What Is a Nurse Log? A Dead Tree Doing Its Most Useful Work

A seedling on a rotting trunk has several advantages over one on the forest floor. Decaying wood holds water like a sponge and releases it slowly, so it stays moist in dry spells when the surface soil does not, which matters enormously for a tiny seedling with shallow roots. It is raised above the litter layer, so seeds are not buried by falling leaves and seedlings are not smothered. It is above the dense herb and moss layer that outcompetes seedlings on the ground, and in many forests that competition is the main cause of seedling death. It is out of reach of some browsing animals and of surface flooding. It carries fungi that form the root partnerships trees depend on, already established in the wood. And its decaying tissue releases nutrients slowly and directly into the root zone. The combination is powerful enough that in some forests the great majority of successful regeneration occurs on dead wood rather than on soil.

The process leaves marks that persist long after the log itself has gone:

• •Colonnades, meaning rows of mature trees standing in a straight line, which mark the position of a single fallen trunk that they all germinated on
• •Stilt roots, where a tree's roots arch clear of the ground because they grew down the sides of a log that subsequently rotted away, leaving a gap beneath the trunk
• •Elevated root collars, with the base of the tree sitting above the surrounding ground level
• •Seedlings visibly lined along a log in graded sizes, which shows colonisation over successive years
• •Distinct plant communities on logs, including mosses, liverworts, ferns and fungi that occur nowhere else in the same forest
• •A decay sequence, since the log passes through stages from hard fresh wood through progressively softer classes, with different species colonising each stage

Regeneration is one function among many. Dead wood supports a very large proportion of forest biodiversity, and the organisms that depend on it are collectively called saproxylic, meaning wood-dependent. Estimates put the number of European species depending on dead wood at several thousand, including beetles, flies, fungi, lichens and the birds and mammals that feed on them or nest in cavities. Many are specialists requiring particular tree species, particular decay stages, particular sizes or particular positions, so a forest with plenty of small brash and no large standing dead trunks supports a quite different and much narrower community. Standing dead trees, called snags, provide nesting cavities that a great many species require and cannot excavate for themselves. Fallen wood in streams creates pools, traps sediment and provides cover for fish. Large decaying logs store carbon and moisture and moderate the microclimate around them. Managed forests historically removed all of it as untidy or as a fire and disease risk, which is why saproxylic species are disproportionately represented among threatened invertebrates in Europe.

Forestry practice has shifted substantially, and deliberate retention of dead wood is now standard in many certification schemes and public forest policies, with targets set for volumes per hectare and for numbers of standing dead trees. The shift required overcoming genuine objections, since dead wood does host some pest species and does carry fuel, and the working compromise generally distinguishes between situations, retaining wood in most contexts while removing it near infrastructure and in specific pest outbreaks. Creating dead wood deliberately has become a technique, with trees ring-barked, topped or damaged to produce snags, since simply waiting produces them too slowly in previously intensively managed forests. Public perception remains an obstacle, since visitors frequently read fallen and standing dead timber as neglect and report it, which is why interpretation signs explaining the practice have become common. The underlying change is a shift from seeing a forest as a crop of living stems to seeing it as a system in which a substantial proportion of the biology happens after a tree dies.

Decaying wood holds moisture, raises seedlings above the litter and the competing herb layer, and already carries the fungi that tree roots need, so in some forests most regeneration happens on dead trunks rather than on soil. Rows of trees on arching stilt roots mark where a log rotted away beneath them. Several thousand European species depend on dead wood, and removing it as untidy is why many are now threatened.