Classics
The following article has been selected because it is deemed very popular or very important to the arboricultural profession and deserves special recognition. It has appeared in a previous Seminar and because of this it is not eligible for earning certification credits; there is no test at the end of this article.
Big Trees
Edited by Len Phillips
What makes some trees grow tall and others do not? Why are the leaves on the tallest trees all about the same size, and why aren’t those tall trees even taller? It all has to do with basic scientific principles at work in nature. The explanation is in the physics of the tree’s vascular system that nourishes the tree from leaf, to trunk, to roots and its ability to hoist water hundreds of feet into the air. How does the maximum tree height vary with the environment and what limits the height of trees?
Genetics
Genetics plays an important role in determining tree size. This fact is obvious. Each species and cultivar of species has a very narrow range of sizes. However, what causes one tree out of a group of the same species to grow taller than the others is the focus of this article. Often it is variations caused by genetics, but more likely it is caused by leaf size and climate.
Leaf Size
Researchers found that tree leaves among all trees vary in size between one inch and four feet, and as much as one inch within a single species or cultivar. All the tallest species of trees, however, have leaves that fall in the range of 4 to 8 inches long.
This is because like most plants, trees fuel their growth and development through photosynthesis. This process uses water, carbon dioxide, minerals, and sunlight energy to produce carbohydrates as a sugar-rich fluid. There is an advantage for a tree to have larger leaves. It can produce more of this sugar-rich fluid that will flow quickly toward the trunk and roots via the phloem system. The faster and more efficiently it flows, the larger the tree can grow. If the tree’s leaves are small, the fluid moves too slowly to be beneficial for a tree to have exceptional growth. As the sugar-rich fluid passes through the leaf phloem toward the stem, it gathers speed as more and more water is pulled in from the leaf through osmosis. Consequently, the longer the leaf, the faster the fluid flows.
When the fluid reaches the tree trunk, however, no more sugar is collected in the phloem, and only water is drawn in from the surrounding tissue. The trunk phloem is longer and consequently, presents more resistance to the fluid’s flow as it travels down the trunk to the roots.
Tree Height
We do know that fast growing trees are able to capture more sunlight and also more energy through photosynthesis and can out-compete other species. Although increased height might provide some trees with better access to sunlight, it also will increase the length and thus flow resistance of the trunk phloem, slowing the movement of nutrients toward the roots. So there comes a point where the optimal limits on leaf size and tree height intersect, indicating the point at which it is no longer advantageous for the tree to become taller or produce larger leaves. This point is slightly more than 300 feet or approximately 100 meters, about the size of the tallest existing trees.
Other Factors
Other factors also play a role in determining tree height such as soil fertility, the frequency of wildfires, and the length of the growing season. However, these factors have only a minor influence on tree growth.
Climate
How does the maximum tree height vary with the environment? Does the photosynthetic energy that trees devote to produce new leaves alter the ability to hoist water hundreds of feet up the trunk of the tree? A recent study determined that tree height, resource allocation, and physiology vary with climate, so climate is likely to be the reason trees grow as high as they do.
The tallest of the flowering trees on record is a Eucalyptus regnans, mountain ash, which has grown to 374 feet (114 m) tall, and it is found in southern Australia. Sequoia sempervirens, the coastal redwood or California redwood is the tallest living tree in the world with one at over 379 feet (115.5 m) tall. It is found in northern California. The climate in both locations is similar with high rainfall, high humidity, and low evaporation rates. The tallest and oldest trees are found in deep valleys and gullies, where year-round streams can flow, and fog drip is a regular occurrence. Cool coastal air and fog drip keep these forests consistently damp year round and the trees do not suffer from evaporation stress. The trees above the fog layer are shorter and smaller due to the drier, windier, and colder climate. The moisture supply is obviously very important in growing extremely tall trees.
Researchers found a relationship between maximum tree height and the ratio of annual rainfall to evaporation. As the ratio increased, so did the maximum tree height. One explanation for this is that the constant supply of soil water allows the trees to allocate fewer resources to growing roots in search of water. Another factor deals with the high humidity that increases the rate of photosynthesis. This causes the stomata to open and take in more carbon dioxide that encourages greater growth by building more wood.
To summarize all this information, among a single species, the tree with the largest leaves and full access to water in the soil and humid air, will grow the tallest.
Sources
* Maciej Zwieniecki, “Tree Physics”, UC Davis News and Information, January 11, 2013.
* Givnish, Thomas, “New Analysis Links Tree Height to Climate”, University of Wisconsin - Madison, Ecology, 2014.
