Anyone who’s ever visited a rainforest in the tropics or the Pacific Northwest will findi it impossible to ignore how tall the trees are. A defining characteristic of a tree is definitely the fact that trees are taller than bushes — but why are trees tall?
Let’s look at why trees are tall and how some trees get so big. If you’re wondering what that tree in your backyard is, be sure to check out the guide to tree ID here.
Factors That Determine a Tree’s Height
Just like height in humans, the height of a tree is a combination of nature and nurture. No matter how well you care for it, a bristlecone pine will never get as tall as a sequoia.
Trees that have better environments and the right genetics will grow taller than starved trees. Factors that influence an individual tree’s height include:
- Lifetime. Trees never stop growing, so it’s intuitive that the tallest trees are also the oldest. Redwoods routinely live 500 to 800 years and may live several thousand years. They’re the tallest trees around at nearly 400 feet. But they’re not the longest-lived trees (that’s the bristlecone pine, which can live 5,000+ years but tops out at just 50 feet. Clearly, there’s more to the story than just lifetime.
- Genetics. It’s obvious at the species level that some species just get taller than others. It’s also likely that certain individual trees have genetics that help them get an edge and grow faster. On the flip side, some trees probably suffer from genetic drawbacks that make soaring heights impossible.
- Water supply. There’s a reason you don’t see the tallest trees in the desert, or even on mountainsides. The redwoods of northern California and the fig trees of the rainforest have something in common — ample water supply. Without water, trees just can’t put their energy towards height. They’ve got other things to worry about!
- Sunlight. If a tree is growing in the shade of a taller tree, it’s not getting all of the energy available from the sun. These sunlight-starved trees just can’t grow as tall as their neighbors.
- Sunlight direction can also influence how tall a tree grows. Trees that have unimpeded sunlight will grow straight up, while a tree that’s searching and reaching for sunlight will grow out first, rather than up.
- Nutrient availability. Soil that’s lacking in vital nutrients, like potassium and calcium, won’t support healthy gigantic trees. Nutrient-poor soils will lead to shorter trees, while nutrient-rich soils give trees a chance to grow taller.
- The right amount of wind.wind helps strengthen a tree’s trunk and prompt it to grow deeper root systems, Trees grown in perfect growing conditions in a greenhouse still won’t get at tall as trees grown in the wild. Why? It turns out that which in turn allows the trunk to support the tree as it gets taller. Just like our muscles, trees need a bit of stress to get stronger.
Let’s look at how these factors may come together to make the redwoods, sequoias, and tropical fig trees so tall. In each case, these trees live in a climate where they can grow year-round. They get ample water and enough sunlight. There’s plenty of nutrients and they have the genetics to grow tall. Finally, they live long enough to reach incredible height.
On the flip side, a bristlecone pine lives 5,000 years but spends its life in a harsh environment where it often can’t grow year-round. There’s not much water and the soil is nutrient-poor. While there’s plenty of sunlight and plenty of time in this tree’s life, it just won’t grow above 50 feet or so.
This all comes together to answer how a tree gets so big. But why is it good to be tall?
Why Do Trees Grow Tall?
The short answer to this question is competition. If you’re in a forest and you’re the shortest tree around, you won’t get much sunlight.
But if you can grow taller than your neighbors, you get more sunlight. This, in turn, gives you more energy so you can grow faster, grow more, and most importantly — reproduce more.
It’s always back to survival of the fittest, isn’t it?
Trees that grow tall can outcompete their neighbors and get to spread their genes.
Then Why Aren’t All Trees Super Tall?
You may be looking at the ash tree out your window and wondering why it’s not taller, then. It’s got plenty of sunlight and should have enough nutrients. Why isn’t it taller?
Of course, that’s partially genetics talking. But why do some trees genetically limit their ultimate growth?
Let’s go back to the idea of water. If a tree is 400 feet tall, that’s a long way to pull water up through its trunk. That’s a lot of work. Transporting water from the roots up to the leaves gets harder and harder the taller a tree is.
Since trees don’t have hearts to pump water around, the water is actually pulled towards the leaves as it moves to replace water that evaporated through the leaves.
Redwoods get around this problem by absorbing up to 40% of their water from the fog in the air, rather than the groundwater. But most plants lack this adaptation.
In other words, for many plants, too much height leads to water stress, reduced photosynthesis, and ultimately reduced growth.
In arid areas, like the savannah or prairie, there’s not enough water to grow very tall. It’s too risky to have to pull water up that far. Plus, why grow 300 feet tall when you’ve got no neighbors?
Finally, in windy areas, it’s dangerous to get too tall! Harsh winds can easily topple trees that grow too tall.
It takes a special set of circumstances to allow a tree to keep growing the way a redwood does!