In one of the early gwersi of the bardic grade, there is a meditation in which we are contacting the Goddess/ Spirits, asking not only what they can give us, but also what we can give back. As I was doing this meditation, I was asking the question and with the power of an electric shock I got the answer: “Study Ecology”. It shocked me so much that I came out of the meditation, and was so stressed about it that I did not even know how to react. There I was, a 42-year old programmer with a full time job in one of the most expensive cities of Europe: Surely it was impossible to stop working and start studying. I decided to ignore the experience for a while, but it kept returning in every meditation I did afterwards, so I started looking around and found the course “BSc Environmental Science”, which could be studid in the evenings. Not quite ecology, but close.
I asked the college to send me the application pack, and then left it lying for another 3 months in the hope the course would be full by the time I applied, but when I finally sent in the application, there were still spaces, and I started the course. In hindsight, it was one of the most rewarding experiences of my life, even though it was also one of the hardest, as juggling working and studying left little time for anything else. But the mixture of geology, geography and biology the comprised environmental science was a bardic journey through the elements and life itself.
At the end of the course, I had to write a literature review about a topic of my choice. At that time, a friend of mine who studied horticulture, was taking me along to the botanical gardens and arboreta in the South of England, where we spent hours identifying plants. And we found that this is easy in summer, in winter it can be a challenging experience, as there are no fruits, leaves or flowers that make identification easy.
One thing I noticed, however, was that horse chestnut trees were easy to identify: Most of them show a distinctive right-hand, counter-clockwise branch twisting around the tree trunk. This twisting starts early in the tree development, when it is still quite steep, and becomes more prominent as the tree gets older, until it can reach as much as a 45 degree angle. It made me curious: Did all horse chestnuts do it, and if yes, why? So I started looking into how trees grow, which became my final literature essay, and this seminar shares my knowledge.
Below is the image of 3 different trees: You can see that the trees in the left and right pictures have the distinctive right-hand twist (they are horse-chestnuts, BTW), while the one in the centre has a left hand twist. The latter tree is an unidentified tree from a US national park.
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The carpenters and woodworkers among us know that grain orientation of the wood is a major factor in determining what wood can be used for. As wood dries, the cells shrink and their general orientation becomes more obvious. If the grain orientation of the wood is twisted, the twist will become more pronounced. Where green, wet wood is used in building, it might change orientation later, causing problems with the stability. In the picture below, the doorpost twisted away from the door after building due to the reorientation of the grain during drying.
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If trees grow totally straight, they produce a nice straight grain that is very valuable for the wood industry. However, as any kind of twist in the wood grain decreases the value of the wood, the forestry industry put a lot of research into the reason why trees twist. Unfortunately, trees are slow-growing. long-living organisms and any studies take 20-50 years before they can be finalized. We therefore rely on experiments and research that was started in the 50s-70s of last century. In addition, a lot of research is focused on conifers, as they are the main timber producers. It seems, however, that any kind of results for conifers is also applicable to deciduous trees, so I have not distinguished between those kinds of trees.
As laymen, we never give it much thought how trees actually grow: They just do it and get bigger each year, more branches, a wider trunk. However, if we look at it closely, on a cellular level, it is much more complicated. Animal growth is largely genetically determined, we will become a human being no matter what our environment is, but while trees have that genetic component too, their final shape is influenced a lot more by the environment they grow in.
So what are the controlling factors?
Trees not only need to grow up to the light, they also have to react to wind as well as deal with different water and nutrient content of the soil. Since they are stationary, they might be in competition with other trees around them for the same resources, and unlike animals they cannot move from a bad spot, they have to cope with the location they are in. Its environment therefore determines how fast a tree can grow and what final shape it will take, in a much stronger way than it does for animals.
So how do they react to their environment?
If we look at any part of the tree trunk and branch system, there is a general pattern to be observed. A cross section of the layout of trunks and branches is shown in the image below:
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The Cambium is the actually growing part of trunks and branches. In early spring, the cambium starts to produce two kinds of cells: those growing to the outside and turning into the phloem, and those growing to the inside and turning into the xylem. The cambium always stays in the centre between these two zones and produces new cells as long as the tree is alive.
