Dichotomous key (III) - Of twigs both stout and thin

White ash on left surrounded mostly by red maple in the background, note the ashes much stouter branches

Leaves have typically fallen off the tree in the fall (except on trees that died before the fall or marcescent  trees or evergreen trees), so we can't see if they're simple or compound. But looking at the twig can give us some indication of what they might have been. I know I said that relative characteristics aren't the best, but they can be helpful with gestalt (recognizing something on a subconscious level) and I'll try and add numbers in to support this. 

Stand of staghorn sumacs showing thick twigs (also support the heavy fruit stalks that are persistent)

Compound leaves can be thought of "cheap disposable branches" (Peter Thomas's description in the incredible Trees: their natural history). This allows the trees to put more investment into quick vertical growth rather than into developing energy intensive branches lower down the bole. A somewhat contradictory result is that compound leaves are bigger and therefore weigh more, and so need a more robust twig to support them. Therefore, we might ascertain from looking at a twig if it's simple or compound. While I'm focusing on opposite twigs here, some of the thickest twigs are the alternate compound leaves of sumac.

Butternut on left, staghorn sumac on right. Both have compound leaves attached to stout twigs

Below I've broken down which of the MAD Capped Bucking Horse species are simple and which are compound:

Simple	Compound
Maples (except boxelder) Dogwoods Caprifoliaceae (except elderberry) e.g. Viburnums, Honeysuckles	Boxelder (Ash-leaf maple) Ashes Elderberry Horse chestnut

I went ahead and did some homework and gathered twigs from most of these species. I arranged them in order of increasing diameter, and wouldn't you know it, this corresponds to relative leaf size! In the photo above, from L to R, we have honeysuckle, buckthorn, dogwood, red maple, sugar maple, norway maple, boxelder, white ash, elderberry. This wasn't exactly a perfect sample, but I did try to find "average" twigs.

Okay, so in a rough estimate, we have the following:

Relative twig diameter (1=smallest, 9=largest)	Species (s=simple, c=compound)	Leaf size (from Peterson's Eastern Trees)
1 2 3 4 5 6 7 8 9	Honeysuckle (s) Glossy buckthorn (s) Red-osier dogwood (s) Red maple (s) Sugar maple (s) Norway maple (s) Boxelder (c) White ash (c) Common elderberry (c)	1-2" 1.5-2" 2-5" 2-8" 2-10" 4-8" 4-10" 8-12" 4-11"

It works out quite nicely, at least in my minimum little experiment. I'll be it would be more instructive to measure twig diameter in the summer and measure the weight of all the leaves attached to that twig. But for now, I'm satisfied with the results.