If you’ve been hiking or trekking lately, you may have noticed something peculiar about the trees around you. Trees on an inclined slope always grow straight up; they don’t lean with the slope at all. How are trees able to grow upwards on uneven ground?
Trees always grow straight up because they have gravity-sensing cells that allow them to judge the direction of gravity and grow accordingly. Trees are also encouraged to grow straight upwards to attain their maximum height and compete with other trees for sunlight.
In this article, we’ll explore the mechanisms trees use to dictate the direction of their growth. I’ll also tell you why some trees develop a lean and if there’s anything that can be done about it.
Will Trees Naturally Grow Straight?
We know that under normal circumstances, trees tend to grow straight up. One merely needs to have a look outside to confirm this. Most – if not all – trees grow vertically upwards towards the sky.
But how are they able to do so with such consistency? Well, there are a few complex but interesting mechanisms at play behind the scenes.
Let’s look at what guides plant growth.
Trees Can Detect Gravity
The ability to detect gravity and grow accordingly is called Geotropism. It’s interesting to note that trees actually have dedicated cells for detecting gravity. In fact, all plant stems grow upwards. Even mushrooms exhibit Geotropism.
These gravity-detecting cells harbor a pocket-like structure within them – statoliths. Statoliths are tiny molecules of starch that sit inside an empty-ish cell. They also serve as a secondary purpose of food storage.
The statoliths naturally fall to the bottom wall of their respective cell due to gravity. The tree understands that the cell wall currently in contact with the statolith is the direction gravity is pulling on it.
This information is quickly transported up the root by the ‘auxin’ hormone.
Gravity-detecting cells are found in pockets at the bottom tip of tree roots, the meristem. The meristem also contains stem cells, hence the name. These stem cells divide to create new root cells and elongate the root. The direction of their growth is dictated by the information they receive from the statolith-harboring cells.
While root cells are programmed to grow downwards and into the soil, the shoot cells above the soil are ordered to grow in the opposite direction, away from gravity.
The statoliths’ position inside their cell is influenced by gravity in real-time. For example, if a tree were to fall over to its side, so would the statolith inside its host cell.
This allows the tree to keep track of where the gravity is coming from and where it needs to direct future growth. Interestingly, this is also why you see the occasional bendy tree that straightens up at the top or a near-horizontal tree trunk trying to reach for the sky once again – often found in rainforests.
Larger Trees Can’t Afford To Lean
Geotropism is a well-studied behavior that has been tested extensively. Researchers have fiddled with the gravitational conditions experienced by plants to see how they react.
In one particularly interesting experiment, Dutch astronaut Andre Kuipers took some seedlings to space to see how they would respond to zero gravity.
Without gravity, the confused seedlings started growing in multiple directions simultaneously.
In this sense, it’s pretty clear why geotropism exists. A tree needs to grow upright, and because it’s so heavy, even the slightest tilt can place an immense burden on a tree’s roots.
Smaller trees, on the other hand, can get away with tilts and bends because their roots don’t have a lot of mass to support them.
Larger trees have stronger roots to help compensate, but their height will make them more unstable if they don’t grow upwards. Uncoordinated growth is not an option since the risk of the tree trunk snapping or the roots being uprooted from the soil is too high.
Non-vertical growth also prevents the tree from reaching as high as it would have if it had grown straight up. As you will see in the next section, this is extremely important, especially in dense forests.
Trees Grow Toward Light
There are only two factors that dictate tree growth on a fundamental level: gravity and light.
Light Is Important for Growth and Survival
We know that sunlight is important for trees and all plants in general. But why does it have a say in what direction trees grow?
To help you understand, I first have to put into perspective how important light is for plant life.
Trees gather nutrients from the soil via their roots. But these nutrients, in their raw form, don’t really do much for them. They must be processed and turned into a useful energy source first.
This is where photosynthesis comes in.
Photosynthesis is the process by which plants, trees, and nearly all vegetation convert nutrients from the soil into sugar. Unlike raw nutrients, sugar can be used as an easily-digestible, ready-to-use energy source.
