Thursday, March 12, 2015

Dichotomous key (II) - opposite (and sub-opposite)

Decussate opposite branching - each pair offset 90deg in sugar maple (Acer saccharum).

This post is mostly a reiteration and expansion of some of the notes from the previous post on branching patterns. There are relatively few trees/shrubs that have opposite branches. If we have an opposite or subopposite species we can use the mnemonic MAD Capped Bucking Horse. Usually it's just MAD Cap Horse, but since we're using shrubs here too, I called it a MAD Capped Bucking Horse.

Horse chestnut

Boxelder (Acer negundo) with branches offset at about 90degrees
Both alternate and opposite branches can have the nodes arranged around the stem in another pattern. The first, and far more common, is called decussate. Decussate branches have nodes at right angles to the pair above and/or below. By alternating placement, the tree maximizes the amount of light each branch receives.

Winged euonymus (Euonymus alatus) branch tending towards distichous branching
Distichous branches, on the other hand, run in two parallel lines up the branch. We don't have any distichous opposite branched trees in Vermont. It's a common growth pattern in understory trees that tend to arc out and flatten at the top, like musclewood or hophornbeam, in order to maximize access to sunlight. If you're growing vertically it doesn't make a whole lot of sense as this would shade out the leaves/branches below.

If you're growing mostly vertical, it makes sense to be decussate (branches offset 90deg), but for branches that flatten out, reaching to the sides of the bole, or trunk, for sunlight, a distichous pattern makes a whole lot of sense. While out today I noticed that the lowest branches on sugar maples are showed a functional distichous pattern. That is, for the most part, they had cut off growth of branches that grew along a vertical plane. I noticed on white ash that I looked at afterwards that the branches were mostly following this pattern, and in places where the vertical branches were still growing, the one away from the sun tended to be significantly longer. That way it could still harness sunlight while not shading out leaves adjacent to it.

Here in Vermont, we only find pronounced sub-opposite branching in buckthorns (common buckthorn, as shown above, and glossy buckthorn shown on the previous post). The faster a twig grows, the more offset are its paired buds. This can even be seen in species other than buckthorns. Once I started looking I noticed this on ash, though the distance they're offset was at most negligible. Glossy buckthorn appears primarily alternate. 

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