Native Plants of Hawai‘i - Dicranopteris linearis

Dicranopteris linearis

Gleicheniaceae or False staghorn fern family

Synonyms: Dicranopteris dichotoma, Dicranopteris dichotoma var. emarginata, Dicranopteris emarginata, Dicranopteris emarginata var. inaequalis, Dicranopteris hawaiensis, Dicranopteris linearis f. emarginata, Dicranopteris linearis f. linearis, Dicranopteris linearis var. maxima, Dicranopteris sandwicensis, Gleichenia dichotoma, Gleichenia dichotoma var. emarginata, Gleichenia dichotoma var. tomentosa, Gleichenia emarginata, Gleichenia hawaiensis, Gleichenia hermanii, Gleichenia linearis, Gleichenia linearis var. maxima, Mertensia dichotoma, Mertensia emarginata, Mertensia hawaiensis, Mertensia linearis, Polypodium dichotomum, Polypodium lineaaris, Polypodium lineare

Uluhe

Unuhe

False staghorn fern, Old world forked fern

Natural Range & Environment

Dicranopteris linearis is native to the Old World tropics and subtropics, including: Africa, Asia, Indonesia, Australia, New Zealand, and Polynesia (including Hawai‘i) [Palmer 2003]. In Hawai‘i, it is indigenous to all the main Islands except Ni‘ihau and Kaho‘olawe, where it is found in mesic to wet sites, often covering steep slopes from near sea level to 2,000 m (6,562 ft) [Flora of the Hawaiian Islands].

Uluhe is one of the most common, perhaps THE MOST common, native ferns in Hawai‘i. It can be seen growing both in the open under full sunlight where it is most dense, as well as in the shade of a closed-canopy forest. New hikers are often cautioned to avoid wandering off the trail into an uluhe thicket because the fern can conceal dangerous cliff drop-offs. While restricted to mesic and wet regions, during dry periods, it is vulnerable to wildfires because of its highly flammable dead fronds and thick underlining detritus.

Uluhe fronds have indeterminate growth, allowing just a few plants to dominate large areas (photographs courtesy of John Game CC BY 2.0 and Forest & Kim Starr CC BY 3.0 US). It all starts with a fiddlehead atop a tall stipe (photograph courtesy of acezhou CC BY-NC 4.0) that unfolds and then forks. The process then repeats one or more times, ending with a pair of leafy fronds (photograph courtesy of The Nature Conservancy in Washington (wavolunteer) CC BY-NC 4.0). Uluhe fronds can be hairless to extremely hairy (photograph courtesy of Forest & Kim Starr CC BY 3.0 US). Header photograph of sori courtesy of G.D. Carr ©.

Description

Dicranopteris linearis is a very common long-lived (more than five years) fern that forms dense scrambling (sometimes climbing) thickets. It has long rhizomes which support the indeterminate growth of its very long (up to nearly 20 feet [6 m]) fronds. The density of reddish-brown hairs covering the fronds can vary dramatically between plants and some taxonomists have even classified very fuzzy plants as a separate variety or forma (see Synonyms above). Over time, dead fronds typically produce a thick organic layer beneath the plant(s). Fertile fronds produce sori on their underside, each with about a dozen sporangia.

Palmer [2003] provides a good description of the indeterminate growth of ulule and many ferns in the Gleicheniaceae family. It starts with a tall, often bluish stipe emerging from an underground rhizome, topped by an uncoiling fiddlehead. At its tip is a dormant bud enclosed by two small, leafy segments. Two straight secondary rachises, devoid of pinnae, emerge directly below the dormant bud and grow outward at a 45° angle. These then stop growing and are similarly tipped with a dormant bud enclosed by two small, leafy segments. Branching of these secondary rachises is usually repeated one or two more times before ending in a terminal frond with large, leafy pinnae. Meanwhile, the primary dormant bud begins to grow, repeating the above-described growth pattern.

Dicranopteris linearis is sometimes confused with the two other native species of Gleicheniaceae found in Hawai‘i, Diplopterygium pinnatum and Sticherus owhyhensis pictured below. There is a really great Instagram video by hawaiian.plants and lualimahawaii that shows the differences between the three species!

Diplopterygium pinnatum (photograph courtesy of Forest & Kim Starr CC BY 3.0). Note that D. pinnatum forks only once while Dicranopteris linearis and Sticherus owhyhensis fork repeatedly.

You can often seen the larger frond Diplopterygium pinnatum growing side-by-side with the smaller frond Dicranopteris linearis.

Sticherus owhyhensis (photograph courtesy of ka_y_ak CC BY-NC 4.0). Note the pinnae on the rachis below the fork, whereas Dicranopteris linearis lacks pinnae in this region of the rachis.

Growth Requirements

General

If you are fortunate enough to have uluhe growing on your property or in a landscape, DO NOT dig it up or attempt to transplant it (even if it's growing right where you wanted to place your hot tub)! Perhaps because it's so common in the wild, many people have tried to either transplant or vegetatively propagate (e.g., cuttings) uluhe. Unfortunately, ALL have failed.

