In the English language, the word “to survive” is an active verb. The Ironwood Tree (Casuarina equisetifolia) would seem to have taken the “active” part all-too literally. This Pine tree look-alike poisons or crowds-out its neighbor plants, grows rapidly and thrives in some of the poorest soils on earth, depletes the soil of all moisture, blocks sunlight for any plant growing beneath it, and each tree produces thousands of aggressively fertile seeds each year of its life. This is all done in a good cause, of course: that of its own survival. Fellow mammals, be glad you’re not some hapless plant trying to make a go of it in the Ironwood Tree’s neighborhood.
A solitary Ironwood tree withstands the strong prevailing Trade Winds near Kahului, north Maui.
Description
The Ironwood’s binomial taxonomic name is Casuarina equisetifolia (subspecies equisetifolia). Some other common names include Beach She-Oak, Australian Pine, Beefwood, Swamp Oak, Whistling Pine, and Horsetail Casuarina.
At first glance, Ironwood looks much like a Pine tree (Pinus sp.) with their needle-like foliage and woody fruit resembling a pine cone. However, the deciduous tufts of jointed and grooved “needles” (10-20 cm long and gray-green) are not true leaves: they are actually small jointed branches or twigs that perform the same functions as leaves. Minute tooth-shaped structures protruding from around the tops of each needle’s “joints” are the true leaves. This odd leaf architecture is one of the Ironwood’s many survival tools: the absence of typical leaves and the structure of the needles prevents the loss of moisture in growing environments where water is scarce.
Close-up of the "joints" of an Ironwood's "needles".
Male and female flowers are present on the same plant and, like the leaves, they are reduced in size with a simple structure to preserve moisture. Sepals and petals are missing, replaced by small bracts (or bracteoles). The male (or staminate) flowers have just a single stamen. The female (or pistillate) flowers have a single style and two stigmas. The staminate flowers are arranged into spikes known as catkins; the pistillate flowers are arranged on barrel-shaped heads known as axillary clusters.
Male (staminate) and female (pistillate) flowers of Casuarina equisetifolia. Photo courtesy of Dr. G. Carr, Dept. of Botany & Plant Pathology, Oregon State Univ. Corvallis, Oregon, USA
The cone-like fruit is abundant, especially on healthy mature trees, and forms only on the heads of the pistillate flowers. Immature fruit is green and covered with fine whitish-green bristles. When ripe, it becomes a hard, brown, warty-looking ball of spikes 1-2 cm in diameter, longer than it is wide, with a somewhat cylindrical shape. Each fruit contains a single winged nutlet about 6 mm long; these are cast upon the wind when the ripened cone splits in two.
Needle-like foliage and maturing fruits of the Ironwood Tree.
The trunk of a mature Ironwood tree averages about 0.3 to 0.5 m in diameter. While the bark of an immature tree is smooth, that of a mature tree is thick and rough, deeply furrowed and brittle, easily splitting into thin strips and flakes, exposing the reddish-brown layer of inner bark.
The Ironwood has a spreading, fibrous roots system that is capable of penetrating very deeply into any soil with an adequate supply of subsurface water. Under these conditions, it forms a dense mat of thick aggressive adventitious roots that can take up large quantities of groundwater. The root fibers are also able to form a symbiotic relationship with the nitrogen-fixing actinobacteria Frankia sp. This association results in the formation of nitrogen-fixing root nodules, which enables the Ironwood to colonize immature, nutrient-poor soils. More about this attribute later.
Close-up of immature fruit of Casuarina equisetifolia.
Range
The Ironwood’s natural range is tropical and subtropical seacoasts from Malaysia to Australia, Micronesia, Melanesia, the Philippine islands, and Polynesia. In Hawai’i, it is common in all but the driest and wettest coastal regions of the islands, especially sandy coasts and lowlands (to more than 900 m altitude). It was first introduced to the Islands on Kauai in 1882. More than 70,000 trees were planted on the forest reserves of the island; many others were planted on private lands. Its introduced range (including Hawaii, where it is naturalized) is coastal Florida, Puerto Rico, the Bahamas, and numerous islands of the Caribbean. It is now common on frost-free coastlines almost anywhere in the world.
Habitat
The Ironwood tree’s natural habitat includes foreshore dune fields and sandy flats, exposed sand bars, rocky strands, cleared and disturbed land, roadsides, and vacant lots. It is able to tolerate a wide range of soil types, but it prefers coarse-textured soils such as the dry, highly-saline calcareous soils of sandy beaches and the relatively young nutrient-poor soils developed on weathered volcanic flows. It is very salt tolerant, but it grows best in slightly acidic (pH of 5.0 to 7.7) sandy soils with good drainage characteristics. Although the Ironwood is known to reach its maximum development in areas with slightly depressional topography where adequate groundwater is perennially available, it can only withstand over-saturated soil conditions or complete inundation for short periods of time. Despite this, it is very tolerant of salt spray such as would be found on a windward-facing beach. The tree grows rapidly in hot weather and is able to tolerate annual temperatures as high as 27 degrees C. It can survive the occasional cold snap, but it does not tolerate prolonged cold or frost conditions.
Growth and Lifespan
The average Ironwood tree is 15 to 30 m tall, but many individuals grow to more than 40 m in height. It is a fast-growing (1.5 to 3 m per year) tree. Growth is most rapid during the first seven years of a tree’s life, but declines quickly after that time. This growth rate is only possible when little or no competing vegetation is present. Maximum growth is attained in 20 years. The Ironwood tree’s average lifespan is 40 to 50 years.
