Casuarinaceae, the Sheoak Family
There are three species of Casuarina that have escaped from cultivation and naturalized in Florida: C. cunninghamiana Miq., C. equisetifolia L. and C. glauca Sieber ex Spreng. Only the latter two are listed as noxious weeds in Florida, but all species of Casuarina are Class I Prohibited Aquatic Plants. Recent genetic research has confirmed interspecific hybridization of Casuarina in Florida (Gaskin et al. 2009). Hybrids typically show intermediate characteristics, which can make species-level identification challenging. For that reason, this review will focus on characteristics that will help you to recognize the genus, rather than identification of the various species or hybrids.
Australian pine is a fast-growing, evergreen tree with an open, irregularly shaped crown. It can attain heights of 150 feet, but is usually only 70 to 100 feet tall. The bark is gray-brown to reddish-brown, smooth and peeling on younger trees, becoming rough and furrowed on more mature trees. Older trees also often develop a buttressed trunk base. What appear to be needle-like leaves are actually slender, segmented branchlets. They are usually pendant, olive-green to gray-green in color, and bear minute, longitudinal ridges. The true leaves are tiny, triangular, scale-like structures that are arranged in whorls at the end of each branchlet segment. Species-level identification is based on a number of characters, including length and width of the branchlet segments, the number of ridges in each segment, the number of scale-like leaves in a whorl, and the presence or absence of dark bands at the base of the leaves. The flowers are tiny, inconspicuous and unisexual, with staminate (male) flowers borne in small terminal spikes and carpellate (female) flowers in axillary clusters. Casuarina equisetifolia is monoecious, with both staminate and carpellate flowers on a single tree, while C. glauca and C. cunninghamiana are dioecious, with staminate and carpellate flowers produced on separate trees. In tropical climates, Casuarina spp. may produce flowers and fruit throughout the year, but in Florida, flowering generally occurs twice a year (between February and April and again in September and October). The fruit are tiny (1/5 to 1/6 inch long), single seeded, winged nutlets (samaras), which are borne in woody, ovoid to globose, cone-like fruiting heads measuring 1/2 to 1 inch long.
Of the three Australian pine species found in Florida, C. equisetifolia has the widest native distribution, ranging from the Andaman and Nicobar Islands through Southeast Asia, Malesia and the western Pacific Islands to northern and eastern Australia. Casuarina cunninghamiana is native to northern and eastern Australia, and C. glauca is native only to eastern Australia. All three are cultivated throughout the subtropics and tropics. Australian pine has naturalized in Florida, the West Indies, Hawaii, Japan, the Mascarene Islands, the Galapagos Islands, Mexico and elsewhere in tropical America. While C. equisetifolia and C. glauca are most common in coastal habitats near brackish or salt water, C. cunninghamiana is typically found on the fringes of freshwater rivers and streams and can occur further inland than the two other species.
Australian pine is found in frost-free areas along the coast from St. Johns and Dixie counties southward and in Franklin County in the panhandle. It is most often observed on coastal strands, but also occurs in tidal swamps, beach dunes, rock barrens, maritime hammocks and disturbed coastal sites. Casuarina equisetifolia is the most common and widespread Australian pine species in Florida.
Australian pine is superficially similar to the true pines (Pinus spp.), which can be distinguished by needle-like leaves borne in fascicles of two to five; cones consisting of overlapping, spirally arranged, flatted scales; and resin canals, which give the leaves and branches a distinctive fragrance.
Australian pines produce large quantities of seeds, which are primarily carried by wind, but may also be dispersed by water and sometimes by birds. A single tree produces thousands of seeds that can remain viable up to one year.
Casuarina was introduced to Florida in the 1890s and was widely planted as an ornamental and shade tree, as well as for lumber, ditch and canal stabilization, and as a windbreak for agricultural fields. Because it produces abundant, windborne seed and is very tolerant of poor soils, salt spray and occasional inundation, it has rapidly invaded natural habitats along Florida's coast. Australian pine outcompetes and displaces native vegetation by means of its rapid growth, dense coverage and thick litter accumulation, which has an allelopathic (growth inhibiting) effect on other plants. It encourages beach erosion by displacing deep-rooted native plant species and toppling readily in high winds. Its dense, shallow root system interferes with the nest-building activities of endangered American crocodiles and sea turtles.