Deadwood
fallen beech log being reclaimed by nature
There was a time when foresters used to clear away deadwood. Now it is realised that deadwood is vital to the sustained
functioning of woodland and forest. In any woodland, most of the nutrients are locked-up in the vegetation. Trees absorb
nutrients from the soil over many years and incorporate them into their bodies, that doesn't leave much for anything else.
Removing trees from a woodland is a sure way of preventing regeneration - the soil will eventually become too impoverished to
support new tree growth. In redwood and sequoia forests, it is not uncommon for a new tree to grow beside or even on top of a
fallen tree, taking up the nutrients over several years as the dead tree slowly decomposes. Deadwood is also useful to many
other organisms. Decomposers, such as many fungi and woodlice depend upon it (and in turn so do the other creatures that
depend upon the fungi - slugs, insects and the birds that eat the insects) and dead trees, especially if still standing, provide
shelter for animals and birds and insects such as bees which may nest in dead trees. Deadwood really is a vital part of the
ecosystem. Foresters will make deadwood safe - they cut it up into logs to clear paths and cut dead trees down only if they are
unstable and so pose an obvious risk to passers by, but they seldom remove it.
A tree does not have to be dead to have deadwood! In addition to the heartwood which no longer conducts sap, trees may
carry dead sapwood where a breach in the bark has exposed part of the tree to infection, or where fire or drought have killed
parts of the tree. Fungi and wood-boring insects will slowly decompose this deadwood. Some of these fungi will only grow on
deadwood and so will not infect living parts, others will spread into the living tree if they can and still others will only feed on
living wood as parasites. A forester will know which fungi threaten living trees and which help them - once the dead wood is
broken down the tree can absorb the nutrients released from it and recycle them. The tree may become hollow as the
heartwood rots away, but this makes the tree lighter and allows the wind to pass through, making the tree generally more able,
not less able, to withstand high winds.
Above: a bracket fungus on deadwood on a living oak tree.
Above, left: a beech log being reclaimed by nature! Centre: fungi on a decomposing
log. Right: more fungi on a tree that still stands. Red liquid full of fungal spores drips
down!
Creepy-crawlies! Above: decomposition is seldom a pretty sight, especially when viewed up-close! In addition to the orange
dots of fungus on the exposed parts of this fallen log, lifting a strip of bark reveals many woodlice, some slugs, a millipede and an
earthworm, all resting and waiting for night-fall to become active. On the right, a large orange slug (about 4 inches or 10 cm long)
has become active on a damp evening as dusk rapidly approaches. This slug was not alone - one was eating a dead earthworm,
another was munching on the leaves of ground-herbs. If you are walking around a wood in late evening, on a damp day, then be
careful where you tread or sit - large slugs are plentiful and emerge under these conditions to forage!
Slime moulds can also be
found grazing the fungi and bacteria on and in rotting logs.
The action of the various wood-eating
microorganisms and invertebrate
animals, combined with the effects of
weather (and possibly people passing
by!), will eventually break down any
tree into fragments that join the leaf
litter! The wood of some trees
decomposes quickly, birch wood
decomposes so fast that one has to be
quick to collect a nice birch log before
it's too late. Often the wood rots so fast
that all that remains is the hollow shell
of bark. Other trees take enormous
periods to decompose. Yew wood can
endure for thousands of years before
it finally rots.
Windfall! Toadstools grow from among the roots of a fallen hawthorn tree. These fungi
are possibly mcorhizal - fungi that normally live fused to the roots of trees in mutual
symbiosis. The tree and the fungus exchange useful materials with one another - the
fungus is get at getting minerals like phosphorus, whilst the tree can make sugars by
photosynthesis. Mycorhizae ('fungus-roots') are essential to the health of trees and few
trees grow well, if at all, without them. Some of these fungi are generalists and are able
to form relationships with a number of tree species, whilst others are specific to one
group of tree species. Importation of foreign trees has caused problems - the trees may
grow only very poorly if the native soil does not carry their own species of mycorhizal
fungus, in which case wood chips of the parent plant species can be used to 'seed' the
soil with the required fungal spores.
Both the fallen hawthorn above and the fallen oak below are still alive because they have at least one major root left in the
soil. If left to their own devices they may continue to grow and put out vertical branches that could become new tree trunks.
The tree below fell after a period of heavy rain and probably the clay soil became waterlogged and semi-liquid, causing the
root disc to swivel until the weight snapped the anchoring roots. Soil failure is a common cause of tree fall, especially when an
area consists of regenerating woodland on soil that was un-wooded for a time. Tree roots may eventually knit the soil together.
The remaining upright stem was grafted on the fallen tree. An earlier fire burnt some of the roots and this may have
contributed to the tree's instability.
This fallen oak was only about 60 years old and its trunk may weigh about 3 tonnes and the crown probably at least the
same. A mature oak could easily weigh ten or twenty times as much. It is just as well, therefore, that older trees are less likely
to fall!

