Wrinkles of time

Growth rings on maple twig

I’ve been wanting for a little while now to repeat a post I did early in the life of this blog. In fact, it was the third post I wrote here (not including the “Hello world!” introductory post), two years ago less a week, on January 13, 2008. It concerned a subject that my mom had recently (at the time) discovered in a library book she was reading, though I seem to have neglected the name of the book.

We all know that you can count the rings of a tree’s trunk, once it’s been felled, to figure out the age it was when it died. You can also do the same thing for large branches by taking a cross-section. But did you know you can also observe the passage of years on the twigs at the end of the tree’s branches?

As a twig grows, its extension of length follows the same stop-and-start pattern as the rings in the tree’s trunk (which represent the laying down of new wood each year), since obviously they don’t continue growing in the winter months. At the end of the fall, before the twigs stop growing for the year, trees such as this maple develop flower buds at their tips that will turn into next year’s flowers come spring.

Growth rings on maple twig

Where they set their buds (regardless of whether or not the twig develops side branches) the bark of the twig “wrinkles”. You can’t see it at the point of the flower buds, but you can easily detect it for previous years. Look closely at the two photos above. See how there’s spots along the twig, about the same length as the twig is wide, where there appear to be a whole bunch of ridged rings clustered together? You can also see it at the base of the side branches on the top photo. That’s where the buds formed that year. Once the flower buds have finished and the tree has released its seeds, the twig continues growing (at least, I believe this is how it works; I poked about online to confirm, but couldn’t find anything that mentioned it. It would be hard for the twig to grow while bearing flowers at its tip, though).

Growth rings on maple branch

The rings persist many years. This is from much farther back on the branch; you can compare the thickness of it to the twig that’s coming off the side. The rings are still present, but you can see the ridges of the bark just starting to cross through them.

Maple twig growth

Because they’re a long-lasting physical feature, you can count backwards to examine the annual growth of each twig and branch. Here’s a sample, above. On this small twig I found four sets of rings, set at various intervals along its length. Each ring represents the start of a new year’s growth, with the portion of twig between each ring having grown in that particular year. So if we consider the small flower buds at the end of the tip as 2010’s new growth, then the section of twig just below the flower bud was what the tree produced during 2009, back as far as the first set of rings. The bit of twig between the first and second set of rings was the growth from 2008, and so on.

I only show as far back as the rings formed from the flowers of 2006, but I could trace back a few more years along the branch behind it (it just wouldn’t’ve shown up well in the photo). It’s interesting to note that the tree doesn’t grow the same amount of twig every year. The length it grows depends a lot on weather conditions such as temperature, and the amount of sunlight and rainfall. In optimal conditions, the tree will produce more twig growth, and in poorer years the growth will be shorter. It can also vary from twig to twig. It’s less pronounced on this twig than it was on the sample twig I used two years ago. (Also note how green the background was two years ago!)

Incidentally, data from the University of Waterloo about an hour and a half west of Toronto indicates that rainfall was about average in 2009, above average in 2008 and 2006, and below average in 2007. Doesn’t really help explain the growth pattern, does it?

Growth rings on maple twig

Have a look at your own trees next time you’re walking by. Try comparing them to trees in other areas or other habitat types. See any patterns? Yes? What do they suggest? No? Well, it’s still neat to observe the annual growth spurts, anyway. :)

Time seals all wounds

Carving on live birch

This is a familiar sight on public-use trails. Something in the human nature cries out to leave a mark, something to indicate that yes, I was here. Perhaps it’s our subconscious recognition that our life is fleeting, ephemeral? Perhaps it’s lovebirds applying superstition to an emblem of their love: as long as this shall remain, so shall we. Or perhaps it’s a declaration of possession, this bit of tree belongs to the carver. Whatever the motivation, the poor trees that find themselves suddenly trail-side usually also find themselves dealing with regular wounds.

Animals, when wounded, regenerate the cells that were killed or destroyed by nearby cells of the same tissue type splitting and multiplying to take their space. If tissue has died but was not removed, the dead tissue is sloughed off or metabolized, then replaced. Trees, on the other hand, simply seal off the wound site and dead or decaying wood, and don’t have the ability to regenerate dead tissue.

