Many nutrients are stored in a tree's wood and bark. When a tree dies and then decays, nutrients are returned to the soil, to be recycled and then used by the next generation of trees.

It takes about 20 years for a large log to decay completely.

Insect activity, as well as the growth of fungi, can speed up the decay of dead wood.

As a tree decays, various organisms began to colonize it. The first to arrive are those that invade the tree as it is dying. Next come organisms that live on wood that has recently died. Other creatures, which specialize in different stages of decay, come later, with the last group specializing in wood that has become crumbly.

The species that colonize wood in its earlier stages of decay tend to be more specialized than those that colonize it in the later stages.

Fungi

A tree trunk is made up of an inner layer of heartwood and an outer layer of sapwood. The sapwood contains the most nutrients. Some species of fungus grow in the heartwood, while others grow in the sapwood.

Fungi are usually made up of tiny transparent threads known as hyphae, which are too thin to be seen by the naked eye. We can only see a  fungus when the hyphae have produced fruiting bodies, such as mushrooms or toadstools, or bracket fungi.

Larger species usually appear in the earlier years of a tree's decay, when the wood still has a large supply of nutrients.

Life in Old Trees

Some old trees can take a hundred years to die.

Birds nest in natural holes that develop in these trees. - Woodpeckers create their own holes.

Bats can live in hollow trees.

Beetles can live in the dead wood inside of the tree.

Insects take advantage of the fact that as the live trunk grows outward, more dead wood is being added to their habitat.