Trees on a Fold

“Forest” on Hutton Lake, next to pale sandstone outcrops. Pattern on lake is windblown waves.

Intent on following my serviceberry tree, neglected since March, I returned to the unexpected forest at Hutton Lake—trees growing where there "shouldn’t" be any. 

Off to see my serviceberry.

The Hutton Lake Forest is the patch of trees on the steep slope across the lake.

Hutton Lake lies in the southern Laramie Basin, in southeast Wyoming. To the east are the Laramie Mountains; the Medicine Bow Mountains lie to the west. The result is a double rain shadow, with the Laramies sucking moisture out of summer storms coming from the plains, and the Medicine Bows doing the same for winter storms from the west. The Basin receives only 11 inches (28 cm) of annual precipitation on average—not conducive to forests. Cottonwoods line the rivers, but the rest of the Basin is covered in grass and shrubs.

Yet on the south side of Hutton Lake stands a charming little forest, with two narrow leaf cottonwoods (Populus angustifolia), a patch of aspen (Populus tremuloides), and about twenty serviceberries (Amelanchier sp.), four of which I consider small trees, being taller than 65 in or 1.65 m (my height). All are growing in a narrow zone less than 100 m long on the northeast slope of a small ridge.

Narrow leaf cottonwood on a windy evening (windy is normal here).

Looking down into the aspen grove. The tilted rocks are a clue as to why these trees are here.

The aspen patch includes about a half dozen trees and many saplings. Aspen readily reproduce vegetatively, via root sprouts, so botanists are quick to conclude that stands are single “individuals” (clones). But there's more than one individual here, for example this tree:

This aspen and several smaller ones nearby are growing out of cracks in the sandstone.

The serviceberry tree that I photographed last March, when it was leafless, is now easy to identify, with its distinctive leaves and fruit.

In spite of the wind, some of these leaves ended up in focus. Click on image to see the distinctive oval leaves with obvious veins and toothed margins.

The berries look like little apples, not surprising given that apples and serviceberries are in the same subtribe of the Rose family (Malinae in the Rosaceae). None were ripe, but some were getting close.

Serviceberries have many names including juneberry, saskatoon, shadblow and others. I wish I could tell you the scientific name of mine, but unfortunately specimens from Hutton Lake have been called both Amelanchier alnifolia and A. utahensis. The two are difficult to separate:

“Serviceberries (Amelanchier spp.) intergrade and hybridize readily, making species identification difficult” (USDA Forest Service).

and:

“Identification is best undertaken in the field, with visits during flowering and fruiting seasons, and observations of habitat, habit, presence of congeners [related species], and flowering time relative to sympatric congeners” (Flora of North America).

Sigh. Maybe I will look into this for a future post, but for now there are much more interesting things to ponder … like the ant hordes.

Little brown dots winding up the right side of the aspen trunk are ants (click on image to view).

There were ants everywhere!—on sandy ground, on sandstone rocks, on aspen trunks growing out of the rocks, and on my bare legs. Fortunately they didn’t bite, but they swarmed up my legs whenever I stopped to take photos and notes.

Why so many ants? Why trees? Maybe the reason is the same—a fold in the land.

When the Laramie and Medicine Bow Mountains were uplifted via massive folds and faults, about 60 million years ago, minor folds were created in the downwarped basin. Hutton Lake lies on the northeast side of such a fold—the Boulder Ridge anticline.

From Ver Ploeg et al., 2016. Added arrow points to exposures of steeply-tilted sandstones

Much of the Boulder Ridge anticline lies out of sight, buried under younger sediments. But erosion has exposed it in places:

“On the south side of Hutton Lake the two limbs of the Boulder Ridge anticline are shown [exposed], the beds on the west side dipping south of west at an angle of 15° and those on the north side dipping north of east at angles of 50° to 85°. The steeply dipping beds … outcrop in a prominent ridge along the south side of the lake.” —Nelson Horatio Darton, 1909

Steep northeast limb (side) of Boulder Ridge anticline. 

Steeply-tilted Muddy Sandstone (lower Cretaceous) behind narrow leaf cottonwood.

Front to back: golden currant, serviceberry, Muddy Sandstone, aspen.

It’s not unusual to find trees associated with rocks in an otherwise unforested landscape. Perhaps there are suitable microenvironments where seedlings were able to grow in the absence of competition from grasses and shrubs. Perhaps there’s more moisture here, from snow drifts, or rain running off tilted rocks. We known that fractures in bedrock serve as reservoirs, funneling and storing water that can be accessed by roots.

As for the ants … well … I don’t know why they were so abundant on the loose sandy soil. Maybe it makes for better burrows. In any case, it appears that they also benefit somehow from the folded rocks of the Boulder Ridge anticline.

Walking back to the car, through the din of complaining prairie dogs, I thought about the little forest and how it came to be. How did seeds manage to land in that little patch of hospitable habitat? How did they hit such a small target? Serviceberry, aspen and cottonwood must cast many seeds far and wide—so many and so far that a lucky few will land in just the right place, even if it’s tiny.

White-tailed prairie dogs are summer company at Hutton Lake. To warn their neighbors of invaders, they chatter and cry loudly … until the invader gets too close at which point they quickly disappear down their holes.

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Sources

Darton, N.H., and Siebenthal, C. E., 1909, Geology and mineral resources of the Laramie Basin, Wyoming: U. S. Geological Survey Bulletin 364.

Knight, DH, et al. 2014. Mountains and plains; the ecology of Wyoming landscapes, 2nd ed. Yale University Press.

Ver Ploeg, A.J., Larsen, M.C., Taboga, K.G., 2016. Characterization of evaporite karst features in the southern Laramie Basin, Wyoming. Wyoming State Geological Survey Report of Investigations No. 70, 34 p.