Going outside in autumn, you’ll notice trees covered in leaves of all sorts of colors — yellow, orange, purple, brown, and of course, green. Many trees will be barren, whereas others will spring with life and display lush green foliage. But why do some trees still have their leaves?
Some trees don’t lose their leaves seasonally because they keep replacing dead leaves with new ones in a constant cycle of renewal. This keeps them green year-round. Evergreen trees will feature adaptations such as needle-like leaves that allow them to keep their foliage year-round.
In this article, I’ll explain why some trees don’t lose their leaves and how they differ from those that do. I’ll also dive into how these trees can get by without losing their leaves when the process seems to be a natural part of a tree’s life cycle.
Why Do Some Trees Stay Green Year-Round?
Evergreen trees, as the name implies, are always green. The question is: how can they keep their green foliage in tip-top condition during fall and winter when it seems like all other trees have simply dropped their leaves and gone to sleep?
To get to the answer, we must first understand why most trees lose their leaves in fall in the first place:
Why Deciduous Trees Lose Their Leaves
Deciduous trees are the counterparts of evergreens. They’re similar in most ways, but they lose their foliage regularly. Most deciduous trees lose their foliage sometime between early fall and mid-winter, but there are some exceptions.
On a surface level, it seems pointless for a tree to drop all its leaves in the fall, only for it to spend energy regrowing them in the coming spring. However, there’s actually a very good reason why this happens.
Deciduous trees drop their leaves in the fall to give themselves the best odds of survival in the upcoming winter.
See, fall and winter are two very harsh months for plant life:
- Sunlight intensity and day length decrease
- Freezing temperatures kill leaf cells
- Windy storms can blow trees over
- Underground water freezes
- Rainfall decreases
- Precipitation comes as snow
Let’s talk about sunlight since that’s one of the more important factors.
Sunlight is used in photosynthesis to convert nutrients gathered from the soil into sugar — something trees can use as an energy source. The sunlight needed for this process is captured and absorbed by chlorophyll, which is the green pigment present in leaves.
Leaves are food production organs. You can think of them as little food factories that run overtime during the summer when sunlight is plentiful but stop being productive around fall.
Productivity only declines further as winter sets in. They’re also the live parts of the tree that need to be fed. As the year progresses and sunlight diminishes, they produce less and less food until they’re barely a net positive.
So, from a resource-management standpoint, it becomes a better option for the tree to temporarily rid itself of its leaves and simply regrow them back later.
However, before doing so, it sucks out all the chlorophyll inside to store it for future use, which is why you see leaves turning yellow, purple, orange, and all sorts of wonderful colors.
A decrease in the water supply might also cause trees to lose their leaves seasonally. Rainfall fluctuates throughout the year. Many deciduous trees will shed their leaves during drought and regrow them in the rainy season.
As this happens, water supply also declines in the winter because of reduced rainfall and the presence of frozen underground water reserves in places where temperatures drop far below freezing. So, it makes sense why trees shed in the fall, right before winter.
Sometimes trees lose their foliage when they shouldn’t. You can learn all about deciduous trees and why they lose their leaves at different times in my blog post: Why Do Trees Lose Their Leaves at Different Times?
Why Don’t Evergreen Trees Lose Their Leaves?
Now you know how leaf loss plays a crucial role in nature. But why don’t evergreens have to drop their leaves? Do the same rules just not apply to them?
Well, evergreens have a few tricks up their sleeves to ensure they aren’t affected by the dwindling light and water supplies the same way their deciduous counterparts are:
Evergreens Hold on to Water During Winter
Perhaps the most obvious quality that stands out about evergreen trees is the shape of their leaves. Deciduous trees have large, flat, open leaves that look very much like what comes to mind when you imagine a leaf.
Evergreens, on the other hand, have thin needles for leaves, similar to cacti. These thin leaves have much smaller surface areas than their wide-open counterparts, and surface area is directly proportional to the rate of water loss via evaporation.