The following article has been selected because it is deemed very popular or very important to the arboricultural profession and deserves special recognition. It has appeared in a previous Seminar and because of this it is not eligible for earning certification credits; there is no test at the end of this article.
Big Trees
Edited by Len Phillips
What makes some trees grow tall and others do not? Why are the leaves on the tallest trees all about the same size, and why aren’t those tall trees even taller? It all has to do with basic scientific principles at work in nature. The explanation is in the physics of the tree’s vascular system that nourishes the tree from leaf, to trunk, to roots and its ability to hoist water hundreds of feet into the air. How does the maximum tree height vary with the environment and what limits the height of trees?
Genetics
Genetics plays an important role in determining tree size. This fact is obvious. Each species and cultivar of species has a very narrow range of sizes. However, what causes one tree out of a group of the same species to grow taller than the others is the focus of this article. Often it is variations caused by genetics, but more likely it is caused by leaf size and climate.
Leaf Size
Researchers found that tree leaves among all trees vary in size between one inch and four feet, and as much as one inch within a single species or cultivar. All the tallest species of trees, however, have leaves that fall in the range of 4 to 8 inches long.
This is because like most plants, trees fuel their growth and development through photosynthesis. This process uses water, carbon dioxide, minerals, and sunlight energy to produce carbohydrates as a sugar-rich fluid. There is an advantage for a tree to have larger leaves. It can produce more of this sugar-rich fluid that will flow quickly toward the trunk and roots via the phloem system. The faster and more efficiently it flows, the larger the tree can grow. If the tree’s leaves are small, the fluid moves too slowly to be beneficial for a tree to have exceptional growth. As the sugar-rich fluid passes through the leaf phloem toward the stem, it gathers speed as more and more water is pulled in from the leaf through osmosis. Consequently, the longer the leaf, the faster the fluid flows.
When the fluid reaches the tree trunk, however, no more sugar is collected in the phloem, and only water is drawn in from the surrounding tissue. The trunk phloem is longer and consequently, presents more resistance to the fluid’s flow as it travels down the trunk to the roots.
Tree Height
We do know that fast growing trees are able to capture more sunlight and also more energy through photosynthesis and can out-compete other species. Although increased height might provide some trees with better access to sunlight, it also will increase the length and thus flow resistance of the trunk phloem, slowing the movement of nutrients toward the roots. So there comes a point where the optimal limits on leaf size and tree height intersect, indicating the point at which it is no longer advantageous for the tree to become taller or produce larger leaves. This point is slightly more than 300 feet or approximately 100 meters, about the size of the tallest existing trees.
Other Factors
Other factors also play a role in determining tree height such as soil fertility, the frequency of wildfires, and the length of the growing season. However, these factors have only a minor influence on tree growth.
Climate
How does the maximum tree height vary with the environment? Does the photosynthetic energy that trees devote to produce new leaves alter the ability to hoist water hundreds of feet up the trunk of the tree? A recent study determined that tree height, resource allocation, and physiology vary with climate, so climate is likely to be the reason trees grow as high as they do.
The tallest of the flowering trees on record is a Eucalyptus regnans, mountain ash, which has grown to 374 feet (114 m) tall, and it is found in southern Australia. Sequoia sempervirens, the coastal redwood or California redwood is the tallest living tree in the world with one at over 379 feet (115.5 m) tall. It is found in northern California. The climate in both locations is similar with high rainfall, high humidity, and low evaporation rates. The tallest and oldest trees are found in deep valleys and gullies, where year-round streams can flow, and fog drip is a regular occurrence. Cool coastal air and fog drip keep these forests consistently damp year round and the trees do not suffer from evaporation stress. The trees above the fog layer are shorter and smaller due to the drier, windier, and colder climate. The moisture supply is obviously very important in growing extremely tall trees.
Researchers found a relationship between maximum tree height and the ratio of annual rainfall to evaporation. As the ratio increased, so did the maximum tree height. One explanation for this is that the constant supply of soil water allows the trees to allocate fewer resources to growing roots in search of water. Another factor deals with the high humidity that increases the rate of photosynthesis. This causes the stomata to open and take in more carbon dioxide that encourages greater growth by building more wood.
To summarize all this information, among a single species, the tree with the largest leaves and full access to water in the soil and humid air, will grow the tallest.
Sources
* Maciej Zwieniecki, “Tree Physics”, UC Davis News and Information, January 11, 2013.
* Givnish, Thomas, “New Analysis Links Tree Height to Climate”, University of Wisconsin - Madison, Ecology, 2014.