On The Size Of Twigs

I’ve been working on a twig identification guide similar to the one I made for leaves. Admittedly, I’ve been having quite a bit of trouble organizing the guide. With leaves, it was a pretty simple and straightforward process to break down the trees into smaller groups based on leaf characteristics (like simple/opposite, teeth/no teeth, lobed/not lobed). But most people don’t give twigs much thought and twig characteristics (like bud scales, leaf scars) aren’t as familiar, obvious, or easy to see with the naked eye. I’m not going to go through the whole process of winter tree ID, but I did want to give a quick run down of how you can easily tell the ashes from maples in winter (both have opposite branches) and the ecological reason they appear different.

Leaves, buds, and branches generally come of the trunk of a tree either in pairs (opposite phyllotaxy) or alternating back and forth along the twig (alternate phyllotaxy). Phyllotaxy comes from phyllo = leaf (like phyllodough, those leafy sheets of delicious dough) + taxis = arrangement (like taxonomy, the arrangement of organisms into evolutionary groups). We have less than a dozen trees with opposite branches (and only a couple dozen more shrubs), and we can use the mnemonic:

“Always do the opposite of what your ‘MA’ tells you to do”

to remember the opposite branched trees (where MA = Maple + Ash). To separate the maples from the ashes, we can go back to the shape and size of their leaves.

To quickly separate the maples from ashes in winter, remember that maples have simple leaves and ashes have compound leaves. The leaves won’t be visible on the branch, but the difference in the leaf blade affects the size of the twig that hold those leaves. Compound leaves, which are significantly larger than simple leaves, are attached to much stouter twigs (see the image above). You can think of these compound leaves as a temporary and energetically cheap branches. The “branch” is the petiole + rachis, the “leaves” are leaflets. Here the margin (edge) of the leaf is so deeply cut that the cut extends all the way to the main vein (rachis) of the leaf.

They’re energetically cheaper because branches are made out of wood, which requires an annual investment of energy to grow and develop (the investment is the accumulation of growth rings). Compound leaves side step this annual investment in wood, allowing the trees to put more investment into quick vertical growth rather than into developing energy intensive branches lower down the trunk. Because compound leaves are bigger and heavier than simple leaves, they also require a more robust twig to support them (something codified in botany as Corner’s Rule). As a result, trees with compound leaves tend to have much stouter twigs and far fewer branches.

Therefore, we might judge just from looking at a twig if it’s simple or compound. Take a look at the image above, arranged from the thinnest (left) to thickest (right) are the twigs from 9 different species that grow near my house. This wasn’t the most scientific of studies, but I did try and collect an “average” twig for the photo. The table that follows shows a pretty clear connection between the size of a leaf and the twig that holds it.