1Today I'm going to talk about the eucalyptus tree.
2This is a very common tree here in Australia,
3where it's also sometimes called the gum tree.
4First I'm going to talk about why it's important,
5then I'm going to describe some problems it faces at present.
6Right, well the eucalyptus tree is an important tree for lots of reasons.
7For example, it gives shelter to creatures like birds and bats,
8and these and other species also depend on it for food,
9particularly the nectar from its flowers.
10So it supports biodiversity.
11It's useful to us humans too,
12because we can kill germs with a disinfectant made from oil extracted from eucalyptus leaves.
13The eucalyptus grows all over Australia
14and the trees can live for up to 400 years.
15So it's alarming that all across the country,
16numbers of eucalyptus are falling because the trees are dying off prematurely.
17So what are the reasons for this?
18One possible reason is disease.
19As far back as the 1970s the trees started getting a disease called Mundulla Yellows.
20The trees' leaves would gradually turn yellow, then the tree would die.
21It wasn't until 2004 that they found the cause of the problem was lime,
22or calcium hydroxide to give it its proper chemical name,
23which was being used in the construction of roads.
24The lime was being washed away into the ground and affecting the roots of the eucalyptus trees nearby.
25What it was doing was preventing the trees from sucking up the iron they needed for healthy growth.
26When this was injected back into the affected trees, they immediately recovered.
27But this problem only affected a relatively small number of trees.
28By 2000, huge numbers of eucalyptus were dying along Australia's East Coast,
29of a disease known as Bell-miner Associated Die-back.
30The bell-miner is a bird,
31and the disease seems to be common where there are high populations of bell-miners.
32Again it's the leaves of the trees that are affected.
33What happens is that insects settle on the leaves
34and eat their way round them, destroying them as they go,
35and at the same time they secrete a solution which has sugar in it.
36The bell-miner birds really like this solution,
37and in order to get as much as possible,
38they keep away other creatures that might try to get it.
39So these birds and insects flourish at the expense of other species,
40and eventually so much damage is done to the leaves that the tree dies.
41But experts say that trees can start looking sick before any sign of Bell-miner Associated Die-back.
42So it looks as if the problem might have another explanation.
43One possibility is that it's to do with the huge bushfires that we have in Australia.
44A theory proposed over 40 years ago by ecologist William Jackson
45is that the frequency of bushfires in a particular region affects the type of vegetation that grows there.
46If there are very frequent bushfires in a region,
47this encourages grass to grow afterwards,
48while if the bushfires are rather less frequent,
49this results in the growth of eucalyptus forests.
50So why is this?
51Why do fairly frequent bushfires actually support the growth of eucalyptus?
52Well, one reason is that the fire stops the growth of other species
53which would consume water needed by eucalyptus trees.
54And there's another reason.
55If these other quick-growing species of bushes and plants are allowed to proliferate,
56they harm the eucalyptus in another way,
57by affecting the composition of the soil, and removing nutrients from it.
58So some bushfires are actually essential for the eucalyptus to survive
59as long as they are not too frequent.
60In fact there's evidence that Australia's indigenous people practised regular burning of bush land for thousands of years
61before the arrival of the Europeans.
62But since Europeans arrived on the continent,
63the number of bushfires has been strictly controlled.
64Now scientists believe that this reduced frequency of bushfires to low levels
65has led to what's known as 'dry rainforest',
66which seems an odd name as usually we associate tropical rainforest with wet conditions.
67And what's special about this type of rainforest?
68Well, unlike tropical rainforest which is a rich ecosystem,
69this type of ecosystem is usually a simple one.
70It has very thick, dense vegetation,
71but not much variety of species.
72The vegetation provides lots of shade,
73so one species that does find it ideal is the bell-miner bird,
74which builds its nests in the undergrowth there.
75But again that's not helpful for the eucalyptus tree.