If, after a long and hard winter, we are not quite sure whether a tree/ plant is still alive, it is advisable to cautiously scrape a little bark off: If the cambium beneath it is still green, the tree is alive and might grow again. If it is brown, the cambium has died, which means there is no hope for the tree left.
Both xylem and phloem are vascular tissues, meaning they transport water and nutrients within the tree in a similar way to our blood vessels. The xylem is transporting water and nutrients from the roots upward to the tips of the tree, while the phloem transports water and leaf products back to the roots.
Phloem function: The cells of the phloem are small and have a different structure to the xylem cells. They transport a solution of sugars and hormones down the tree to the roots and do not need to be very wide, since the tree uses most of this nutrient solution in the canopy and only a small part of it is stored or used for root growth. The phloem is therefore a lot thinner and softer than the xylem. When phloem cells reach the outside of the trunk, they turn into first cork and then bark cells which protect the trunk, and are eventually shed as the bark falls off.
Xylem Function: the xylem transports water and basic nutrients (minerals and chemicals such as nitrate) to the canopy. As the tree needs a lot of water and nutrients in spring to initiate growth, the first cells produced for the xylem are relatively big and expand further as they grow. This is the main cause of thickening of a tree trunk: the addition of xylem cells to the inside. As the cells mature, their cell walls harden and they become hollow to make water transport possible. During the summer, the xylem cells eventually grow smaller until growth stops for the winter. This change in cell size creates the tree rings we see and can use to determine the age of a tree, as they denote the yearly change. In bad years, tree rings do not spread wide, as the tree does not have enough energy and resources to create many large cells, so we can also derive a picture of the environmental and climate circumstances for each year from the rings.
Finally, as the tree trunk thickens, the xylem cells are overlaid by new formed cells and die off completely. They are now buried in the centre of the trunk and become the heartwood which stabilizes the trunk. When we refer to wood we therefore mean the xylem cells.
So whether cells become water-bearing xylem or nutrient-bearing phloem is determined by where they are in relation to the cambium, but how do cells know into which orientation to grow lengthwise?
We need to look at the cells not in cross section, but laterally, and then we see a different picture:
I can only upload 3 images per post, so please see this image in the next post
Cells in the cambium are elongated along the trunk and branches. To grow, they are inserted between other, already present cells and push them apart or to the sides (inside/outside). The new cells can grow either with a straight vertical orientation, or they can twist to the left or the right. The vertical orientation during growth creates the wood grain we can see in planks of wood.
In general, the growth orientation is determined by gravity: Cells produce hormones which create a gradient inside the cell. Most hormones gather at the top of the cell, up against gravity, so the cell has an orientation and growth follows lengthwise against the hormone gradient.
In addition, the branches and leaves at the top of the tree produce hormones which are transported downwards inside the tree and suppress upward growth of lower level branches. At the top, the hormones levels are low and do not stop further upward growth, but as the hormones follow the way down the tree their amount grows, and shoot growth is suppressed.
For the gardeners among us: This is the reason why it is recommended to leave only 3 shoots on a fruit tree to further upward growth, with only one main shoot, as otherwise there is too much competition between branches striving for height.
So if the hormone concentration explains the lengthwise orientation of the cells during growth, it does not explain why or how trees twist their trunks. Again, we have to go back to the cells in the cambium.
Usually, trees do not grow in isolation: They grow in clumps, groves, forests. This means that a seedling has to put a lot of effort into growing to the light, faster than the other seedlings around it. Anyone who has ever twisted a rope knows that is gets shorter as the twist gets more pronounced, so growing twisted means that the seedling lags behind in upward growth and will get less light in the long run. In a competitive situation like this, surrounded by older trees and other seedlings, a new tree will grow predominantly straight up to maximize its access to light. In addition, it is sheltered by the trees around it from harmful winds and weather influences, so stability is not such an issue. In contrast, at the edges of a forest or grove or in solitary trees, competition for light is not so fierce for a tree seedling, but there is a lot of exposure to wind and weather.
