Ulmus thomasii – Rock Elm

Trees Canadensis: Rock Elm on Second Line Road in north Kanata.
Rock Elm on Second Line Road in north Kanata

Rumours of Rock Elm’s demise are greatly exaggerated!  Although uncommon, Rock Elm is widespread in eastern Ontario and may be locally abundant.  It seems to be slightly more resistant to Dutch Elm disease and many healthy, mature trees can still be found.

Rock Elm is a large tree typified by rugged growth form.   It prefers neutral-alkaline substrate, often found on rocky limestone/marble uplands and also in rich mesic forest conditions, appearing to show significant shade-tolerance in a deciduous understory.  Its crown can be narrow and upright especially on dry rocky sites, or broader on richer soils.  Tends to retain branches along lower half of stem, which hang rigidly and provide characteristic contrast to umbrella-like growth form of the very common American Elm (Ulmus americana).  Twigs often have thickened corky growths, which can be very conspicuous in winter, but some trees nearly lack this trait altogether.  Buds tend to be pointier and lighter coloured yellowish/brown than American Elm, with leaf veins more closely-spaced; leaves often begin to turn yellow in colour along margins in autumn while the centre of leaves remains green.  Fruits are much larger than American Elm and the fruit/flower clusters are arranged in racemes.  This species often sprouts colonies of vegetative-clone suckers via surface roots.

Trees Canadensis: Rock Elm on Scotch Corners Road in Lanark.
Rock Elm on Scotch Corners Road in Lanark.

Good locations for viewing this tree in eastern Ontario include: Reveler Conservation Area (near Crysler), Summerstown Forest (near Cornwall), Portland Conservation Area (near Verona), Trillium Woods (look on the top of rocky ridges), and McCarthy Woods (Ottawa).  Rock Elms are prevalent in Lanark County around Carleton Place and can often be spotted from the road.

Read about the 2020 Carleton University Project: Rock Elm – A Species in Decline?
Here are selected photographs with location coordinates in iNaturalist:
http://www.inaturalist.org/observations/5189096 (old tree on rocky upland, Reveler CA)
http://www.inaturalist.org/observations/5188633 (root suckers in field, Reveler CA)
http://www.inaturalist.org/observations/2701296 (very large tree, Merrickville)
http://www.inaturalist.org/observations/2549179 (very large tree, Merrickville)
http://www.inaturalist.org/observations/5137678 (very large tree, Merrickville)
http://www.inaturalist.org/observations/3026808 (young tree, McCarthy Woods, Ottawa)
http://www.inaturalist.org/observations/3027466 (adolescent tree, McCarthy Woods, Ottawa)
http://www.inaturalist.org/observations/2701098 (mature tree, Ottawa greenbelt)
http://www.inaturalist.org/observations/5152184 (mature tree, Carleton University)
http://www.inaturalist.org/observations/5145153 (mature tree, Carleton University)
https://www.inaturalist.org/observations/7724681 (mature tree on Scotch Corners Road)
https://www.inaturalist.org/observations/10086692 (mature tree in South March Highlands)


Trees Canadensis - Rock Elm at Richardson Ridge development.
Rock Elm at Richardson Ridge development.
Trees Canadensis - Pointy, reddish-yellow-brown buds on seedling.
Pointy, reddish-yellow-brown buds on seedling.