What impact does the Australian pine have on the local plant community?

Story & Photos By Chloe Hardman

This dense mat of Casuarina needles can prevent other plants from growing.

Tall trees waving in the winds are a common sight along the beautiful beaches of the Turks & Caicos. Locally known as the cedar, these trees offer pleasant shade. But have you ever stopped to wonder where these trees came from?

History

Very few people alive today will remember a time when there were no cedar trees on the Islands. Historical records tell us the cedar trees were introduced to Florida in the late 1800s from Australia. They probably reached the Turks & Caicos around a similar time. This origin gives rise to another name for the tree: the Australian pine.

Although they look like pines, these trees are not true pines. The scientific name for these cedar trees is Casuarina equisetifolia. Having leaves reduced to tiny scales around thin branchlets gives the appearance of needles. What look like cones are the fruiting heads which are dispersed by birds, wind and water. One pound of these fruiting heads can contain up to 300,000 seeds. This is just one feature that makes the cedar tree spread so rapidly.

Spreading fast

Have you noticed an increase in the number of cedar trees on the Islands? Many people have. Bambarra and Whitby Beaches on Middle and North Caicos, respectively, are areas along which you may have noticed the trees spreading over the years. You may have also seen them along roads or wherever land has been cleared.

Why do these trees grow so well along beaches and roads? Colonising bare sandy soil is not easy for many plants. It is a dry, salty environment with few nutrients. Casuarina trees have a partnership with microbes, helping them to fix nitrogen from the air into the soil. Being salt tolerant and fast growing also helps.

Impact on other species

Being such fast growers gives Casuarina trees a competitive advantage over some other plants. There is concern that they displace native vegetation. In Florida the tree is considered a serious pest and is thought to interfere with turtle and alligator nesting. When a species spreads rapidly and causes harm it is classed as invasive. I am interested to find out what impact the Casuarina is having on the native plant community in TCI.

To investigate this I have been recording the plants growing under the Casuarina and comparing this to the plants growing outside of it. Many people told me “nothing grows under the Casuarina,” so I was surprised when I found a range of species growing under it. However the abundance of plants does appear to be lower under Casuarina, especially in areas that have been recently cleared.

Combination of threats

Casuarina is a plant that likes to grow where humans have disturbed the environment. It is often the first plant to grow back in a cleared area and puts down a carpet of needles and shade making it harder for other plants to grow. There is no doubt the dune communities would look very different without it. The further into the bush humans take bulldozers, the more Casuarina will grow and change the landscape.

Values and choices

The Turks & Caicos Islands are home to some truly unique natural habitats. Nine species of plants are found no where else on earth except these Islands. In contrast Casuarina trees are found on hundreds of islands across the world and form monocultures in areas which once were diverse.

What type of landscape do you value? On one hand, the Islands could end up looking like many other places in the world. On the other hand, leaving important areas of unique natural habitats whilst sensitively developing other areas could help preserve natural heritage. The choices the people of Turks & Caicos Islands make in developing the landscapes will determine which route is taken.

My research into the invasive plants on Turks & Caicos is part of my MSc degree in Conservation Science at Imperial College London, UK. I would like to thank the Turks & Caicos National Trust for working with me, particularly B. Naqqi Manco, Ethlyn Gibbs-Williams, Melanie Visaya, Bob McMeekin and Miranda Jones. The trip would not have been possible without the fantastic support of the Royal Botanic Gardens, Kew, UK, especially Martin Hamilton and Marcella Corcoran. Also, to the many people who have talked to me about the local plants, thank you for your time and insight.