Wednesday, May 8, 2019

Tree-following Stand-in: Singleleaf Ash (or is it?)

Obviously I’m not in Laramie, Wyoming, where it’s currently snowing. Instead, I’m 350 miles to the south and 3000 feet lower—in Fruita, Colorado. Unfortunately I forgot to visit the still-unknown tree that I'm following before I left home, but maybe that's just as well. I'm pretty sure it hasn't changed since last month's report.

Yesterday I considered various candidate stand-ins. Though this is dry country, there are many to choose from—maybe a tree in my friend’s yard or along the Colorado River path or in one of Kenton’s very cool native landscaping projects. But then I remembered the Singleleaf Ash and my mind grew dreamy. That would be my tree.
Fraxinus anomala, McInnis Canyons just south of Fruita, 2012.
“It had the distinctive one-winged fruits of ash trees! But could this be an ash? with only simple leaves instead of the compound ones typical of the genus?? and growing in the desert??? Yes! ... evolution has reduced the compound leaf to a single leaflet, hence the name. The scientific name suggests that the botanist who first described this tree was surprised as well: Fraxinis anomala.”
So wrote a fledgling blogger seven years ago (yikes!). I kept reading …
“Single-leaf ash, Fraxinis anomala, is a tree of the high-deserts of western North America. It grows from westernmost Colorado, where this photo was taken, west to easternmost California and south into northern Arizona and the northwest corner of New Mexico.” (1)
I knew the Singleleaf Ash grows in the dry canyon country just south of Fruita. But how easy would it be to find one? It's hardly common, requiring more water than most desert plants.
“… [some] ‘desert’ trees avoid arid conditions by growing where more water is available. Desert washes look dry much of the year, but often there is water fairly close to the surface, within root-reach. Tree cover often is greater in washes.”
When I hiked the Pollock Bench Trail in the McInnis Canyons National Conservation Area, I scanned each wash along the way. Soon I spotted my tree, plus a great perk for my field assistant.
While she frolicked in the muddy water, I again pondered the unusual leaves. In the absence of other evidence, it's really hard to believe this tree is an ash!
As I wrote seven years ago, most species of ash have compound leaves, meaning with multiple leaflets. The Green Ash leaf below, with seven leaflets, is typical.
Fraxinus pennsylvanica, courtesy TreeLib.
Fraxinus anomala is indeed an oddball among the ashes. Its undivided leaves are guaranteed to confuse a botanist unfamiliar with this tree. In my googling, I came across an essay by Craig Holdrege, a kindred spirit, who noted:
“To my amazement, I recognized the fruits as ones I knew as belonging to ash trees (the genus Fraxinus, Figure 2). Later I saw specimens with flowers that were also characteristically those of an ash. But ash trees always have divided leaves—so I thought—and the leaves of these shrubs clearly had simple round blades and long leaf stalks.”
Craig didn’t mention the problematic common name, which apparently didn’t bother me either seven years ago. But now I can’t ignore it. Why is this tree called Singleleaf Ash? After all, it has far more than a single leaf.

Maybe the answer seems obvious … it’s because most ash species have leaves with multiple leaflets, whereas Fraxinus anomala leaves have only one (2). But if you think about it, the common name doesn’t truly describe the situation. Though the leaves of Fraxinus anomala are sometimes described as simple (not compound), more likely they’re actually evolutionarily-reduced compound leaves, now with just one leaflet. Consider the California or Two-petal Ash (Fraxinus dipetala) below, thought to be a close relative of F. anomala (3). It’s easy to imagine a simple evolutionary step from multiple leaflets to just one.
Fraxinus dipetala; poto by Keir Morse (CC BY-NC-SA 3.0).

Therefore … [FANFARE!!!!] … Fraxinus anomala should be called the Singleleaflet Ash! Not impressed? Nor am I. This is just my mind entertaining itself. Let’s move on to something more interesting—the trees.

Three trees grew next to a shallow wash with low sandstone steps (above)—two on the left and one on the right. Does the second tree from the left look different? a bit more yellow? Yes … because it was covered in young samaras (seed pods).
I noticed that both the single leaflets and the samaras are shorter and broader than is typical for ash species (compare photos below). Is the same developmental thing going on? It’s fun to speculate, but for an answer, we’ll have to wait for the evo-devo folks to take an interest in ash trees.
Leaflets and samaras of the Green Ash (source).
Singleleaf Ash samaras from my 2012 visit to McInnis Canyons (different tree).


(1) I’ve since learned that the range of Fraxinus anomala extends into Mexico. It also has been reported for Wyoming, my home territory, but I could find no documentation online … something to investigate when I get back.

(2) Some individuals of Fraxinus anomala produce three or even five leaflets, but these are uncommon.

(3) Wallander, E. 2008. Systematics of Fraxinus (Oleaceae) and evolution of dioecy. Plant Systematics and Evolution 273:25-49.

For this month's tree-following news, and information about joining us, see May's gathering, kindly hosted by The Squirrelbastket.