We’ve got a problem in Southwest Florida — and there are problems in the Caribbean, South Pacific and elsewhere — with a tree properly known as the casuarina (pronounced caz-u-ar-ina). You probably know it as “Australian pine” and that’s half correct. It is from Australia, but it is not a pine. It’s not even a close relative of pines, although it appears to have needles like a pine and its seeds are produced in half-inch, cone-like structures. As a family, true pines are limited to the Northern Hemisphere, except for those we introduced to New Zealand and elsewhere. As a species, humans have a propensity to spread plants and animals around. The trouble is, exotic plants are like Jekyll and Hyde: In one setting, they can be beautiful and useful, but in a new ecosystem they can become a disaster.
The name casuarina refers to several related species of trees, three of which have been introduced to Southwest Florida. All are locally called Australian pine, but they’re also known by such names as beefwood, ironwood, and she-oak.
The one most of us are most familiar with is known scientifically as Casuarina equisetifolia because it thrives along our beaches. It is sometimes known as “beach she-oak” due to its habitat, drooping, hair-like “needles,” and wood that is as hard as oak. The word “casuarina” may not immediately have meaning – unless you are from Australia and know of the large ostrich-like bird called the cassowary. The casuarina’s long “needles” are said to resemble the long, hair-like feathers of the cassowary. The name equisetifolia also refers to those “needles,” a reference to their superficial similarity to the stems of a primitive and quite unrelated plant we know as “horsetail” or “scouring rush” whose scientific name is Equisetum. The name equisetum came from an imaginative botanist who likened the stems of scouring rush to the coarse hair (setum in Latin) on a horse’s (equus) tail. Both the “needles” of casuarinas and the stems of scouring rushes are coarse and appear “jointed.”
However, the “needles” of the casuarina are not really needles. They are highly modified stems. A close look reveals branching stems that emerge from what appears to be a joint, which is really a ring of tiny, tooth-like vestigial leaves.
Begin at the tip of a casuarina “needle” and follow it back. The needle-like structure is green, thus capable of undergoing photosynthesis to produce sugars for the plant. A green stem may end at another green stem, or at a slightly larger reddish-brown stem. At the next level is an even larger, slightly red stem. Stems as large as four inches in diameter are covered with flaky, gray-brown bark. On larger trees, the trunk — sometimes two to three feet in diameter — is nearly smooth and mottled with red and gray-brown bark.
The joints where the vestigial tooth-like leaves emerge to ring the green needle-like stems are obvious. Horizontal lines marking the joints are often still evident on stems two inches or more in diameter, giving stem surfaces an interesting texture. These joints remain active areas for potential stem development, giving the casuarina the ability to make a comeback from the stump of a damaged tree. Many casuarinas that were snapped off in the 2004 hurricanes are already showing a flush of new growth, testament to this tree’s resilience.
In addition to re-sprouting after disaster, the seeds of casuarinas are borne in brown, cone-like fruits that are readily carried by wind and waves. Seeds of beach she-oak are tolerant of saltwater, and hurricanes actually help to spread this casuarina to new areas.
In its native Australia, the hardy casuarina is used for lumber. It provides shade in a hot environment and wind blowing through its green stems hums much the same as wind blowing through the needles of pines. The casuarina is tolerant of salt spray and some fire. It readily recovers from damage or pruning and can be shaped as desired, even grown as a hedge.
So what is there not to love about this tree from the land Down Under? Plenty.
Because of its salt tolerance, easily dispersed seeds, and ability to send up new shoots from roots, casuarina can rapidly invade disturbed habitats. Once established, it out-competes native plants. A grove of casuarinas often has almost no other plant life growing beneath the trees. This may be a function of the dense layer of casuarina “needles” that accumulates, but it may also be due to chemicals produced by the tree that inhibit the growth of other plants. Isolated casuarinas often have some other plants growing near them, including native vines, such as Virginia creeper, and exotic weeds, like the poisonous rosary pea.
Australian pines have little value for wildlife, except for occasionally being tapped by wintering yellow-bellied sapsuckers and serving as perches for ospreys and other hawks. Indeed, they have negative value. Their branching pattern does not provide good nesting sites for most birds; their fruit is not a good source of food for wildlife; and in coastal areas, their shallow roots extending onto beaches can become a death trap for sea turtles attempting to dig nests beneath them.
Historically, casuarinas were often planted in this region to stabilize canal banks and beaches before it was realized that they out-compete native sea oats and other plants, which perform a much better job at stabilizing sandy soils. Casuarinas also were planted as a windbreak because they are rapid growers that can reach nearly 100 feet in height. Regardless, they can also snap easier than shorter, stouter native trees, as evidenced on Sanibel Island during the 2004 hurricane season.
As with other exotic plants, casuarina has few natural enemies in North America. One interesting pest is the casuarina spittlebug, so named because the insect produces enough of a frothy, spit-like liquid that it can hide inside as it feeds on the casuarina sap near the bases of the needle-like stems. If a spittlebug is removed from its “spittle,” it will produce more to build a new hiding place. For the tree, however, the spittlebug is little more than a nuisance that doesn’t slow this tree’s pace of invasion.
Because the casuarina is so detrimental to the environment, it is officially listed in Florida and elsewhere as an undesirable invasive exotic that should be removed wherever possible and is prohibited from being planted. At this point, eradication is a formidable task. Thousands of seeds each year can be released by a single tree to waves and winds — seeds that drift well beyond city streets and manicured yards.
Casuarina: A Beauty and a Beast by Dr. Jerome A. Jackson was originally published in the March 2005 edition of Expressions magazine.