In simpler words, the larger a leaf, the faster it will lose its water content. As a result, needle leaves have a much better capacity for water retention, which helps solve one of the major problems experienced by trees that stay active through winter: low water supply.
The water retention of needle leaves is further boosted by the wax-like coating that typically covers them.
Evergreens Can Withstand the Windy Winters
Low surface area also makes wind less of a problem.
Temperature differences that drive air movement (and hence, create wind) are at their highest during the winter, and winds pose more of a threat to trees that have a larger overall surface area since there’s more surface to push against.
Being vertically upright also allows a tree to achieve its maximum height, which is a huge advantage for competing with other plants over access to direct sunlight. Simply put, trees don’t want to be leaning (usually). They want to grow up straight, which is why wind is a nuisance.
Fortunately for evergreens, their needle-like leaves don’t add much surface area for the wind to push against, so they can stay relatively stable, even during the windy winters.
This becomes even more important during storms and hurricanes, which actually have the speed to topple and uproot trees.
Evergreens Are Built to Tolerate Ice, Snow, and the Cold
Broad-open leaves are exposed to the elements. Any of these leaves that live to see the worst of winter die to the freezing cold.
Winter frosts are notorious for killing off vegetation. In fact, the US hardiness zone system is dictated largely by how intense these frosts are and how cold temperatures can get.
Metabolism slows down as the temperature decreases, but that’s not why the cold is so deadly. The reason actually has more to do with the chemical properties of water.
See, unlike most elements, water expands when it freezes. When temperatures dip far below the freezing point of water (32°F or 0 °C), the water inside leaf cells freezes up.
This water expands and forms sharp crystals, which tear through sensitive cell walls and cause cells to burst open. The result is mass cell death, and frost-damaged parts of the leaf turn black a few hours later.
Needles can much better tolerate the cold, and they aren’t killed by frost in the same way for many reasons:
- There’s less surface area exposed to the cold.
- There’s less sap to freeze.
- Needle leaf cells have much better pressure tolerance than those that make up open leaves. Even if water happens to freeze internally, its expansion is less likely to tear open these more resilient cell walls.
- Evergreen trees have water between cells. This water outside would have to freeze up before cold can get to the water inside of cells, acting as a buffer.
It’s also important to consider how evergreen trees are structured.
Deciduous trees have most of their foliage near their top to absorb the most sunlight. Evergreens, on the other hand, look like a cone. The tree is widest at its bottom and reduces to a peak at its top.
This, combined with the low surface area of needle leaves, ensures snow has a way to fall off the tree and doesn’t accumulate on its foliage.
Evergreens Continue to Carry Out Photosynthesis During the Winter
Admittedly, evergreens cannot carry out a lot of photosynthesis during winter because of the low-intensity sunlight and reduced temperature.
Still, every little bit helps when you’re starved for resources. The fact that evergreens can keep carrying out photosynthesis throughout the year means it’s worth keeping foliage around. Evergreens also have highly dense foliage built to collect as much sunlight as possible, which certainly helps.
Evergreens Can Tolerate Harsher Conditions
Evergreens, with their needle-like leaves and snow-resistant structure, can tolerate much harsher conditions than deciduous trees, which helps them keep their foliage all year round.
While deciduous trees have to drop all their leaves to preserve resources and go into hibernation mode for the winters, evergreens can soldier on through the cold while keeping their foliage in decent shape.
Sure, they get a little roughed up by the elements, but they almost always make it through winters without too much of a problem. And, as a result, they don’t have to spend a high amount of energy the consecutive spring to regrow foliage.
Do Evergreens Ever Lose Their Leaves?
Evergreen trees do lose their leaves. A typical needle leaf has a lifespan of three to four years. This is much greater than broad leaves, which don’t even last a full year.
The bigger difference, perhaps, is that needles aren’t lost all at once. Spent evergreen needles are constantly being dropped and replaced with new ones, so the overall foliage density of the tree isn’t noticeably affected by season.
Why Some Trees Hold On to Dead Foliage Over the Winter
This is an entirely different scenario, but it’s not uncommon. Sometimes, deciduous trees retain their foliage over winter. But why is this?
