Most trees have wide, flat leaves that are perfect for capturing the sunlight they need to photosynthesize and produce food. Although leaves vary from plant to plant, very few have developed needles the way pine trees have – so why is that? Why do pine trees have needles?
Pine trees evolved needles in a response to their environment. Pine needles are much more durable than normal leaves, which is why pine trees do not shed their needles each winter, but can instead keep them year after year. Pine needles are better at withstanding frost and will hold onto more water than standard leaves, making them ideal for cold environments.
What Are The Biggest Advantages Of Pine Needles?
Let’s look at the advantages that are granted by needles in more detail, so we can understand why pine trees evolved this way. They are so different from the leaves that most other trees have, and there are some good reasons for that.
Needles Withstand The Cold Effectively
One of the biggest reasons that conifers have evolved needles and that needles are so useful is that they are very good at withstanding the cold. They are much thicker than standard leaves, and this gives them superb cold resistance.
They are also waxy, which means that the cold does not penetrate them as much, and the way they cluster traps air and reduces the chill of the wind.
In a flat leaf, the water is close to the surface and will freeze readily, destroying the leaf’s delicate structure. In a pine needle, because the leaf is thicker, the water is less exposed to freezing temperatures and may not freeze as readily, or do as much damage to the tough leaf if it does.
All in all, they are very good in cold conditions, where survival depends on the tree’s leaves not getting ruined by frost.
Needles Don’t Need To Be Shed Annually
You are probably well aware that most trees shed their leaves when the cold weather starts – some much earlier than others. Pine trees, however, retain their needles, which may last for around four years, after which they will probably be replaced with fresh ones.
The annual leaf shedding takes up an enormous amount of a tree’s energy. Every spring, despite a winter without food, it has to pour its resources into producing new leaves so that it can start to photosynthesize again and keep growing – which is a massive waste of energy.
Conifers, by contrast, do not gain as much energy because they don’t have broad, flat leaves that capture a lot of sunlight, but they do save a huge amount of their resources because they don’t have to replace their entire stock of leaves every year.
Where conditions are harsh, with long winters and short summers, this is a far better approach. It allows trees to make the most of the sunny days, and they don’t have to shed their leaves afterward to save energy through the dark months – so this is great.
In such environments, the payoff of broad leaves is reduced because the summers are so short, while the cost of shedding and regrowing them is increased because the winters (when the tree has no leaves) are much longer.
This is why pine trees survive in cold conditions much better than other kinds of trees. If you have ever spent time in a cold country, you will be aware that pine trees flourish in this sort of environment, and little else does.
Needles Retain More Moisture
Needles have a further advantage over the leaves of normal trees; they hold onto moisture better.
Normal leaves lose a lot of moisture. This means that the tree has to constantly suck up more through its roots and transport it through its stem, and a significant quantity will be lost to the surrounding air.
This is not the case with needles. They are designed to prevent water loss and have three advantages over standard leaves:
- They are thin – this reduces their surface area, so there is less space for water to evaporate from.
- They have a waxy coating – which allows them to retain water very well. If you have ever tried to soak a pine needle, you have probably noticed that they are quite buoyant and water resistant, and they never really become saturated. This waxy coating forms a barrier that prevents water from being lost to the surrounding air.
- The stomata are positioned differently – the stomata are the openings where leaves can exchange gas, but through which water is also lost. On most leaves, these are exposed to moving air currents all the time, and so more moisture is lost. On pine needles, the stomata are set inside pits, and this means that the air they are in contact with is unmoving most of the time. This reduces evaporation.
Different kinds of pines have their stomata organized in different ways, but all are tucked away, unexposed to moving air that could quickly strip water from the leaves.
This water resistance also adds to their ability to withstand cold weather well and helps the tree to survive even when all the water is frozen and therefore unavailable for the roots to absorb.
Where a tree with normal leaves might run out of water and die, a tree with pine needles will retain much more of its water stocks and should be able to wait out a freeze effectively.
That makes a massive difference in freezing conditions, and could also help pine trees to survive in droughts or dry environments too. Needles may not take in light as efficiently, but they do give the tree an advantage in terms of moisture retention.
The disadvantage of having thinner leaves is slower growth, while the disadvantage of losing water could mean dying of thirst – so pines have opted for the slow and steady that increases their chances of survival when water is scarce.
Needles May Reduce The Damage Done By Storms
This might sound like a strange one, but it’s true – needles can help pine trees to handle weather storms. Again, we can see how pine trees have evolved to survive in extreme conditions and withstand difficulties.
So, why do the needles help?
Because they are thin, the wind can whip past them and they will create minimal resistance. Flat, broad leaves are much more wind resistant, and this means that trees that have them get buffeted by strong winds to a far greater degree.
When a strong wind blows against an oak tree, it will rock and groan because the wind is catching in its leaves and every single one is a broad, resistant surface. This puts the tree under a lot more strain.
By contrast, when the wind hits a pine tree, it can whistle straight through its foliage. The only broad face is the trunk, so there is a lot less strain on the tree. None of the needles will produce any significant air resistance when subjected to a strong wind.
In some instances, this could mean the difference between staying upright and being uprooted by the storm.
Needles Are Less Appealing To Insects
What about in terms of predators? Well, we all know that insects are not fussy feeders and different bugs specialize in eating all kinds of different leaves. Every kind of tree has different insects that specialize in turning it into their breakfast.
Obviously, this is harmful to the tree, but there isn’t much they can do about it. Green leaves are simply attractive to insects and they will get eaten.
Pine needles are not very appetizing to most insects, perhaps because they are tough, dense, and waxy. Where most leaves are soft, thin, and easy to eat, pine needles are a much harder meal to tackle.
Of course, there are still insects that feed on pine needles, but they are quite specialized. Pine needle weevils, pine butterflies, the silver spotted tiger moth, and conifer sawflies all attack the needles, but most insects will leave them alone.
The waxy coating is probably a big part of this, as it may be difficult to bite through and digest for insects that haven’t specifically evolved to do so. That means that pine trees are much less troubled by grazing insects than many other kinds of trees.
Conclusion
Pine trees have evolved to withstand harsher climates than many other kinds of trees. They are ancient, and this evolution would have been much needed on a colder planet, where water would have been in short supply, and storms might have been considerably more common.
The needles help pine trees to conserve water, withstand the cold, hold up to storms, and avoid excessive damage from most insects. They also let a tree keep its energy reserves high because it doesn’t need to grow new leaves every year. While broad, flat leaves may have some advantages, needles are clearly a very viable alternative!