Age Before Beauty
Examining the relationship between tree size and tree age
Nature inspires people to write. It probably always has, and if you pick up a book of nature writing from North America you're almost certain to come across someone writing about a walk through the woods. It's more than cliche at this point, but there seems to be something innate about a forest that makes us feel and think about the world.
These thoughts and feelings often manifest as connections with the past, which isn't surprising. We think of trees as old beings. And for the most part, they are. Even a tree of a short-lived species can live a long time compared to us (if it's left undisturbed). But not all trees are old. Some are older than others. When we walk through the woods, we tend to assume the oldest trees are the largest ones, and again, this makes sense - trees continue growing until they die. So, shouldn't the largest trees be the oldest?
It's important to find out, because old trees, like old forests, have irreplaceable value. They provide critical habitat for many species, they link younger forests to the evolutionary past through their seed production, and within society they take on important socio-cultural meaning in aesthetic and religious contexts. Despite this importance, old trees are disappearing globally due to climatic and environmental changes.
Conservation has, historically, made similar assumptions about tree age and size as most hikers. That is, if we protect the largest trees in the forest then we'll also be protecting the oldest ones, by default. There is some logic to this given that studies have previously reported a positive statistical relationship between tree age and tree size (size in this case meaning the diameter of the trunk). But there are also other studies indicating that not all old trees are large, and some show that many large trees aren't old.
These conflicting findings highlight the need for a systematic study of the relationship between tree age and tree size. A new study, recently published in Current Biology, did just that. Here, researchers gathered tree-ring data from a database known as the International Tree-Ring Databank. It's the most comprehensive source of tree-ring data globally, and it's well suited for this kind of research. The authors of the study compiled age and size data for more than 120,000 trees at over 5,000 sites globally, to answer three questions: 1) Is tree size a strong predictor of tree age? 2) What are the distribution patterns of both small and large old trees? and 3) What factors influence this distribution?
So, what did they find? In general, tree size increased with tree age in young trees, but this relationship weakened as trees got older. In fact, less than 2% of old trees - trees older than 264 years - in the study had a diameter greater than 100 cm (about 39 inches), while about two-thirds of these trees were less than 50 cm (about 20 inches) in diameter.
The authors also identified locations where old trees were more likely to be found. What's interesting here is that the distribution patterns were different for old trees that were small versus old trees that were large. Small old trees were more likely to be found in areas that were cold, dry, and limited soil fertility, while the opposite pattern was found for large old trees. Human activity also played a role in distribution patterns, having a negative effect on the number of large old trees at a site, but not small old trees. And with respect to conservation, 45% of large old trees were found in protected nature preserves, compared to only 29% of small old trees.
A number of biological phenomena explain these findings. The first is that some tree species prioritize a longer lifespan over higher growth rates. They invest more of their metabolized energy into chemical and anatomical defense, rather than expanding in size. The result is a stronger resilience to stressful environments, but a smaller tree overall. And here we see a kind of feedback loop, where trees in more stressful climatic environments are exposed to fewer harmful insect pests which can further contribute to a longer lifespan. Small trees are also less likely to be harvested for timber.
There's a lot to digest here, but a few conservation implications are clear. Not all old trees are large, and not all large trees are old. We can't rely on tree size as a perfect indicator of tree age. Of course, we can always use tree core samples to count the rings and assign an accurate age, but this require special fieldwork equipment, time and effort. It isn't efficient in many scenarios. Luckily, there are other characteristics we can use. Old trees, regardless of their size, tend have similar features, including: large holes and cavities in their trunks, spiraling growing patterns, crown dieback, and a higher presence of fungi and invertebrates. Once you see a few old trees, you start to notice the pattern.
Like words in a book, the trees in a forest can be read, if you know what you're looking for. And learning to read the forest can change your experience of it. Maybe the woods where you go hiking are older than you think. Or younger. In any case, it's important for the scientific and conservation communities to know which is which, and for that we'll need more studies like the one discussed here.
To cite this article, use the following:
Lockwood, B. (2024). Age Before Beauty: Examining the relationship between tree size and tree age. Brief Ecology Newsletter, 3. Retrieved from: https://benlockwood.substack.com/p/age-before-beauty
There’s a couple of instances where I would immediately think a smaller tree is an older tree; trees growing at or just above tree line and trees living in short growing seasons (closely spaced growth rings). Interesting that closely spaced growth rings (more rings per inch) are weaker than wide growth rings due to “spring wood” vs “summer wood”. I’m very intrigued about your comments on “spiral growth” being an indicator of age. I’ve not heard that explanation before.