Growth form variation in Rock Elm (Ulmus thomasii)

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wrecsvp
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Growth form variation in Rock Elm (Ulmus thomasii)

Post by wrecsvp » Sun Jan 19, 2014 6:44 pm

Books often claim that Rock Elm has a non-elm-like growth habit e.g. more like green ash or pignut hickory. I find this to be sometimes true and sometimes not, with a significant percentage of Rock Elm I encounter looking as if they could fall within the (very large) growth form spectrum of American Elm.

Here's a photo from last spring to illustrate this point (taken ~10km NW of Aylmer QC):
The tall tree on the right is a "non-elm-form" Rock Elm with upright habit, narrow crown of horizontal branches, thick cork on twigs, and sprouts from roots.
The very different-looking tall tree on the left is also Rock Elm, but this one has something of an elm-like growth habit I would reckon. Note the much wider somewhat vase-like crown, strongly pendulous branches, light cork on twigs (absent in places) and lack of root sprouts.

The funny thing about this photo is that I was invited to see the tree on the right by the landowner who knew its identity as a thomasii but didn't recognize the tree on the left as also being a Rock Elm until I pointed it out. If anything I would say the left tree is more typical of the growth form, but it looks not very different at a glance than American Elm and is apt to be confused with it.

Thankfully, I find the buds of Rock Elm (yellowish-brown, narrowly conical, ~symmetrical, pointy) to be a foolproof ID feature when the caricature traits of the species (e.g. cork, monopodial growth form, root sprouting) are absent.
L-R (foreground): Rock Elm (tall), Slippery Elm, American Elm, Hop-Hornbeam, Rock Elm (tall, with small root-suckers behind)
L-R (foreground): Rock Elm (tall), Slippery Elm, American Elm, Hop-Hornbeam, Rock Elm (tall, with small root-suckers behind)

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