has a way of combining the sublime with the practical, and the fiery reds, yellows and oranges of autumn trees stand out as an excellent example of this convergence.

    Why Falling Leaves?

    As winter descends, trees in temperate and boreal zones face punishingly cold temperatures and frigid winds, conditions that would damage leaves, so trees have to reduce themselves to their toughest parts—stems, trunks, branches, bark. Leaves must fall.

    Evergreens can hang on to their leaves through winter, because their foliage is coated in a wax that helps protect against cold, and their cells bear anti-freeze chemicals that ward off winter’s worst woes. Not so for broadleaf, or deciduous, trees. The fluids that flow through their leaves are thin and susceptible to freezing, the tissues tender. Winter cold dooms the leaves, and trees save energy by getting rid of them. Let’s take a look at the process.

    As the dark mantle of winter’s dwindling days falls, trees can sense the loss of light. Thanks to chemical light receptors—phytochrome, which detects red light, and cryptochrome, which is sensitive to blue—trees can register day-length changes of as little as half an hour. When they do, they undergo chemical and physical changes that produce autumn hues.

    How Pigments Play a Part

    It’s all about chlorophyll—the green pigment that allows plants to absorb sunlight and turn it into food that can be stored for winter dormancy, much as a bear stores fat for hibernation. During the growing season, trees create chlorophyll as fast as they use it up, so leaves stay green. But as daylight declines, trees slow the production of chlorophyll until, finally, it stops. Producing more would be a waste of energy because, as temperatures near the freezing point, the process of photosynthesis slows to impractical levels.

    While the green pigment ebbs from the leaf, other pigments hidden in the greenery during warm months begin to appear. Carotenoids—which produce the yellow, orange and brown colors in the flowers of daffodils and the roots of carrots, in the rinds of pumpkins and the peels of bananas—are present in leaf cells throughout the growing season, but they’re masked by the green pigment. Once the chlorophyll disappears, the carotenoids give leaves a burst of color.

    Trees produce another pigment group, the anthocyanins, primarily in autumn. These pigments give red and purple to such things as blueberries, cherries, red apples, concord grapes, and plums. And autumn leaves. Possibly, their presence helps to lower the leaf’s freezing point, giving it some protection from cold and allowing leaves to remain in place longer, giving trees more time to absorb nutrients.

    You can tell by the color of an autumn leaf what kind of pigment a tree specializes in. Oaks, dogwoods, black tupelo and some maplestend to turn red, brown or russet because they produce a lot of anthocyanins. Hickories, aspensand some maples (the most erratic of fall trees, obviously), are big on carotenoids, leading to the trees’ brilliant golds and yellows.

    The precise timing of the color shift apparently is genetically controlled. Oaks, for example, are among the last trees to change color. Others, such as sourwood, flip the switch to fall colors as early as August. However, weather and soil moisture can affect the quality of fall color. A severe summer drought can delay fall color by a few weeks. A warm spell in autumn also tones down autumn colors.

    Best Weather for Colorful Falls

    Michigan’s Upper Bond Falls shows why fall scenery is sought after by tourists. Good places to find it include most of New England, the Appalachian Mountains and, for golden aspens, the Rockies.

    The most-dazzling displays of scarlet and crimson occur in autumns marked by warm, bright days and cool, crisp nights with temperatures above freezing. Sun-lit autumn days stimulate leaves to produce sugars, and chill nights close the veins leading into and out of the leaf, locking in the sugars—which in turn lead to the production of anthocyanins and their crimsons and violets. Yellow and gold colors vary little from year to year, however, because leaves contain carotenoids at all times.

    Losing Leaves

    Eventually, autumn leaves must fall. By the end of summer, they may be damaged by insects, disease or general wear and tear and ready for renewal. They are equipped to self-destruct. At the point where leaf stem meets twig or branch is an array of cells called the abscission layer. As autumn days shorten, this layer begins to choke off the veins that move water into the leaf and food into the tree. Once the leaf is completely choked off, the layer becomes dry and flakey and, through decomposition, detaches the leaf from the tree.

    The Autumn Gardener

    Nature seems to abhor waste, so it is no surprise that though leaves may fall to earth, they still have not outlasted their ecological role. As they decompose, their nutrients trickle into the soil and feed future generations of plant and animal life. Quite likely, fallen leaves are a key factor in the survival not only of trees, but of forests as a whole.

    This means that you need not militantly rake up every fallen leaf. In fact, leaving them on the ground is actually a very good thing to do for wildlife.

    With their leaves gone, the trees are ready to take on winter’s slings and arrows. Naturalist Henry David Thoreau imagined it this way in his journal entry for October 29, 1858: “Nature now, like an athlete, begins to strip herself in earnest for her contest with her great antagonist Winter. In the bare trees and twigs what a display of muscle.”

    Still waters in the north woods give a double view of autumn terrain. Trees colors in fall are often brilliant and beautiful, but they serve a practical purpose: helping trees conserve energy.

