Although trees in primeval forest do not grow in great thickness, it is not easy to walk on the forest bed, as it is covered by thousands of broken branches and hundreds of fallen trunks. Some of these are covered by small green spots of moss, others are overgrown by moss slowly transforming into yellow and orange rotten wood. Trees that had fallen down several dozens or hundreds years ago remain small bulges with lush vegetation. Visitors to Bialowieza Primeval Forest are always amazed by the look and number of dead trees.

In forest environments heavily influences by humans, one cannot find any dead trees, as they simply do not have time to die and rot in natural way. Most of the trees are cut down when they reach maturity and the rest before they come of age. Trees which are broken, knocked over by the wind or damaged by insects or fungi are quickly removed. Only few trees, growing in inaccessible areas, where their removal would be both to difficult and expensive, can live long enough to wither and decay.

Dead wood is a natural and essential component of the forest ecosystem. This was the case for millions of years. A multitude of species specialized in feeding on dead wood, and the number of species eating these "dead wood eaters" is even greater. Those groups of coexisting organisms are dependant on a supply of dead wood.

Let us try to describe this coexistence in a simple way. Bacteria and fungi live on the dead wood. Many species of insects feed on such bacteria and fungi. Those insects serve as a food for other insects, arachnids, amphibians, reptiles, birds and mammals. In addition fungi which decompose dead wood are a host for other species of fungi which parasitize on them. The organic matter that was once a tree, passes through a chain of organisms and returns to the environment, where bacteria and fungi decompose it to simple chemical substances, that are again used to "build" new generations of trees.

There are animals species specialized to such an extent that they are unable to exist in an environment without dead wood. Two Polish woodpeckers are an example of such specialized animals.

White-backed Woodpecker feeds mostly on soft, rotten wood of standing and fallen stems. The woodpecker seeks for large larvae of insects living in decomposing wood. The hollows, where it dwells can only be scooped out in large, dead trees. White-backed woodpeckers require around 200 dead trees per 1 hectare of forest. Therefore it inhabits areas where every fifth or even fourth tree is dead.

Three-toed Woodpecker gathers 90% of its food on dead and dying spruces and scoops out hollows only in dead tree trunks. This species requires 20 large, dead spruces per 1 hectare of forest which gives a sum of 4000 such trees on a breeding territory of one pair of birds.

Two species describes above are unable to survive in forests where human influence is high and inhabit only a few areas in Poland, where foresters do not remove any dead trees.

In a natural forest of temperate climate zone, dead trees and their parts can make up to 1/3 of total biomass of trees. In the National Park of Bialowieza, within an area of one hectare there are on average 410 cubic meters of live trees, and 52 to 93 cubic meters of dead wood.

Dead wood can have many forms. Starting from withered branches of a living tree, long dead standing trunks, fallen logs, crumbled twigs and rotting remains overgrown with fungi and plants, finishing with dried mould dust carried by the wind. Each of this forms can be utilized by a couple of dozens or even hundreds of different species. Dead wood as a resource is in many ways exploited by over 40% of species inhabiting a forest. It is often the case that a dead wood is “"used" by more species than when it was alive.

Dead wood:

• is a source of nitric compounds (needed for growth of plants) slowly released to the soil;
• keeps carbon atoms bound, which delays the release of carbon dioxide to the atmosphere;
• participates in creating and enriches the structure of forest floor and soil.

Fallen trunks influence the way in which water is gathered and drained, they obstruct the flow of small brooks and soak up with water themselves. Rotting wood can absorb six times as much water in comparison to its own mass. Forest with large quantity of dead wood can stop or delay the flow of considerable amounts of rainwater which reduces the risk of flood. Water gathered in this way serves as a reserve for the times of drought and therefore reduces the risk of fires.

Hollows in tree trunks are an important component of the forest structure. In Poland there are 9 species of woodpeckers nad 32 other bird species that nest in hollows. Moreover, hollows are also inhabited by other animals: wasps, hornets, bees and dozens of other insects and arachnids, as well as mammals like: martens, squirrels, fat dormice, garden dormice, forest dormice, common dormice, yellow-necked mice and a couple of bats species.

In a natural forest of temperate climate zone you can find dozens of hollows in trees on one hectare of forest. From among them only 5% to 10% is occupied by birds each year. Not all hollows are appropriate for birds to nest in but the best ones are used by different birds in following seasons for many years.