Trees usually hold on to dead foliage over winter because something went wrong while the leaf-dropping process was underway. As mentioned earlier, deciduous trees typically drop their foliage in the fall – sometimes later in the year, but almost always before the winter sets in.
However, this is a time-consuming process. The tree first has to suck the chlorophyll, other useful pigments, and nutritious sap out of the leaf to preserve as many resources as possible.
It then commences the development of special abscission cells. These cells create an abscission layer at the point where the leaf meets the tree. As this layer develops, the leaf loses structural support and is cut off.
There Was an Early Frost
Sometimes, when winter sets in too early, leaves can die due to frost before the process is finished.
Dead leaves cannot carry out the instructions given to them by the tree. As a result, no abscission layer is formed, and the leaf remains intact.
This is usually not problematic. The leaf will either fall off on its own, get carried off by the wind, or stay in its place until spring when new growth forces it off. And on the bright side, you can marvel at the beauty of the unusually colored foliage for a little longer.
I should mention that this process can also take place if you experience an unusually warm fall. The warmer temperatures trick the tree into thinking it’s not time to shed its leaves when suddenly, the first frost strikes.
The Tree Received Too Much Fertilizer
Nitrogen-based fertilizer, in particular, is the likely culprit.
Like plants, trees need three main macronutrients: nitrogen, phosphorus, and potassium. And out of the three of these, it’s nitrogen that fuels their growth the most.
Trees get greedy when they see lots of nitrogen in the soil and begin using it all up to grow and create more foliage.
When this happens in the fall, the tree gets conflicting signals. On the one hand, it wants to use the available nitrogen to fuel its growth, but on the other, it needs to drop leaves.
Either way, you can safely leave the tree alone. It would be impractical and inconvenient to do anything about the remaining foliage because it’s not actively doing any harm.
There are only two noteworthy problems:
- Snowfall will accumulate on the remaining foliage. Too much snow can cause branches to break.
- More surface area means more overall wind resistance. A tree with foliage is more likely to be blown over during storms or fast winds.
However, both of these scenarios are quite unlikely. So unless you’re concerned about the tree falling over and causing damage to your property, I wouldn’t sweat it.
Should I Get an Evergreen or a Deciduous Tree for My Property?
If you’re wanting to get a few trees on your property, you may be wondering whether to go for deciduous or evergreens. On the surface level, evergreens seem like the obvious answer. They’re tougher and won’t go barren during the winter, so they must be the better option, right?
Indeed, evergreens won’t lose their foliage (which can be a deal breaker in certain cases), but there’s more to consider:
- The two types look different: Deciduous trees concentrate most of their foliage at the top. Evergreen trees do so at the bottom. Which looks better is purely subjective.
- Deciduous trees provide more shade: Their broader leaves and greater top foliage allow them to shade your property during the summer while letting in sunlight during the winter.
- Evergreens make for better wind blocks: Their winter foliage allows them to shield your property against the wind when it matters the most.
- Evergreens shelter birds and animals during winter: The foliage provides shelter and insulation against the cold. This is great if you’re a bird lover and would like to see more around your property.
To be honest, you can’t really go wrong with either choice. Planting trees is a great way to make your property more lively and interesting. You and your neighbors also get improved air quality, reduced pollution, and wild shielding when you plant trees, and positioning them strategically to block sunlight will allow you to save on air conditioning.
Not to mention the increased property valuation trees can add to your home.
Many trees lose their leaves during fall and winter because of harsh conditions. However, some trees, such as evergreen trees, can keep their green foliage all year long because of a few special adaptations.
Needle leaves allow evergreens to store water and nutrients while keeping wind contact relatively low. Needle leaves are also less prone to frost damage, something that can kill broad leaves in hours.
Some deciduous trees may hold on to some leaves throughout winter if the first frost happens too early or the tree is fed too much nitrogen-heavy fertilizer. Either way, this isn’t usually a problem.