Cercocarpus ledifolius / curl-leaf mountain mahogany

Adjectives: drought tolerant, exposed, fire, interesting bits, native, seed bank, tree, winter botany

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Curl-leaf mountain mahogany is a tree or shrub, ranging from 3 to 35 feet at maturity. Of all the plants on this site, this has one of the more interesting and restricted distributions in the western US.

Young plants often appear shrubby because they still have branches growing low, near the ground. The branches are often twisted and the twigs are stiff. In many cases, the overall crown appears wide, spreading and curved, but this depends on where it is growing. The gallery photos illustrate a range of possibilities.

The whole plant is pleasantly aromatic, so get close enough to brush against it next time you are out.

If you were to cut one down, or sample a branch, you’d find the heartwood to be dark reddish-brown, i.e. the color of mahogany. It is not, however, related to true mahogany.

The leaves of this plant are thick and leathery, simple with smooth margins and pointed at both ends. They are dark-green on top and silvery underneath, and persistent over the winter, i.e. the plant is evergreen.

Flowers have both male and female parts but no petals. They occur in small clusters (usually 2 or 3). As such, pollination is mostly by wind, although some insects do visit the flowers. The flower consists of a small tan tube from which protrudes a long, plumelike style covered in luxuriant tan hairs. I hope to get some pictures of this in the 2020 season, so if you are reading this before they appear, please come back.

The fruits of the curl-leaf mountain mahogany are short, and the spiral, silver-haired seed plumes are eye-catching. Off hand, I would say that neither the flowers nor the fruits immediately scream “Rosaceae”, but that is indeed the family.

This mountain mahogany reproduces almost entirely from seed, and the seeds can remain viable in the seed bank for 25 years or so. Resprouting after major disturbances, in particular fire, is rare and occurrence of these trees is generally inversely related to fire frequency. On the other hand, the bark is thick enough to improve survival after “light” fires. The seeds are also robust enough to sprout quickly after such a fire or logging.

Curl-leaf mountain mahogany is often found high on our mountains, isolated or in small groves. It occupies sites with generally poor soils, especially with respect to nutrients. These are hard places to live, and these trees have a couple of important adaptations. First, they are quite drought tolerant, slow growing and long-lived. The root system is shallow so not the major source of drought tolerance… keeping in mind that “shallow” for a tree can still be 4 feet or more. Second, Like Ceanothus velutinus, curl-leaf mountain mahogany is a nitrogen fixer, associated with actinorhizal nodules. Unlike snowbrush, however, the mahogany adds N to soil and increases growth of other plants, for example, limber pine (Pinus flexilis).

Interesting bits – one of the major cool things to discover using Cercocarpus ledifolius as your “model” is how its leaf morphology contributes to its drought tolerance. The leaves, as noted earlier, are thick and leathery. Just what does that mean? They are green on top and silvery underneath. Can that be translated into morphology or anatomy? What does curling do?

Amazingly, an undergraduate student (Amelia L Kellogg) asked (and answered) these questions for her senior thesis project at the University of Illinois – in 1915! Click the image for a larger version.

Here is what Ms Kellogg had to say about this drawing:

“The leaf of this plant is peculiar in cross-section in that there is a prominent mid-rib from which the blade curls back on either side, giving the entire structure [the] appearance [shown here]. This mid-rib contains a single vein with a definite xylem and phloem broken by narrow bands of parenchyma cells… Below the mid-vein is a crescent of spongy parenchyma meeting the small crowded pallisade-like cells above – which comprise the bulk of the internal tissues. The lateral veins proceeding from the mid-rib pass slightly upward and are distributed through the mesophyl. Each of the lateral veins is surrounded by a sheath of heavy-walled cells, one layer deep. A conspicuous layer of hypodermis occurs just below the epidermis [effectively blocking any stomates on the upper surface]. It is composed of large thick-walled cells abundantly supplied with tannin. This is also true of the cells composing the bundle-sheaths. The epidermis consists of a single layer of very small cells with a well developed deposit of cutin on the outer surface. Stomates are abundant on the lower surface of the leaf. Transpiration is reduced by a dense mat of unicellular clothing hairs and the peculiar curling of the leaf. The hairs are entirely restricted to the under surface of the leaf on either side of the prominent mid-rib…”