The Ecology of Skirts

Explanation: What good are dead leaves? This is the ecology question of the week.

As some plants grow, the lower leaves and branches die back, and instead of falling off they remain on the trunk or main stem and form "skirts" of dead cover.

First, why do some plants do this and others don't?

It may have to do with protection of the main stem from harmful insects, bird excavators, and other organisms that might tear into the bark and the growing tissues of the plant. It might also help prevent ferns, strangler figs, and other epiphytes from attaching to the trunk and potentially damaging the plant. But as to why some plants have this feature and others do not is unknown, and as far as I know, is not predictable.

Second, what other ecological function might such "skirts" provide, and how might that affect the plant?

Well, where plants form such skirts in many kinds of ecosystems, the skirts are often used by some invertebrates, birds, and even mammals, including bats, as good hiding cover and a safe place to nest, roost, or den. Whether the plant benefits from such a relationship is largely unknown.

So mostly, such functions of skirts are unstudied, and could make for great Master's or Doctoral dissertation research.

Here are a few examples of "plants with skirts" that I have encountered on some travels:

Massive hanging root tangles of a Ficus (fig) tree.

Ficus trees grow by sending down aerial roots and eventually strangling their host tree.

What is left is often a hollow column of aerial root tangles, forming a type of "skirt" which can serve as hiding, resting, and feeding habitat for many wildlife species.

In central Africa, the Maned Owl is purported to use such tangles of vines as roosting sites.

Ntchisi Forest Reserve, Malawi, Africa.

An ancient holdover found in New Zealand is this katote tree fern (Cyathea smithii) in New Zealand.

Many members of this tree fern family (Cyathaceae) form skirts of dead fronds around the trunk.

Pelorus Bridge, New Zealand.

Another tree fern, another skirt, this one from the tropics of northeastern Australia.

Also genus Cyathea (possibly species C. rebeccae, C. robertsiana, or C. cooperi).

Mount Lewis, North Queensland, Australia.

These palms growing in the southern Arizona Sonoran desert sported a thick skirt of dead fronds that refused to "leave home."

California fan palm (Washingtonia filifera).

Arizona, USA.

Hiding in the skirt of the palms above was a Cactus Wren, apparently using the palm skirt as hiding cover and perhaps as a nesting substrate.

Arizona, USA.

The main star of this week's EPOW episode is desert spoon (Dasylirion wheeleri), often mistaken for a yucca.

Desert spoons typically grow low to the ground and form a dense rosette of spiked leaves that likely help protect the trunk from herbivores.

The skirt might also help the plant conserve water in the hot dry desert environment.

And it certainly provides habitat for lizards, snakes, mice, and other small denizens of the Sonoran ecosystem.

Arizona, USA.

Finally, here are several more images of the kinds of skirts created by desert spoon:

Addendum:
     Wildlife Biologist Tom Kogut has offered the following additional information:
     One plant that Bruce left out in his astute and perceptive commentary is swordfern (Polystichum munitum), which also forms a skirt at the base of the live leaves. Most of the Cryptomastix devia (Sensitive/"Survey and Manage" snail) specimens I've located were found under these swordfern skirts which, as Bruce points out, provide hiding cover as well as a moist microsite for diurnal "roosting." Many other mollusks, arthropods and amphibians can be found under these swordfern skirts as well. When surveying the Tower Rock Thin timber sale (one commercial thinning unit located adjacent to the Cispus Center [in the Cascade Mountains of southern Washington state, USA]), I was surprised that we only located one C. devia site despite many nearby locations, and quite a bit of good bigleaf maple [Acer macrophyllum] habitat with lots of swordfern. I believe the reason was that intensive elk browsing on swordfern essentially eliminated "skirting" of mature ferns, and thus greatly reduced suitable habitat availability for C. devia. In that case, there was also a paucity of down wood, so there was no opportunity for down wood cover to compensate for the loss of the swordfern skirts.

EPOW - Ecology Picture of the Week