Birch tree wound

Trees wounds can be caused by being scraped, eaten by animals, broken branches, fire or insect attacks. The outer layer of the tree, the bark, is effectively dead and damage to this layer doesn’t result in injury to the tree. For a tree to suffer a wound, the injury must occur to the live wood containing the tree’s food and water transport systems (called the phloem and xylem, respectively). For some species of trees, like beech or maple, the bark is often thin and easy to penetrate (which makes them great for carving your initials into), while for others, such as many pines, the bark can be much thicker.

Trees use two methods to seal a wound. The first is compartmentalization. New wood growing around the edges of the wound creates a sort of “callus”, which effectively walls off the wounded wood from the rest of the plant. This prevents decay and infection from spreading to other parts of the tree. In the above, and below, a branch was broken off at some point in the tree’s life. The large rolls at the sides of the hole is the result of the tree sealing over the wound site with “callus” wood.

Old decaying tree

In addition to sealing off the wound site, a tree will also try to prevent the spread of infection by using certain chemical and physical responses to pathogens at the wound edge (the way our immune system sends out white blood cells to attack intruders). The exact mechanisms by which a tree does this are not well understood, but often the long-term health and survival of the tree depends on how well it accomplishes this. If pathogens are able to slip past, the whole tree may become sick. As with animals, usually vigorous, healthy trees are able to ward off infection successfully.

It used to be that the use of special tars or paints were recommended to dress tree wounds, such as those from pruning, to protect them from infection and to speed “healing”. In fact, research has suggested that these dressings did little to help, and may actually hinder a tree’s ability to seal a wound, as they may prevent drying and encourage fungal growth, and may interfere with callus growth. Similarly, filling a hollow tree cavity with the intention of increasing the strength of the trunk used to be fairly common practice, but isn’t often done anymore. It’s generally accepted that a tree’s own mechanisms are more successful than ours.

Carvings on live arbutus

This is the trunk of an arbutus, observed when I was out on Vancouver Island last summer. They’re beautiful trees, with their deep red bark and bright evergreen (but broadleaf, not needle) leaves. Their trunk is cool and silky-smooth to the touch, like a giant piece of hand-worn worrywood, it’s incredibly soothing to run your hand across. I wish we had them here. Just looking at these images and remembering makes me feel calm.

I was so intrigued by this tree, because it seemed to have an unusual method of “healing” wounds. Rather than growing in from the sides to leave a noticeable, sharp scar, like in the first photo, the arbutus almost seems to be lacking bark altogether and just keeps building up layers of wood, filling in wounds, more like how an animal would regenerate cells. Perhaps they fill in their wounds using the same method as the deciduous trees I’m used to seeing, but form smooth, uniform wood where the wound edges meet, due to the lack of rough bark. (Note, Kim, Bert and Ken were here.)

Broken limb on arbutus

This is the end of a broken branch, after the tree has healed over the wound site. It resembles an amputated limb to me, an even more uncanny resemblance to animal healing. I tried doing a bit of research on how arbutus trees deal with wounds, but I couldn’t find anything very helpful. I will admit that I didn’t spend hours hunting for an answer, so there may be something out there I didn’t get to.

Old fencing

Out in the woods behind my parents’ house there’s an old fence that used to bound part of a field, a long time ago. It predates my parents’ ownership of the place, so it’s several decades old. The tree it was secured to has grown over and around the wires so now it looks like they were drilled straight through the trunk. The scarring created by the wound-sealing process as the tree grew is visible only as a thin line, so the whole thing sort of now resembles a bit in a horse’s mouth.

Funny bark pattern on beech

Blackburnian and I encountered this weird beech tree while out at the Rouge. I have no idea what has happened to its trunk, since it doesn’t seem like the usual pattern of wounding from any sort of animal or insect attack I’m familiar with. It makes me think of parched, cracked soil, but I doubt that dehydration is the cause here. Maybe a beetle or fungus infestation?

Tree Gall

This last one is usually referred to as a “burl”, a large growth affixed to the trunk, or sometimes large branches or roots, of a tree. The cause of burls isn’t clearly understood, although it may be due to physical trauma, or insect or fungus infestation. Burls are prized as carving wood, as they have interesting grain patterns that create beautiful finished wood pieces. Check out the size of this one spotted in the Missouri Ozarks!