The presence of sunlight is a key requirement for photosynthesis, without which the process cannot take place. (I say sunlight, but other light sources can also do the job.)
A tree’s leaves contain chlorophyll, which promotes sunlight absorption. Chlorophyll is also the pigment responsible for a leaf’s green color.
If you take the sunlight out of the equation, no photosynthesis occurs. A plant that never sees the sun will eventually die of starvation, no matter how fertile the soil is. It simply won’t be able to use the nutrients it gathers.
Of course, things usually don’t get to that point in nature. Even if the sun is blocked off by other trees, reflected sunlight will pick up the slack and still allow the sun-deprived vegetation underneath to convert some of its nutrients into food.
And that’s not all. Sunlight keeps the exposed foliage healthy, pest-free, and protected against infections and diseases.
Trees Can Bend Toward Their Source of Sunlight
In areas where sunlight is allowed through the canopy, you may see smaller trees leaning toward the openings, seemingly as if they’re reaching for the sun – and they are. Plants and trees can bend at will if it means they’ll be closer to sunlight.
Auxins are a class of hormones found in trees. These hormones play a major role in determining how the tree will grow, both above and below the soil. They also influence the rate at which cells divide and grow.
Auxins exist all over the tree body but are found in higher concentrations on sides that don’t get exposure to sunlight. The sun-deprived side of the tree then grows at a faster rate compared to the side that receives its fair share of sunlight.
The result? The tree gradually starts bending toward the sunlight. This behavior is called phototropism.
Phototropism is not only displayed by trees but by almost all plants and flowers. Even houseplants display this behavior; look around, and you may notice that yours seem to be growing toward the windows.
Taller Trees Compete for Sunlight
It should come as no surprise that trees compete for sunlight. Quite aggressively, in fact — especially taller trees in densely crowded areas such as rainforests.
They try to grow taller than each other and reach above the canopy. Any growth that isn’t completely vertical won’t help the tree reach its full potential and, consequently, its ability to compete for sunlight.
This aggressive competition is a major reason trees in dense rainforests are taller than anywhere else. They often sacrifice trunk width in favor of height so they can gain access to more sunlight.
Rainforests have trees as tall as 60 meters (196 ft), which is over twice as tall as the average tree. They also tend to have most of their foliage near the top since growing foliage below the canopy would provide inefficient returns in sunlight absorption.
Why Do Some Trees Have a Lean?
Even though most trees grow straight up, you’ll still see unexpected growth patterns from time to time. Why, despite the existence of gravitropism and phototropism, do some trees develop a lean?
Fast Winds
Trees that grow in naturally windy areas try to combat the force exerted on them by digging their roots deeper into the soil. However, the unprotected trunk above the soil can develop a lean, especially if the offending winds blow without pause.
Younger trees with weaker foundations are particularly susceptible to constant winds because external forces can easily influence their growth. Of course, younger trees also have thinner, less bulky trunks.
As you can tell, there’s not really much to be done about this problem. It would be impractical to protect trees against the wind, which is itself a force of nature.
The good news is that trees subject to strain from fast-blowing winds adapt and become sturdier in response to the excess pressure exerted on them. They develop special reaction wood, which has a different structural composition to regular wood.
It’s also called stress wood since trees develop it only when placed under stress.
In particularly windy areas, you’ll find trees growing in all sorts of strange directions and orientations – and these trees still rarely fall.
Natural Disasters
During storms and hurricanes, trees will experience the force of extremely fast winds, even in areas that are usually calm.
There is a high risk of trees falling during natural disasters. Trees break and collapse when wind speeds exceed 42 meters per second (94 miles per hour / 151.2 kilometers per hour).
Keep in mind that this threshold is when trees are expected to break. Winds of such speeds are still likely to create structural abnormalities in the trees that aren’t snapped in half.
A Lack of Sunlight
Lack of sunlight will cause trees to grow in unpredictable ways in search of it.