Ironwood trees towering over mature Coconut Palms, Kihei, south Maui.
Competitive Advantages and Invasive Behaviors
In addition to its abilities to propagate and grow quickly, and adapt to and thrive in a wide range of harsh environmental conditions, Casuarina equisetifolia possesses several other competitive advantages which it employs in the cause of its own survival.
It has few natural enemies (including predatory insects) in its Northern Hemisphere range. In part, this may be due to the high tannin content of its foliage and saplings, making them toxic and carcinogenic.
Once Ironwood trees colonize an area, they form dense monotypic stands under which almost no other plant species is able to grow. These nearly impenetrable thickets substantially reduce the amount of sunlight reaching the under-story and produce a thick blanket of branch and leaf litter that completely covers the ground. This radically alters the light, temperature, and soil chemistry regimes of beach habitats. All this serves to displace resident and native coastal plant species, especially those specifically adapted to beaches, mangrove swamps, and other ecologically sensitive coastal environments.
Opportunistic Ironwood saplings colonizing the stabilized sand dunes at Kanaha Beach, north Maui.
Ironwood trees are believed to employ allelopathy to eliminate competing plant species, as evidenced by the near absence of other plant species in the under-story once a mat of litter develops around the trees. Allelopathy is the release (by a few species of plants) of chemicals that inhibit the growth and development of competing plants. The chemicals involved are usually terpenes or phenols, and are present in large quantities in the roots, stems, leaves, fruits, or seeds of the allelopathic plant. They render the soil beneath the offending plant ecologically sterile, reducing its food value for competing species.
Nitrogen Fixation — The roots of the Ironwood tree are able to concentrate (or “fix”) atmospheric nitrogen (N2) in root nodules that contain large colonies of symbiotic bacteria of the genus Frankia sp. Mature nodules are filled with bacterial filaments that actively fix N2. This phenomenon is known as root symbiosis; it results in N2 fixation rates that are comparable to those of nodulated legumes. The ability to form N2-fixing root nodules enables Casuarina equisetifolia to colonize and flourish in young and nitrogen-poor soils such as those that have developed on weathering volcanic flows and stabilized sand dunes. However, prolonged water-logging inhibits the growth of root nodules.
An isolated stand of Ironwood trees "sculpted" by strong prevailing winds, Ha'iku, north Maui.
This excerpt from the Hawaiian Ecosystems at Risk (HEAR) project website nicely summarizes the invasive nature of the Ironwood:
Casuarina equisetifolia is one of the most “highly invasive, extremely problematic weeds in Hawaii. This species should not be planted anywhere in the state of Hawaii… because the damage that it causes (e.g. to native ecosystems) is permanent and irreversible. It is impossible to have just a few of these plants: Planting just one has effects outside the area where it’s planted; it’s not just the immediate vicinity that can be permanently negatively affected. Nor is it the immediate future that is the only consideration: a planting of a reproducing, wide-spreading species [such as this] can potentially last forever, affecting areas both near and far over time.”
The branch of an Ironwood tree displaying its modified leaves that resemble the needles of a Pine tree.
Economic Benefits
The Ironwood tree is in wide use throughout the Developing Countries for windbreaks and erosion control, reforestation, and rejuvenating depleted soils. However, it is considered a noxious weed in the Hawaiian Islands and as such is not cultivated for agricultural or industrial uses.
Ironwood is extensively used as firewood in Hawaii. Although it is difficult to ignite, it will burn hot (even when green) and the resultant ashes retain their heat for a long time. It has a high calorific value (4,950 calories or 8,910 BTUs) and has been called the best firewood in the world.
Early promoters of the cultivation of Ironwood in the Hawaiian Islands believed that it would serve as a good source of hardwood, pulpwood, and tannin. The abundant sprouts and saplings were intended to serve as a source of forage for cattle. Ironwood has proven inadequate for every one of these applications. Although its wood is hard, heavy, and strong, with a fine texture and tightly interlocked grain, it is brittle and difficult to work with because of its tendencies to crack, split, and shrink substantially when cured. The logs are difficult to saw, producing offsize timber with low market value. Pollen and sawdust pose a problem to humans as they cause respiratory irritation. Its pollen can cause allergic reactions such as eye irritation, sinusitis, and throat irritation.
The following are LINKS to more-detailed online information about the Ironwood Tree.
http://www.hear.org/alienspeciesinhawaii/species/casuarina/index.html — Invasive weeds in Hawaii: Ironwood, Australian Pine
http://www.hear.org/species/casuarina_equisetifolia/ — Hawaiian Ecosystems at Risk: Casuarina equisetifolia
http://en.wikipedia.org/wiki/Actinorhizal_plant — Wikipedia: Actinorhizal Plants
http://www.issg.org/database/species/ecology.asp?fr=1&si=365 — Global Invasive Species Database: Casuarina equisetifolia
Tags: actinobacteria, Actinorhizal, allelopathy, Australian Pine, Casuarina, Casuarina equisetifolia, equisetifolia, Frankia, ironwood, ironwood tree, Ironwood trees in Hawaii, nitrogen fixing plants, root symbiosis, She Oak, Swamp Oak
