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. :)

Growth rings

Twig1

Proving that there’s always more to learn about nature, last week while visiting my parents, my mom shared a tidbit she’d recently read in a library book she was working on. Apparently, you can track the annual growth of twigs on living trees using structural markers, much the same way you could count rings on a stump of a felled tree. Intrigued, I headed outside to check it out for myself.

I had noticed that the buds seemed to be out on the maple trees in the front yard, and wondered whether the warm spell had encouraged them to grow. When I paused to think about it, though, I didn’t really know at what point in the winter buds start appearing on trees. I kind of thought March, but it could be earlier. So I looked it up.

Turns out, trees actually form next year’s buds at the end of the summer. In thinking about this, it makes sense, because deciduous trees are dormant over the winter, going into a low-energy stasis (much the way the Red Squirrel does!). The buds on most trees are formed at the base of the current year’s leaves, but they’re tiny, barely noticeable. It’s during the late winter, as the days start warming up and the sap starts to flow again, that the buds begin to swell and develop. Some early swelling can take place in warm spells mid-winter, and I guess that’s what I’d been seeing.

I’ve also generally assumed these buds to be leaves, but they’re actually the flower buds – the leaves develop in the early spring, after the flowers are blooming. Although Silver Maples have both male and female flowers on the same tree, they tend to produce a majority of one gender or the other, resulting in either very low, or very high seed crops on a single tree.

Twig2

Back to the purpose of my outing, when I took a close look at the twigs of the Silver Maples in the yard, I could immediately see the rings in the bark that indicated the base of previous years’ growth. In the above photo you can see a number of rings circling the twig just under the bud. The rings are formed at the base of the buds (you can see some rings underneath the little stem supporting the buds in this photo, as well), and the twig’s new growth starts from the terminal buds. As the twig grows, the rings from where the terminal bud had been remain.

Twig3

The amount of growth from year to year seemed to vary greatly, and the amount of growth from twig to twig was also extremely variable. Here you can see the “growth rings” from the preceding three years (the 2008 rings will be at the tip of the current twig, when it starts to grow this spring, so the rings shown here indicate the base of the 2007 and 2005 growth – the base of the 2006 growth is harder to see because of the knobbly bit). As you can see, the twig grew considerably more in 2007 than it did in 2006 or 2005. In this case, I think in 2006 it started growing a side twig after the terminal twig was broken off, resulting in the funny bend, which may also have reduced its total growth that year.

I was so pleased with the discovery, I had a look at a few other tree species to see if it was as easy to detect. It was. Below is a twig from the Chokecherry in front of the house. The annual growth of this twig was more even between years, but I notice it also didn’t grow as much as the maple did in a year – about 5cm compared to the 15cm in 2007 for the maple.

Twig4