Trees next to larger buildings or structures typically have their sunlight exposure restricted to a certain direction. As a result, they only receive sun at a particular time of day. If this amount of sunlight isn’t enough, the tree will likely grow outwards to catch more of it.
Unfortunately, there isn’t much to be done in this case. With smaller garden plants and houseplants, you could just move them to a more sunny spot.
However, if one of the trees on your property is shaded by another tree’s overgrowth, you can trim the taller tree’s foliage to allow more sunlight through.
The good news is that this type of non-vertical growth usually doesn’t present a safety hazard. When a tree grows sideways in search of sunlight, it compensates for the increased stress on its trunk by creating compression-resistant wood on the side, supporting the weight of the new growth. This is also a form of reaction wood.
Unstable Soil
Trees rely on their roots to anchor them to the ground and provide resistance against external forces. What good is this anchor, though, if it can’t hold on to anything?
Unstable soil is another major reason that trees develop arches and bends. Soil fails to provide adequate support when it’s too loose. This can happen over time as the result of natural erosion.
Soil that is too compact can also make for an unstable foundation, although this is less common. Dense soil restricts root growth to the upper parts of the soil and prevents tree roots from reaching essential nutrients. If the trees can’t stretch their roots deep enough to provide a good anchor, they may not be strong enough to reach their full height.
In this case, the best thing to do is to provide the leaning tree with external support to prevent the impending fall and promote a healthier soil environment.
Disease
We think of trees as sturdy, resilient, and near-indestructible. But they, too, suffer from their own illnesses. An untimely infection, especially during the earlier years of growth, can lead to structural abnormalities down the line.
Thankfully, trees can usually hold their own against pathogens and recover in time.
Physical Damage
Lastly, physical damage can also cause trees to lean. Strong as they may be, trees are not completely safe from the destructiveness that accompanies human ingenuity.
The collapse of a nearby structure or a high-speed car crash, for example, is likely to leave most trees with noticeable damage.
If chunks of the tree trunk are ripped out, leading to a sudden loss of support, the best way to deal with this is to stake it. This is best done with smaller trees; staking large trees is far too impractical.
Staking is particularly beneficial for younger trees since it guides upcoming growth. By staking your trees, you can help them grow in a uniform and balanced manner.
What Should I Do About a Leaning Tree?
If you have a leaning tree on your property, you should consider the underlying cause of the lean and see if you can fix the problem without removing the tree altogether. Look out for the following causes to determine what caused your tree to lean:
- Passive winds: If constant winds have caused your tree to develop a lean, you may not have to remove it. Trees in windy conditions develop stress wood, which is stronger than regular wood.
- Natural hazards: Trees on your property damaged by natural hazards should be removed if the angle of their lean is too great. Slightly tilted trees may be supported using staking methods.
- A lack of sunlight: Trees that grow outwards in search of sunlight also supplement their efforts with stress/reaction wood. They are unlikely to fall.
- Unstable soil: Trees caused by unstable soil are a safety hazard. You should consider supporting the tree or having it cut down, especially if it’s uprooted on one side. However, if you can fix the soil chemistry to be more balanced and secure for the roots, you may be able to save the tree.
- Disease and trunk damage: Diseased and damaged trees are also a cause for concern. Although trees have exceptional healing abilities, they take a long time to recover. Provide support to the tree or have it cut down if the damage is too great.
Trees that develop their lean over a long period should be relatively stable because of reaction wood. However, trees that suddenly lean to either side likely have major stability issues, such as loose roots, and should be dealt with immediately.
Most experts agree that a tree with a lean of more than 15 degrees to the vertical is a problem warranting prompt attention.
Final Thoughts
Trees always grow straight because special gravity-sensing cells in their roots allow them to tell the direction of gravity. Roots grow towards gravity, whereas the shoot grows against it. This was confirmed in an experiment where seedlings under zero-gravity grew in random directions.
Trees grow straight up to reach their maximum height efficiently and compete for direct sunlight in crowded areas such as rainforests. Sun-deprived trees can bend toward their nearest light source.
Trees that develop a lean of over 15 degrees to the vertical suddenly should be dealt with immediately.