    That display of botanical muscle might still need a bit of help from the gardener. Water trees and shrubs through autumn, so they can begin winter with a head start on moisture. Even in winter, trees, especially young ones, can benefit from watering every three or four weeks when temperatures are above freezing. Dousing them early in the day gives them more time to absorb the water before night freezes the soil. As cold weather begins, wrap tree trunks with crepe-paper tree wrap or burlap until spring to prevent sun scald, which occurs when sunlight on a subfreezing day warms a tree trunk to as much as 40 some degrees above freezing, allowing ice to form in the tree cells during night cold and producing dead tissues that in spring will crack open.

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    Curious Kids is a series for children. If you have a question you’d like an expert to answer, send it to [email protected] You might also like the podcast Imagine This, a co-production between ABC KIDS listen and The Conversation, based on Curious Kids.

    Why do leaves fall off trees? – Emma, age 5.

    Great question! The short answer is that leaves fall off trees when they aren’t doing their job any more.

    A leaf’s job is to turn sunlight into food for the tree. To do this, the leaf needs water. This water comes from the soil, and is sucked up through pipes in the trunk and branches all the way to the leaves – this can be a very long way for tall trees!

    It’s a very long way from the ground all the way up to the leaves of these gum trees! Flickr/Geoexplore, CC BY

    If there isn’t enough water, the leaf can be damaged and stop working. The tree doesn’t want to waste all the good things in the leaf, so it takes the nutrients from the leaf back into the stems and roots. This way, they can be recycled.

    When the leaf is empty, the tree stops holding onto it and it falls to the ground, or blows away in a gust of wind.

    Baca juga: Curious Kids: how can a tiny seed actually grow into a huge tree?

    What are deciduous trees?

    Some trees lose their leaves every year. These trees are called deciduous trees, and they lose their leaves in response to the seasons. Deciduous trees mostly come from places where winter gets cold and snowy.

    When it is very cold, the water in the tree can freeze – the leaves stop working and can even be damaged by the ice crystals. These trees know to prepare for this, and start taking nutrients out of the leaves when the days get shorter in autumn – this is when we can see them changing colour.

    It makes sense for trees to lose their leaves before winter in places where it gets very snowy. Flickr/Aine, CC BY

    But there are deciduous trees in tropical places where it never gets cold. Winter in these places is very dry. When the rainy season ends, the tree knows that it will not have very much water for a few months, so it lets go of its leaves.

    Trees hibernate too

    When the tree is leafless, it can’t make food. But it doesn’t get hungry. Instead, it rests.

    Just like a bear goes into hibernation and snoozes all through winter, trees have a long sleep until the water in the pipes starts moving again. This can be in spring, or when it starts to rain again. Then, they wake up and put out new leaves, so they can start making food again.

    Some trees hold onto their leaves all year long. These trees are called evergreens, because they stay “ever green”. But the leaves on these trees all die and fall off eventually. That happens when the leaves are old or damaged. Leaves don’t work very well after they’ve been munched on by an animal.

    Leaves are really important for the tree, but sometimes it’s better for the tree to let them go. They can save all the good bits and when there is enough water, they can use them to grow brand new leaves.

    Baca juga: Curious Kids: Where did trees come from?

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    ‘Field Notes:’ What Determines When Leaves Fall?

    Listen Listening… / 2:41 ‘Field Notes:’ What Determines When Leaves Fall?

    Every autumn, deciduous trees and shrubs shed their leaves. In some years, leaves are shed earlier than in others. Why does this happen? What determines when leaves fall?

    Leaves are organs that enable plants to capture sunlight and turn it into useable energy in the form of sugars and other carbohydrates. In much of North America, however, winter is too cold for these functions, so most plants become dormant.

    Leaves serve no purpose when trees are dormant, and they may be a liability. Strong winds and heavy snow can put tremendous pressure on limbs that have leaves. Some of the worst tree damage occurs during late-spring snowstorms when wet snow collects on trees that have already leafed out. So for trees that produce broad leaves, it’s better to spend the winter bare.

    As autumn progresses, trees prepare for winter by stopping the production of chlorophyll, the green pigment that captures light. The leaves gradually change color as nutrients are withdrawn and transferred to roots and stems. At the same time, enzymes digest the cells at the base of the leaf stalk forming an abscission layer or scar. When digestion is complete, the leaf falls off.

    Dormancy is brought on by a change in the levels of plant hormones. Short day length is the most important environmental cue that stimulates the onset of dormancy and leaf fall. For that reason, in cities, trees closest to street lamps are often the last to lose their leaves. Drought will also hasten the onset of leaf fall, causing trees to shed their leaves earlier following a dry spring and summer. On the other hand, superabundant watering or hard pruning stimulates vigorous growth which delays the onset of dormancy.

    Some years, you may notice that many trees continue to hold their leaves much longer than usual. A wet spring and early summer can stimulate growth and delay the onset of leaf fall. If an abnormal period of prolonged cold weather hits before most trees drop their leaves, leaf tissues can freeze and be killed before sufficient enzymes are produced to cause abscission and leaf drop. As a result, many trees may enter winter still carrying some of their leaves. Trees that have been overly pruned and watered can hold onto almost all their leaves, and are most at risk of breakage from severe winter storms. Leaves without abscission layers will eventually fall, and unless next year’s are damaged by an unseasonable cold snap, we can be sure to see new leaves appear again in the spring.

    “Field Notes” is produced by the Montana Natural History Center.

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