| Today I'm going to talk about the eucalyptus tree. |
| This is a very common tree here in Australia, |
| where it's also sometimes called the gum tree. |
| First I'm going to talk about why it's important, |
| then I'm going to describe some problems it faces at present. |
| Right, well the eucalyptus tree is an important tree for lots of reasons. |
| For example, it gives shelter to creatures like birds and bats, |
| and these and other species also depend on it for food, |
| particularly the nectar from its flowers. |
| So it supports biodiversity. |
| It's useful to us humans too, |
| because we can kill germs with a disinfectant made from oil extracted from eucalyptus leaves. |
| The eucalyptus grows all over Australia |
| and the trees can live for up to 400 years. |
| So it's alarming that all across the country, |
| numbers of eucalyptus are falling because the trees are dying off prematurely. |
| So what are the reasons for this? |
| One possible reason is disease. |
| As far back as the 1970s the trees started getting a disease called Mundulla Yellows. |
| The trees' leaves would gradually turn yellow, then the tree would die. |
| It wasn't until 2004 that they found the cause of the problem was lime, |
| or calcium hydroxide to give it its proper chemical name, |
| which was being used in the construction of roads. |
| The lime was being washed away into the ground and affecting the roots of the eucalyptus trees nearby. |
| What it was doing was preventing the trees from sucking up the iron they needed for healthy growth. |
| When this was injected back into the affected trees, they immediately recovered. |
| But this problem only affected a relatively small number of trees. |
| By 2000, huge numbers of eucalyptus were dying along Australia's East Coast, |
| of a disease known as Bell-miner Associated Die-back. |
| The bell-miner is a bird, |
| and the disease seems to be common where there are high populations of bell-miners. |
| Again it's the leaves of the trees that are affected. |
| What happens is that insects settle on the leaves |
| and eat their way round them, destroying them as they go, |
| and at the same time they secrete a solution which has sugar in it. |
| The bell-miner birds really like this solution, |
| and in order to get as much as possible, |
| they keep away other creatures that might try to get it. |
| So these birds and insects flourish at the expense of other species, |
| and eventually so much damage is done to the leaves that the tree dies. |
| But experts say that trees can start looking sick before any sign of Bell-miner Associated Die-back. |
| So it looks as if the problem might have another explanation. |
| One possibility is that it's to do with the huge bushfires that we have in Australia. |
| A theory proposed over 40 years ago by ecologist William Jackson |
| is that the frequency of bushfires in a particular region affects the type of vegetation that grows there. |
| If there are very frequent bushfires in a region, |
| this encourages grass to grow afterwards, |
| while if the bushfires are rather less frequent, |
| this results in the growth of eucalyptus forests. |
| So why is this? |
| Why do fairly frequent bushfires actually support the growth of eucalyptus? |
| Well, one reason is that the fire stops the growth of other species |
| which would consume water needed by eucalyptus trees. |
| And there's another reason. |
| If these other quick-growing species of bushes and plants are allowed to proliferate, |
| they harm the eucalyptus in another way, |
| by affecting the composition of the soil, and removing nutrients from it. |
| So some bushfires are actually essential for the eucalyptus to survive |
| as long as they are not too frequent. |
| In fact there's evidence that Australia's indigenous people practised regular burning of bush land for thousands of years |
| before the arrival of the Europeans. |
| But since Europeans arrived on the continent, |
| the number of bushfires has been strictly controlled. |
| Now scientists believe that this reduced frequency of bushfires to low levels |
| has led to what's known as 'dry rainforest', |
| which seems an odd name as usually we associate tropical rainforest with wet conditions. |
| And what's special about this type of rainforest? |
| Well, unlike tropical rainforest which is a rich ecosystem, |
| this type of ecosystem is usually a simple one. |
| It has very thick, dense vegetation, |
| but not much variety of species. |
| The vegetation provides lots of shade, |
| so one species that does find it ideal is the bell-miner bird, |
| which builds its nests in the undergrowth there. |
| But again that's not helpful for the eucalyptus tree. |