Casuarina equisetifolia L. (Casuarinaceae)

Drought and salt resistant tree from Australasia widely planted in the tropics and sub-tropics. In Florida it is spreading on sandy shores and in areas disturbed by hurricanes. It forms dense stands and destroys reptiles breeding sites.


Species characteristics

Life form, size, lifespan

Evergreen tree reaching a height of 30-40 m.

Taxonomy, variation and plasticity

The var. equisetifolia and var. incana Benth. differ in height and stem straightness the latter being smaller and a poorer stem form and a more open canopy. Many morphological features are very variable.

Reproductive biology

The species is monoecious or dioecious with 2-3% of the individuals being monoecious. Males start to flower in the second year whereas females one year later. wind pollinated. The fruit is like a woody cone, 1 to 2 cm in diameter. They contain 70-90 seeds and take 3-4 months to mature. The seeds (achenes: 600 to 2000 seeds per g) have a membranous wing and are wind-dispersed. They can also be water-dispersed. Germination rate varies between 30 and 84 % but seeds are only viable for a few months. Usually, there are two main flowering and fruiting seasons although some fruits are often produced all year round, but in Hawaii and Puerto Rico flowering and fruiting are irregular. Seeds are produced annually.

Resilience and resistance

Adaptations to dry climate include scale-light verticillate leaves and modified needle-like twigs for reducing transpiration by means of sunken stomata lying in grooves, hairs, thickened cells and cuticle. Roots have nodules with symbiotic nitrogen fixing bacteria. It is not resistant to frost and has low fire tolerance but is salt tolerant. Adapted to growth on sand dunes as lower branches root and upright branches develop. Stilt roots and prop roots are sometimes in waterlogged areas. Does not suffer from browsing. When cut the tree sprouts profusely. It is often propagated by cuttings and in India by layering from low spreading branches.

Environmental requirements and successional status

A light demanding species which needs open sites for establishment, free from plant competition. It forms permanent stands in saline coastal environments and sand dunes.

Products and uses

The strong and heavy wood is durable in the ground and submerged in saltwater. It is a very good firewood and fine for charcoal whereas it is not adequate for fine carpentry. Widely planted for amenity purposes, coastal reclamation, medicinal purposes, tannin, dyes, pulp and timber.


Status in native range

Range and abundance

Native to Australasia. In Australia the var. equisetifolia occurs along a narrow coastal strip of tropical Australia whereas var. incana extends south (to 32S). Altitudinal range: sea level to 100 m.

Climate

Var. equisetifolia occurs in the hot humid and sub-humid zone (monsoonal) whilst var. incana is found in the warm sub-humid zone. Frost occasionally occur inland in the most southern part of var. incana range. Typical rainfall lies between 1000 and 1500 mm a year.

Site requirements

Found in dry and exposed habitats, typically coastal sand dunes. Var. incana may also occur around the bases of rocky headlands. Usually it is the only woody plant in vegetation dominated by grasses and herbs. Sometimes found associated with eucalypts.

Weediness

Not recorded.

Pests and diseases

Seeds are carried off in large quantities by ants. Seeds and seedlings are attacked by insects and crabs. It suffers from many fungal diseases.


Status in invaded regions

History of introductions and intensity of invasions

In Florida, var. incana Benth., was introduced around 1900 and planted by farmers and fishermen near homesteads and citrus groves for shade or as windbreak. In the 1930s trees planted to reclaim swampland as it was erroneously thought that the trees would dry the swamps. Extensively planted for reforestation in many parts of the world mostly on coastal dunes as well as in new urban areas of Malaysia. It was introduced to India as early as the 1860s but its provenance is unknown.

Patterns of invasion and time-lag

C. equisetifolia first started to spread along shell beaches and open sands, and by the 1950s it was only found in scattered locations in the Everglades National Park. Two successive hurricanes in 1960 and 1965 created the disturbed areas necessary for the rapid spread of C. equisetifolia. The species didn't survive well in areas of prolonged flooding and its spread inland was restricted by fire.

Often found in nitrogen deficient soils in urban areas where the top-soil has been removed. For instance, with Acacia auriculaeformis, it dominates the pioneer vegetation in urban areas of Malaya.


Site and climate

Floristic region and vegetation types

Coastal herbaceous swamp and broad-leaved hammock communities.

Pests and diseases

In Malaya it does not appear to suffer from pests although it harbours Icerya purchasi, a pest of orange and lemon. Many pathogens threaten extensive plantations in many parts of the world. In India a stem borer kills shoots and seedling damping-off by Rhizoctonia spp. occurs in nurseries. In Puerto Rico natural regeneration is rare because ants eat nearly all the seeds.

Impact on ecosystem

In Florida its roots can prevent turtles from digging nests for their eggs, and sometimes turtles are trapped by the roots. On coastal areas subject to wave erosion, toppled trees act as groynes, increasing loss of sand. Under the dense shade cast by C. equisetifolia canopy a thick mat of twigs is formed which prevents regeneration.

Impact on humans and related activities

The large quantities of air-borne pollen cause respiratory irritations. In Florida this large tree (up to 30 m) is considered as a public safety hazard. During hurricanes the fall of C. equisetifolia can damage property or block evacuation roads. In northern areas frost-killed trees become a nuisance due to the fall of branches and crowns. It often forms dense stands which are nearly impossible to walk through.

Control

Its threat to the Everglades National Park was quickly realised and a control programme was initiated. Seedlings were easily uprooted and larger trees killed by cutting or girdling and treated with the herbicides. However, the herbicides used in the 1960s, 2,4,5-T and 2,4-D, were and other herbicides have since been used with limited success. The spread of the species had been contained by the early 1980s but as seeds are continuously brought in from mature stands inside and outside the park, continuing seedling control is required. At present resources are not available to deal with the large dense stands.


Ecological differences

Existence of ecological equivalent species and competitive interactions in invaded regions

C. equisetifolia invades coastal areas of Florida which are free of woody plant species.

Differences in status and ecology between invaded and native ranges

No obvious differences are apparent, although no diseases have been reported from Florida suggesting that they have little impact on the invading population.


Selected references

* Little, E.L. & Skolmen R.G. (1989) Common forest trees of Hawaii (native and introduced). Agric. Handbook 679. US Forest Service, Washington.
** LaRosa, A.M., Doren, R.F. & Gunderson, L. (1992) Alien plant management in Everglades National Park: an historical perspective. In Stone, C.P., Smith, C.W. & Tunison, J.T. (Eds) Alien plant invasions in native ecosystems of Hawai'i: management and research, pp. 47-63. University of Hawaii Press, Honolulu.
* # Midgley, S.J., Turnbull, J.W. & Johnston, R.D. (Eds) (1983) Casuarina: ecology, management and utilization. CSIRO, Melbourne.
# Pinyopusarerk, K. & House, A.P.N. (1993) Casuarina: an annotated bibliography of C. equisetifolia, C. junghuhniana and C. oligodon. ICRAF, Nairobi.
* Ratnasabapathy, M. (1974) Acacia auriculaeformis and Casuarina equisetifolia - the urban invaders. Malay. Nat. J. 28, 18-21.
* Vietmeyer, N. (1986) Casuarina: weed or windfall? Am. For. 92(2), 22-24.
Troup, R.S. (1921) Silviculture of Indian trees, Vol. 3.
# Wilmot-Dear, C.M. (1985) Casuarinaceae. In Polhill, R.M. (Ed.) Flora of tropical East Africa. Balkema, Rotterdam.

Pierre Binggeli

May 1997