The Australian pine is an aggressive pioneer species causing coastal erosion in many coastal and island ecosystems.
Casuarina equisetifolia is a member of the genus Casuarinaceae. There are 45 species native to Australia and Asia, however it has been planted on tropical shore all around the world.
It is a tall slender, deciduous tree with needlelike twigs, dark-green in color. These twigs are not leaves however they fulfill the function of leaves. The actual leaves are tiny and found at the nodes of the twigs. The Casuarina can grow to over 100 feet in height. The names of Casuarina equisetifolia have all originated from its resemblances. Casuarina refers to the resemblance the foliage has to Cassowary feathers and the species name of equiestum is due to the resemblance of the foliage to the horsetail. The common name of Australian pine can be misleading as the Casuarina is not a member of the pine family at all however it does resemble one.
The Australian pine thrives in salty areas, such as sandy coastlines, as it is extremely salt tolerant. It can also be found growing in a variety of soil types at low elevations inland.
The Casuarina has medicinal properties, the following are as cited by David Wells in the 1994 edition of Seashore Plants of South Florida and the Caribbean. The bark has astringent properties and when made into a tea has been used as a remedy for diarrhea and dysentery. The brown fruits when ground and mixed with nutmeg have been reported as a home remedy for toothache.
The Australian pine was originally planted in the coastal environments it is now a problem in, to help stabilize those shorelines. It was first introduced to Florida in the late 1800’s and the Bahamas in the 1920’s. It sheds its needlelike twigs regularly and this creates a mat over the ground vegetation that can help to prevent wind erosion. The nodules of the Casuarina’s root are nitrogen-fixing and this helps to create more fertile soils.
These positive effects were the reason the Casuarina was planted as a coastal shelter bed on the Nashan Islands, on an area of shifting dunes, with positive results, as cited by Seashore Plants of South Florida and the Caribbean. There was a 60% reduction in wind speed and a 12% reduction in evaporation, which led to a stabilization of the sand ultimately resulting in a 3-fold increase of agricultural yields.
The negative effects however, are outweighing the positives in many areas. The Australian pine has a very shallow root system, this means that in hurricane force winds they are the first trees to be felled. The dense shade caused by the thick Casuarina canopy stop ground vegetation getting the sunlight they need. The mat of foliage also has negative effects by again stopping the sunlight reaching the ground, soil-stabilizing vegetation and also having negative allelopathic effects.
Allelopathy is caused by allelochemicals (a type of biochemical) that can either have positive or negative effects on neighboring plants. In the case of the Casuarina, the foliage has negative allelopathic effects, the toxins suppress the growth of the understorey canopy.
These negative effects are the cause of much coastal erosion round the Caribbean and South Florida. Many areas are not only prohibiting the planting of these trees now, but also encouraging their removal.