The understory of our forests is graced with lots of ferns in a variety of shapes and sizes. Ferns (along with trees and wildflowers) are classified as vascular plants, meaning that they all have special channels for conducting fluids from the roots in the soil through the stem to all the above-ground parts. That trait distinguishes them from mosses and algae, which lack those conducting channels and roots.
Licorice ferns (Polypodium glycyrrhiza) prefer moist conditions, and often grow around stumps and the bases of trees in decorative clusters of rather stiff, evergreen fronds. But, unlike most ferns, they are not restricted to the forest understory. They grow well in the mossy sides of tree trunks, sometimes several meters above the ground. Their spores are dispersed by wind and they sometimes land high in the tree canopy, where the young ferns find root-hold and grow as epiphytes. In fact, licorice fern has been documented in the crown of one of the largest redwood trees in California, at heights of well over 50 meters.
There are dozens of species of Polypodium around the world. Our licorice fern is distributed on the west coast of North America, from southern Alaska to California, with a reported outlier population in Idaho.
The fronds do not seem to be subject to serious browsing of the fronds by herbivores. Mountain goats, deer, and grouse may take occasional bites, but there is little documentation of such activity. In general, ferns are not eaten extensively by vertebrate herbivores.
Licorice ferns have long been known to humans for the intense sweetness and licorice flavor of the rhizomes. Rhizomes are substantial underground stems that generally grow horizontally, not vertically. They are not roots, although internet sources often conflate or confuse the two terms. Fern roots are thin and fibrous, taking up water and nutrients from the soil and decomposing organic material and transferring them to the rest of the fern. The roots of licorice fern also make mycorrhizal connections with fungi, and thus may pick up nutrients and other dissolved material from other plants in the neighborhood.
My curiosity was aroused by the well-documented sweetness of the rhizome. Why would an underground stem be so sweet? Some plants put sugars in their fruits, to entice birds and mammals to eat the fruit and disperse the seeds, a type or interaction in which both sides benefit. (Of course some insects also feed on the sugary fluid without benefiting the plant). But presumably licorice ferns wouldn’t benefit from having consumers dig up the rhizome and eat it. So what might be the functions of that sweetness? The sweetness molecules are thought to be “polypodocides”, glucose-containing compounds, some of which are hundreds of times sweeter than sucrose. These molecules are likely to be a means of energy storage for winter, since they contain sugars and are speculated to function also as antifreeze. Those functions would have direct adaptive value. The quality of sweetness (and the licorice flavor) might be side-effects, of no adaptive value in and of themselves. Clearly, more investigation is needed.
• Mary F. Willson is a retired professor of ecology. “On The Trails” appears every Wednesday in the Juneau Empire.
