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.
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.
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.