Brown rot and white rot

Wood consists largely of cellulose, hemicellulose and lignin, which are all materials making up the cell walls. Cellulose and
even more so lignin are especially tough and to digest these materials a would-be wood-eater needs specialised enzymes.
The so-called brown rot fungi only decompose the cellulose component, leaving the brown lignin behind. Lignin forms a brown
powder when crystallised and as the enzymes degrade the cellulose the wood shrinks and begins to crack and eventually
crumbles  into dark brown cubes. These fungi are mostly basidiomycetes. For example,
Serpula lacrymans causes a dry root,
mostly in softwoods. White rot fungi may be basidiomycetes or ascomycetes and degrade lignin (and sometimes cellulose as
well). Degrading the brown lignin leaves behind the white cellulose skeleton of the wood.

Other denizens of decaying wood

In Britain alone, some 1000 or so species of animals have been found living in dead wood. Many of these are invertebrates:
specific types of beetles, such as the stag beetles and cardinal beetles, flies, woodlice, centipedes, spiders and snails. Some
of these, such as fungus gnats, feed off the fungi, whilst other feed on the wood itself. Many insects possess cellulase
enzymes to digest tough plant cellulose.
Fallen beech
Fallen beech
Fallen beech
No thing lasts forever, not even the giants! A fallen beech tree (Fagus sylvatica). Being hollow was not the problem in of itself,
since the central deadwood contributes little to mechanical support and trees often resorb the nutrients released from rotted
heart wood. However, this tree also had some of the outer living wood missing near the base and had been further damaged by
fire. Beech trees have shallow roots and normally only live for about 200 years (relatively short as far as trees go) despite
growing rapidly to reach enormous size. Often growing on chalky slopes with thin soil, the shallow roots often fail to support
these giants in the end. In high winds and especially when heavy rain weakens the soil their root plates often rotate, flooring
the tree and tearing many of its roots. It is more unusual to see such a large tree toppled by mechanical failure of the main
stem. Some trees survive falling if they still have roots in the soil and a continuation of the conducting vessels to the crown, or if
the crown produces new roots where it embeds in teh soil. Here the conducting pathway from the roots to the crown have been
completely severed!
Beech tree carcass
Above: new life from old - The crumbling remains of another long-dead beech tree. A viable sapling has taken root in the
nutrient-rich remains of the dead trees exposed root plate!
Dead wood need not consist of logs rotting on a damp woodland floor, for dead branches ('aerial logs') may occur on living
trees (such as old oaks with 'stag's head' crowns) or on dead trees which remain upright. These dead branches attract a
different range of living organisms.
Rot holes provide homes for entire ecosystems, with dry rot holes supporting different
systems than we rot holes.


Article updated: 27 Dec 2016


More on decaying wood.
stag beetle, 3D Pov-Ray model
Stag beetles breed in rotting wood. There
are about 1200 species worldwide,
varying considerably in colouration and
jaw size, and all belonging to the family
Lucanidae (order Coleoptera). The larvae
feed on dead and rotting wood, often
buried beneath the ground, where they
feed and grow for as long as 6 years.
This beetles can be large, up to 12 cm in
length in some species. The adults
generally live for a few months only,
reproducing and then dying.

The males have exaggerated jaws, also
called antlers due to their resemblance to
deer stag antlers. Although the males will
use their jaws for defence, opening them
wide in a threatening manner if alarmed,
their bite is reportedly weaker than that of
the much smaller jawed females. Thus,
there are costs to having exaggerated
jaws, but their real use is when males
fight over a rotting log. The males will
clash, attempting to throw each other off
the log or to overturn their opponent. The
winner gets to mate with a female
attracted to the log, who then lays her
eggs in the rotting wood.

Antler size in stag beetles varies
enormously and shows
positive
allometry
, meaning that larger males
have disproportionately larger antlers.
stag beetle, 3D Pov-Ray model
stag beetle, 3D Pov-Ray model
stag beetle, 3D Pov-Ray model
More views of the 3D model: click each image to
enlarge!