Narrowleaf mountain mahogany's adaptations to thin soils and herbivores

By Jeff Mitton

My perch on Muley Point allowed me to watch the first rays of sunlight strike the monoliths in Monument Valley and watch the warm glow penetrate the dark canyon of the San Juan River.

It had been a chilly night in late March, but now the warmth of a bright morning brought out the beauty of the early flowers on the Colorado Plateau.

I had never developed a fondness for narrowleaf mountain mahogany, Cercocarpus intricatus. From years of walking around it, I had the impression that it was a rough, ragged, prickly, scratchy shrub without sufficient greenery to throw shade. But on this morning, each shrub displayed many dozens to a few hundred minute flowers, and this colorful display transformed my disdain to interest.

I suspect that the dense, intricate branching and stiff, scratchy twigs of narrowleaf are an adaptation to protect interior leaves from herbivores."The evergreen leaves of narrowleaf are short with rolled back margins, less than an inch long. The flowers are radially symmetric, about 0.3 inches long, with no petals but with yellow to buff sepals that form a cup containing 1 pistil (female) and 10 to 20 red and yellow stamens. The small seeds have feathery plumes, about an inch long, that catch the wind to disperse the seeds. This and several other species are called mountain mahogany due to their dark wood, but they are shrubs or small trees in the rose family.

Three species of mountain mahogany have overlapping ranges on the Colorado Plateau, though on a smaller scale they occupy slightly different environments. Narrowleaf has the smallest leaves and generally the lowest elevational range. It ranges from sandstone benches below the pinyon — juniper woodland up into stands of ponderosa pine. Most shrubs are wider than tall, 3 to 4 feet in diameter and very densely branched; intricatus means intricately branched.

Alder-leaf mountain mahogany, Cercocarpus montanus, has deciduous leaves that do not curl at the margins, and it grows up to 9 feet tall and has a more open-branching growth form. It commonly overlaps the elevational ranges of both narrowleaf and curl-leaf mountain mahogany.

Curl-leaf mountain mahogany, Cercocarpus ledifolius, grows to over 20 feet tall with open branches and trunks up to 5 inches in diameter. Its evergreen leaves are about 1.5 the size in the other species, and the margins of the leaves are curled, but not as tightly as in narrowleaf. This species grows near the others, but it also extends higher to mix with spruce and fir forests.

All three species have fungal nodules on their roots that allow them to fix nitrogen. Their symbiotic relationships enable them to occupy sites with shallow, infertile, rocky soils and, consequently, they are frequently found on sandstone ledges, rocky ridges, or steep and rocky hillsides.

Although the descriptions above of the three species are apt for some populations, other populations display a continuum of leaf sizes and shapes, growth forms, and plant heights. Analyses of these populations have revealed that they are populations of hybrids. Studies of growth and reproduction have shown that the hybrids tend to grow faster than either of the parental species, and it has been proposed that the hybrids are better adapted to intermediate habitats. But the hybrids have low fertility, so hybrid populations do not seem to be expanding.

All three species of mountain mahogany are valued range plants because their leaves are highly palatable and nutritious. Bighorn sheep, pronghorn, elk, deer and cows all rely on mountain mahogany for winter browse. The taller growth habit of curl-leaf and alder-leaf allow them to grow beyond the reach of browsers, but the shorter narrowleaf is always within reach. However, I suspect that the dense, intricate branching and stiff, scratchy twigs of narrowleaf are an adaptation to protect interior leaves from herbivores.

Jeff Mitton, mitton@colorado.edu, is a professor in the Department of Ecology and Evolutionary Biology at the University of Colorado. This column originally appeared in the Boulder Camera.

March 11, 2016