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My hope for this expedition to the San Rafael Swell was to find flowers on scarlet monkeyflower, which grows in hanging gardens on vertical walls. I had made similar trips in previous years but had not seen monkeyflowers in bloom. This time I spotted flowers growing from the wall, but they were not monkeyflowers. The flowers had the classic form of columbines.

Alcove columbine or small flowered columbine, Aquilegia micrantha, is a species that is endemic to hanging gardens on the Colorado Plateau. That is, it is restricted to the red rock, canyon country surrounding the Four Corners (where CO, UT, AZ, NM meet), and in that vast expanse, it is found only in hanging gardens. The flowers are usually white to cream. In terraces just above the floor of the canyon, alcove columbines were growing in soft sand. In addition, large, healthy, flowering columbines were rooted in small cracks 5 to 20 feet up an overhung wall, with no soil in sight. 

A colorful alcove columbine flower found in a hanging garden.

Alcoves form in sandstone, on the steep to vertical surfaces of escarpments and canyons. Sandstone is porous and permeable, so water falling on horizontal surfaces penetrates the stone, moving vertically until it settles on an impermeable layer. Thin, impenetrable lenses of limestone are the petrified remains of bottoms of ancient lakes and ponds. Water moves horizontally on the lens until it encounters vertical walls of escarpments and canyons, where it usually escapes through cracks. The floor of the alcove is the impenetrable lens and water moving over the lens erodes the sandstone to form an alcove.

Many widespread species are found in canyons and alcoves, but biologists who frequent canyon country come away with the impression that the plant communities in alcoves are made unique by the presence of endemic species. A study of 84 hanging gardens found 201 species of vascular plants. Of these, 12 are endemic to the Colorado Plateau and 7 are endemic to the hanging gardens on the plateau. 

Surveys of hanging gardens report several species of columbines: alcove columbine (A. micrantha); golden columbine (A. chrysantha); western red columbine (A. formosa); desert columbine (A. desertorum). However, only alcove columbines are specialists to and are endemic to hanging gardens. The other are found in alcoves, but they occur in other environments as well. Alcoves above entrenched creeks or rivers are mesic, shady and cool in comparison to the surrounding slickrock desert.  Endemics to these patchy, isolated and unique environments have long distance dispersal, tiny wind-blown seeds, and the ability to flourish in very shady, mesic, and relatively cool environments. Populations in hanging garden can be separated by large expanses of arid, sun-blasted rock desert.          

Hanging gardens of columbines form where water seeps from alcove walls.

Some descriptions of alcove columbines describe their range of flower colors as white to cream. But in other places, including the population that I found, flower colors include white, cream, yellow, blue and raspberry. To explain this diversity of colors in some but not all populations, Al Schneider at swcoloradowildflowers.com reminds us that even distantly related columbines hybridize. Given that at least three widespread species of Aquilegia are found in hanging gardens, it is possible that hybridization of alcove columbines is the source of floral color diversity in some populations of A. micrantha.  A specific example is seen in the hanging gardens of Zion National Park. Golden columbine (all yellow) hybridizes with western columbine (yellow petals, bright red sepals and spurs) to produce hybrids recognizable by subdued red hues in their sepals and spurs. 

Environmental conditions vary considerably between hanging gardens. Some produce abundant water year-round while others dry up in summers. Some are in the sun while others are usually in the shade and a few never experience direct sunlight. Many provide water that is potable, while others have white precipitates indicating high concentrations of calcium carbonate or black stains produced by precipitation of minerals such as oxides of iron and manganese. So not all hanging gardens would support the same community of plants. 

Surveys of plants in hanging gardens have noted one general association of endemics. Alcove columbine, A. micrantha, is usually found in the same gardens with scarlet monkeyflower, Mimulus eastwoodiae, and cave primrose, Primula specuicola. This association of three endemic species suggests to me that they have evolved to very similar conditions, and in that process, eroded traits needed to thrive in the relatively harsh environments outside their alcoves.