Romanchak et al. [2006] conducted an important horticultural study of uluhe. (Here is a link to a 2005 online presentation of their experiments. To summarize their results, they found that:

No adventitious roots formed after eight weeks in a frond layering experiment using auxins.
There was zero survival after eight weeks for a division experiment where rooted uluhe rhizomes with at least two fronds were dug up and transplanted to 3-gallon pots containing native soil.
In a follow-up experiment to determine if the division experiment failure was due to separation from the mother plant or disturbance to the root system during transplanting, sections of wild uluhe were severed from their parent plant but otherwise left undisturbed. After four weeks, all severed sections were dead.
Propagation from spores successfully yielded many young sporophytes. However, the process took two years to produce ferns only three cm in height.

These results would seem to indicate that the only cultivation path for uluhe is spore propagation, a difficult multi-year endeavor. Or do they? In my search for online uluhe photographs to repost, I came across this one, an uluhe fiddlehead emerging all alone from the scorched earth of a wildfire, seemingly proof that uluhe can regrow from just its rhizomes (without its fronds). Meanwhile, Romanchak et al. [2006] follow-up experiment showed that uluhe rhizomes with their fronds (an important difference?) when severed from their parent plant will die, presumably because the parent plant supplies the severed portion with something essential. I would hypothesize that something is most likely supplemental water, something the severed rhizome was unable to provide enough of to keep its attached fronds from drying out and dying. However, what would have happened if Romanchak et al. had immediately removed all the fronds from the severed rhizomes in their experiment? Would they have witnessed something like the scorched earth photograph?

The answer to how to cultivate uluhe? (Photograph courtesy of Forest & Kim Starr CC BY 3.0 US.)

Unfortunately, David and I lack the resources to conduct the experiments needed to show that severed uluhe rhizomes — with their fronds immediately removed to prevent water loss — can resprout (like the scorched earth photograph). However, we hope some researcher reading this with the resources will pursue these experiments and possibly discover a quick and easier uluhe cultivation pathway using severed rhizomes. Until then, my top-line advice still holds: If you are fortunate enough to have uluhe growing on your property or in a landscape, please keep it in place undisturbed.

Pests and Diseases

In the 1990s, there were increasing reports of widespread uluhe dieback throughout Hawai‘i. Closer inspection revealed that the introduced alien two-spotted leafhopper (Sophonia rufofascia), first discovered in Hawai‘i in 1987 [CTAHR], was to blame. Since then, the damage appears to have lessened but is still common [Palmer 2003].

Photograph courtesy of W. Naganime ©.

Uluhe dieback on Maui (photograph courtesy of Forest & Kim Starr CC BY 3.0 US).

Uses

Ancient Hawaiian

According to Degener [1973], "The Hawaiians in former times occasionally drank an infusion of the uluhe as a laxative." There is a somewhat similar statement made by Hubbard [1952] on page 6 (unnumbered): "An infusion was prepared from the ULUHE, which the Hawaiians drank as an emetic, according to Fowler" [1940]. Unfortunately, I (Koebele) have been unable to access Fowler [1940] directly to confirm Hubbard's statement. Additionally, an emetic is a substance used to induce vomiting, not bowel movement.

Modern

Uluhe fiddleheads are used in the floral industry as a cut flower accent [Floral Design Institute]. Modern lei are also made with uluhe [McDonald 1978; Maui Nui Botanical Gardens].

Outside Hawai‘i, Dicranopteris linearis has numerous (mostly traditional) uses. Refer to Useful Tropical Plants for a comprehensive listing.

Special Features and Information

General

The Gleicheniaceae is a small fern family with 125 to 160 species within six to eight genera [Britannica; WikipediA]. Most species exhibit indeterminate growth of their fronds and are found in the Old World tropics, although the family is also present in warm regions of the New World [Britannica].

In Hawai‘i, there are three native species of Gleicheniaceae, the indigenous Dicranopteris linearis, and the endemic Diplopterygium pinnatum and Sticherus owhyhensis (see Description above).

Dicranopteris linearis is considered to be an equal-opportunity competitor that suppresses the sprouting and growth of both native and alien seeds and seedlings, respectively, with its dense frond thickets. It is able to dominate large areas, including phosphorus-poor sites, accounting for 74% of a site's above-ground net primary productivity while only containing 14% of its living biomass [Russel et al. 1998].

In contrast to other parts of the world, in New Zealand, Dicranopteris linearis is confined to geothermal active sites within the Taupo Volcanic Zone on the North Island, where it is designated as Threatened – Nationally Endangered [New Zealand Plant Conservation Network].

Hawaiian Name

Dicranopteris linearis shares the names, uluhe and unuhe, with a similar-looking endemic fern, Sticherus owhyhensis. Additionally, another endemic fern, Diplopterygium pinnatum, has the name uluhe lau nui [Palmer 2003]. (See Description above.)

Etymology

Dicranopteris comes from the Greek words, dikranos, meaning "two-pronged," and pteris, meaning "fern," referring to the forking growth pattern of the plant's fronds [Palmer 2003]. The species name, linearis, is Latin, meaning "having the form of a line," [Wiktionary] possibly referring to the plant's indeterminate growth behavior or its narrow pinnae.

BPK