It is those factors that influence the way a tree grows: Light, wind, snow, availability to water and nutrients.
In many trees, there is a genetic disposition toward twisting grain created by cells growing at an angle between 8 and 15 degrees from the vertical. Often, grain orientation changes from year to year: In one year, grain grows in a right-hand orientation, in the next in a left-hand orientation. This is called interlocking grain, and gives the trunk greater stability in wind than as if all cells are straight. Wood produced from those trees will be straight, despite the change in cell orientation, as the cells pull against each other during drying.
However, as the tree, and with it the canopy, grows, the environment puts more strain on a tree. A denser and bigger canopy is more prone to be catching wind than just a few small branches. Snow in the winter is caught more on large branches than on small, to the point where it is too heavy for the tree to bear. If additional wind twists a branch, it might snap off, or in extreme cases the whole tree breaks down.
It is in those cases that twisting grain to one particular side gives the tree a better survival chance. If the wood grain has an angle towards the predominant wind direction, wind forces tearing at the branches are better dissipated around the trunk and give more stability to torsion as if all grain is straight. Again, if we imagine the twisted rope: the fibres of the rope can cope easily with further twist, and pull the fibres back into position after the rope has been twisted. Straight grain cannot bend as well and dissipate those forces, so the tree is in greater danger of being snapped off in strong winds.
This also means that trees often start to produce more twisted trunks when they get exposed to wind. For example: Trees growing in a sheltered position in a forest predominantly grow straight, with only as much twist as is needed to balance the canopy. However, if trees are suddenly exposed to wind, e.g. after a windfall in the forest creates a clearing or if they exceed their neighbours in growth, they will start producing twisted grain to cope with a new environment.
The canopy size also has an influence: A small canopy does not catch a lot of wind shear, while large, widely branching canopies act similar to large sails, which means that a tree with a large canopy is twisted more than one with a small canopy. That is the reason why the tree usually produces more twist in the trunk and branches as it ages: It is a reaction to increasing canopy size.
Interestingly enough, trees are able to sense the kind of wind shear and snow burden even during dormant times. A study in which branches were weighed down in winter showed that trees increased the twist of grain and branch thickness in the following summer, for up to several years afterwards. They had “remembered” that there was a hard winter where branches were twisted more than usual, and reacted to it. So we can say that trees are never really inactive, even in winter, and have a memory that will be activated during the growth seasons.
Another reason why trees twist is related to the water and nutrient content in the soil. Trees cannot choose their place of germination, and once they are established, they have to deal with the location they are in. Sometimes, this location is less than ideal: They might grow on a spot where one side is muddy and one is rocky: A lot of water on one side, and not enough on the other. If the tree would grow straight up, roots on the wet side of the tree would have water in abundance, while roots on the other side of the tree would have little water to spare. The branches supplied by the roots below them would therefore be unbalanced: the ones with access to sufficient water would grow better than those with access to little water. For the tree, it would mean that the canopy would become one-sided, and the tree would be in danger of falling over by the sheer weight of its canopy. When the vessels grow in a circular motion around the tree, however, branches on all sides have equal access to water, however much it might be, leading to the formation of a balanced canopy even though the root system is unbalanced.
So looking at trees, very often we find that twisted grain is found more in trees that grow solitary, exposed to weather and wind, or in places that are not very favourable. While there is a genetic factor in the growth of a tree trunk, meaning that even in the best position a tree might have a twisted grain, in general twisted grain is a survival strategy that protects the tree from environmental strain and imbalance.
So look at the solitary horse chestnuts in our parks, see their huge canopies, and you know why they twist.
Two things are in abundance in the universe: hydrogen and stupidity.
- you've answered a question that's been bothering me for a couple of months now, since I noticed the pronounced twisting in certain spruces that grow near the peaks of mountains, in contrast with how straight the same type of tree is when it is more sheltered. I'm looking at all the trees on the property with new eyes, now, and I so appreciate the fact that you explained it all in clear, concise, thorough terms. That made it possible to actually absorb and understand the information the first time, rather than having to struggle through a lot of unnecessary verbiage. I am really glad you posted this! 
, but this is good stuff to know! 
